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Intra-mammary pressure changes in the lactating cow. Merriman, Mo Hendra J.G.S. 1960

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THE INTRA-MAMMARY PRESSURE CHANGES IN THE LACTATING COW by MoHendra J. G. S. Merriman B.V.Sc, Panjab University, India A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN AGRICULTURE i n the division of Animal Science We accept this thesis as conforming to the required standard Members of the Division THE UNIVERSITY OF BRITISH COLUMBIA April, i960. In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree th a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood tha t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed,without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver $, Canada. Date ABSTRACT The object of this study was to establish the nature of the int r a -mammary pressure changes that occur i n the udder of the lactating dairy cow during different phases of lactation. The author was, i n addition, interested i n the study of the various factors responsible for these changes. Results have shown marked and rapid fluctuations i n the intra-mammary pressure at the time of milking. Stimulation of milk ejection by manipulation of the udder caused a spectacular increase i n the i n i t i a l intra-mammary pressure. On milking a drop i n the pressure was recorded. The extent of this f a l l i n pressure was found to be influenced by the amount of milk removed from the udder. On complete milking the intra-mammary pressure dropped to the minimum le v e l . The pressure i n the hind quarters was found to be i n most cases higher than i n the fore-quarters indicating that the l e v e l of the intra-mammary pressure i n each quarter was under some influence of the quantity of milk present. A f a l l i n the intra-mammary pressure of a milked quarter had no effect on that of the other unmilked quarters. Intra-mammary pressure subsequent to the injection of oxytocin before milking as well as after complete milking was recorded. The administration of oxytocin under both of these conditions resulted i n a rise of the pressure, which, however, was not well marked. Considerable effect of the conditioned reflexes and of the emotional state of the animal on the intra-mammary pressure was observed during this work. - i i -TABLE OF CONTENTS Page ACKNOWLEDGEMENT i ABSTRACT i i I. INTRODUCTION 1-3 II. LITERATURE REVIEW Ii A. General h 1. Anatomy of the mammary gland U - 6 2. Physiology of lactation 6-10 3. Effects of secretions of the different endocrine glands on lactation (milk secretion) i n farm animals 10-13 B. Specific lit - 15 Milk ejection and intra-mammary pressure . . 15-26 III. MATERIALS AND METHODS 27 A. Materials 27 1. Experimental animals 27' 2. Hormone products 27 3. Apparatus 27 U. Functional description of the apparatus . 27-29 B. Methods 29 1. Standardization of apparatus . . . . . . 29-31 2. Procedure of recording 32 3. Hormone administration 33 iu Milk y i e l d 33 5. Precautions . . . . . . . . . . . . . . . 33 - 3U Table of Contents Page 17. RESULTS AND DISCUSSION 35-38 A. Physiological Studies 39 1. The intra-mammary pressure changes before and after complete milking 39 - U5 2. The effect of increasing time intervals on the intra-mammary pressure 1+6-50 3. The effect of p a r t i a l milking on the intra-mammary pressure 51-53 U. The effect of complete milking of the front quarters on the intra-mammary pressure of the unmilked hind quarters 5U - 56 5. The effect of milking each individual quarter on the intra-mammary pressure of the other quarters . . . . . . 57-58 B. Pharmacological Studies 58 - 60 1. The effect of administration of oxytocin prior to milking on the intra-mammary pressure 61-63 2. The effect of the administration of oxytocin subsequent to milking on the intra-mammary pressure 61+ - 65 V. GENERAL DISCUSSION 6 6 - 7 0 VI. BIBLIOGRAPHY 71-78 ACKNOWLEDGEMENT The writer wishes to thank Dean B.A, Eagles, Chairman of the Division of Animal Science, for providing the f a c i l i t i e s with which to conduct this study. Sincere appreciation i s expressed to Dr. W.D., Kitts, Associate Professor of Animal Husbandry, for suggesting this problem and for his direction, assistance and criticism during the course of this study. Thanks are expressed to Dr. A.J. Wood and Dr. J.C. Berry for their advice and Mr. J.C. McGregor for his assistance during the experiments undertaken. - 1 -I. INTRODUCTION Lactation i s the complex physiological mechanism of milk production. Various theories have been extended to explain the onset of lactation after parturition. Lactation i t s e l f comprises different phases which may broadly be grouped under two headings: milk secretion and milk removal. The two phases of lactation, though parts of the same process, are quite distinct from each other; a fact that was not recognized u n t i l recently. Therefore, the latte r phase, the phase of milk removal did not receive much consideration for compre-hensive investigation. Milk after secretion i s accumulated i n the mammary glands and requires to be removed periodically for efficient lactation. For convenience the phase of milk removal may be sub-divided into: (a) passive withdrawal and (b) milk ejection. It i s a well known fact that complete l e t down of milk or milk ejection i s essential to obtain the maximum quantity of milk present i n the mammary glands. Without milk ejection, only the milk present i n the gland cisterns i s obtained on milking which does not include the major portion of the milk stored i n the alveoli and smaller ducts of the mammary gland tissue. Milk ejection which has been recognized as a neuro-hormonal mechanism, i s stimulated by the act of nursing or hand manipulation of the mammary glands. On milk ejection the pressure within the mammary glands i s suddenly increased to i t s maximum. After complete milking the intra-mammary pressure f a l l s to the minimum level and starts r i s i n g again gradually u n t i l the next milking. The impact of the level of the intra-mammary pressure on the secretory effeciency of - 2 -of the glandular tissue as also on the composition of the milk secreted has been pointed out by Ludwin (39), Gasnier et a l . (31), Csiszar (17), and Donker et a l . (19). The precise evaluation of the effects of the intra-mammary pressure changes on these aspects of milk secretion i s , however, not available from the present literature. Before these effects can be ascertained, a thorough study of the changes i n the intra-mammary pressure i t s e l f i s necessary. This study has been undertaken by a number of investigators working on different animals but the results presented are inadequate and leave a l o t of ground for further investigation. The present work i s an attempt to understand the fundamentals of the problem of the milk ejection and intra-mammary pressure changes i n different phases of lactation. These experiments were performed on the dairy cow since this animal i s the best specimen for milk production. Moreover, most of the information collected from the study of the intra-mammary pressure changes i s intended to be used to understand and improve the milk production of the cow, an animal which alone contributes the most to meet the human needs of milk. The work reported here i s by no means a complete answer to the important questions related to the problem of milk ejection though the results obtained may prove to be of considerable interest and value. In addition to the observations on the natural milk ejection, the effects of injecting oxytocin, the hormone responsible for milk ejection were also studied. - 3 -The findings of this work support the v a l i d i t y of certain theories about the intra-mammary pressure proposed by a number of workers. It i s hoped that on further investigation when more knowledge i s available about this problem, i t w i l l be possible to improve the present practises of the dairy industry to make the industry more productive and profitable. Besides, the knowledge obtained could be useful i n understanding the problems of lactation of the other species. - k -I I . LITERATURE REVIEW A. General !• Anatomy of the mammary glands The anatomy of the bovine udder has been the subject of study by many investigators and has been dealt with comprehensively by Sisson (60) and Turner (65). The udder of the cox* comprises of four mammary glands and i s located i n the inguinal region. Each half of the udder i s situated on either side of the median l i n e and each of i t s quarters has a separate channel and opening to the exterior. Structure of the Udder (a) Macroscopic:- Within the udder there are two types of tissues: i . glandular or parenchymatousj consisting of alveoli and duct system, and: i i . the stromal tissue consisting of fibrous and adipose connective tissues. The teat of each quarter at the tip has an opening called the papillary duct or streak canal which i s l / U to 1/2 inch i n length. The streak canal i s normally kept closed by a circular or sphincter muscle. This arrangement effectively blocks the escape of milk from the udder between milkings. Above the streak canal the cavity of the teat widens to form the teat cistern which at i t s upper end opens into the gland cistern through a circular opening. The gland cistern, also called the lactiferous sinus, i s irregular i n shape and has numerous pocket-l i k e processes into which the larger ducts open. The teat cistern and the gland cistern of each individual gland communicate with - 5 -each other freely through the circular opening, (b) Microscopic;- The larger ducts rapidly divide into smaller and smaller branches. Ultimately the finest ducts end i n elongated or pea shaped dilatations called alveoli. The alveoli are grouped together i n clusters within a distinct fibrous capsule. Such a grouping i s called a lobule. These lobules themselves group together to form the lobes separated by broader connective tissue septa. It i s believed that the sinus-like dilatations along the duct system are storage spaces for milk as i t accumulates i n the udder during intervals between milkings. The epithelium of the alveoli and the terminal ducts i s made up of a single layer of columnar and glandular epethelium. The other ducts of the gland and the teat cisterns are lined with two layers of epithelium. The nature of the sub-epithelial tissue around the alveoli and inter-calary ducts has been a subject of controversy. Richardson (55) has shown this tissue to be myoepithelium and not the smooth muscle that was thought to be responsible for the contraction of the alveoli during milk ejection. Circulatory system;- The udder i s ri c h l y supplied with blood. Most of the blood to the udder i s supplied by the external pudic arteries. Cranial, epigastric and perineal arteries also contribute. The more important veins of the udder are the external pudic veins. The other route i s the sub-cutaneous abdominal or milk vein. The lymph vessels of the udder are - 6 -numerous and they pass chiefly to the supra-mammary lymph gland. Nervous system:- The main nerves supplying to the udder are f i r s t and second lumbar nerves and inguinal and perineal nerves. These nerves are mainly sensory but they also contain some fibers from the sympathetic nervous system. 2» Physiology of lactation At parturition, i n i t i a t i o n of lactation takes place when the mammary glands of the mother begin to secrete milk. Cowie et a l (9) have presented a classification to describe precisely the different phases of lactation. According to this, the phases of lactation may be set out as follows: (a) Synthesis of milk by cells of alveolar epithelium. (b) Passage of milk from cytoplasm f milk of alveolar epithelium into secretion alveolar lumen. (a) passive withdrawal (b) milk ejection Milk removal Lactation Under this scheme lactation can be divided into two main phases - milk secretion and milk removal. The present study undertaken for this thesis deals with milk removal. However, to make a brief mention about milk secretion would not be out of place. - 7 -Several theories have been put forth by different workers, to explain the i n i t i a t i o n of lactation at parturition. These theories are reviewed by Smith (62) and are mentioned here b r i e f l y . 1. Mammary growth inhibition theory;- According to this theory lactation i s inhibited by the products of pregnancy promoting mammary development. Support for this theory came from Nelson (U3) and Nelson et a l (Uk)» Lactation during pregnancy i s inhibited by cellular growth and lactation i s init i a t e d when the stimulus for growth i s removed at parturition. 2. Uterine distension inhibition theory;- Mechanical distension of the uterus by growth of the foetus has been considered as the factor causing inhibition of lactation. Surgical removal of the foetus i n late stages of gestation initiated lactation. But i f the uterus was distended with paraffin after removal of the foetuses i n rats, they f a i l e d to lactate. This was shown by Selye (58). Evidence against this theory has come from Bradbury (3). 3. Placental inhibition;- Nelson (U3) showed that presence of placental tissue i n the uterus inhibited lactation. On the other hand, Selye (58) observed no inhibition of lactation i n lactating mice after they had received placental implants. U. Estrogen inhibition;- According to this theory the relatively high level of estrogen i n the circulation during pregnancy inhibits lactation by a direct inhibitory effect on the mammary gland and also by preventing the secretion of prolactin (the lactogenic hormone) by the hypophysis. The basis for this theory was the - 8 -various reports on the suppression of lactation by the injection of estrogens. These reports were presented by Folley and Kon (23)» Robson (57) and Smith and Smith ( 6 l ) . 5. Progesterone inhibition:- The estrogen theory was c r i t i c i z e d by Meites and Turner (Ul and U2) who held the view that secretion of prolactin was not inhibited by high level of estrogen i n blood which i n fact at i t s high concentration i n blood stimulated prolactin secretion. But the inhibitory effect noticed during lactation was only due to the simultaneous presence of progesterone i n the blood during pregnancy which n u l l i f i e d the action of estrogen. At parturition, the sudden f a l l i n the progesterone level permitted the secretion of prolactin bringing about the onset of a copious milk secretion. Therefore, corpus luteum was held responsible for inhibiting lactation. Folley and Malpress (25 and 26) advocated a modification of the estrogen inhibition theory put forth by Nelson (U3). They proposed a double threshold concept of estrogen according to which the effect of estrogen on the pituitary gland depends upon the level of estrogen i n the blood circulation, a high level being inhibitory to pituitary function while a low level i s stimulatory. Recent observations of Cowie et a l . (10) during experiments on ovariectomized goats with estrogens and various dose levels of progesterone led these authors to conclude that while some of their results followed Folley and Malpress double threshold theory, others were more i n accord with the progesterone inhibition theory presented by Meites and Turner. - 9 -I n i t i a t i o n of milk secretion has been reviewed by Folley ( 2 2 ) . This review points out that: (a) Measurement of the prolactin content of the pituitary does not give any indication of the rate of prolactin release and i s , therefore, of no real guidance. (b) Low levels of estrogen i n the circulation activate pituitary function while higher levels inhibit i t . (c) Lactogenic doses of estrogen may be deprived of lactogenic activity by a suitable dose of progesteronej the combination thus inhibits milk secretion. This inhibiting influence acts during pregnancy. Diminishing levels of progesterone after parturition allows estrogen to assert i t s lactogenic activity resulting i n i n i t i a t i o n of lactation. A theory on the i n i t i a t i o n of lactation has been proposed which i s based on the neuro-hormonal mechanism concerned i n milk ejection. Gaines ( 2 9 ) observed that the milk ejection response of the mammae to post hypophysis hormone was induced only after parturition and not during pregnancy. Colostrum i s secreted by the cells and f i l l s the ducts of the mammary glands during pregnancy long before the contractile mechanism i s sensitive to p i t u i t r i n (oxytocin) or to the stimulus of milking. Gaines pointed out that the contraction of the gland under the influence of p i t u i t r i n (oxytocin) runs more closely pa r a l l e l to the appearance of an actual outflow of milk than does the formation of milk (colostrum) by the secret-ing c e l l s . Therefore, he considered oxytocin to be an important factor i n the onset of lactation. - 10 -Petersen (!?2) proposed a modification of Gaines' theory and suggested that the post parturition milk flow i s not due to an i n i t i a t i o n of secretion at that time but rather to a beginning of ejection of the alveolar contents. During the lactation the stimuli of suckling and milking maintain a high level of secretion. 3. Effects of secretions of the different endocrine glands on  lactation (milk secretion) i n farm animals. Smith (62) has b r i e f l y reviewed the effects of the secretions of the different endocrine glands on lactation i n farm animals. i . Anterior hypophysis and lactation j Experiments to increase milk production i n farm animals by suitable anterior pituitary extracts have been carried out. Azimov and Krouze (2) showed that the milk production of cows could be increased by injecting suitable extracts of anterior pituitary. Folley and Young (28) supported this contention by injecting anterior pituitary extract i n cows, otherwise, i n declining lactation. But this increase i n yield i n cows was possible by using anterior pituitary extracts of the bovine origin as injections of sheep or pig pituitary extract rather decreased the yield probably due to antibody formation. No augmentation of yield was observed when cows were treated with this anterior pituitary extract at the peak of their lactation. The galactopoietic effects of anterior pituitary extract have been shown to bear insignificant relation to their prolactin - 11 -content. This was shown by Young (77) who has also given a review. Folley and Young (27) indicated that galactopoietic effects of anterior pituitary extracts appeared to be associated with their content of the diabetogenic factor. When the identity of this factor with the growth factor was revealed, further experiments by Cotes et a l (6) showed that the growth hormone appeared to be the a l l important factor for galactopoietic activity of the crude gland extracts. The observations, however, do not suggest that prolactin and adrenocorticotrophic hormones have no role to play i n the galacto-poiesis. Donker and Petersen (18) and Chung et a l (5) have confirmed the galactopoietic effects of the growth hormone. Folley et a l (2k) showed that increase i n milk yield results from the administration of the anterior pituitary extract to cows and goats i n which lactation had been a r t i f i c i a l l y induced by estrogen but where estrogen had f a i l e d to i n i t i a t e lactation. The effect of the anterior pituitary extract was both lactogenic and galactopoietic. Ovarian hormones and milk secretion* The influence of estrogen and progesterone i n i n i t i a t i n g and maintaining lactation i n collaboration with prolactin and other hormones of the anterior pituitary has already been discussed under theories of milk secretion. Reineke et a l (5k) suggest that after treatment with estrogen and progesterone i n cows and heifers, to induce lactation, there should be a period of treatment with estrogen alone to stimulate the anterior - 12 -pituitary to secrete prolactin. i i i . Thyroid hormone and milk secretion; It has been known that the administration of desiccated thyroid or thyroxin to lactating cows results i n an increase of milk production upto 20 per cent as shown by Ralston et a l (53). Response varies with many factors such as age, stage of lactation, levels of dose etc. Iodinated proteins have been used to increase milk production. It has to be understood that the galactopoietic effect of thyroid preparations i s not a specific one on the alveolar cells but i s dependent on the role of thyroxine on the general metabolic rate of the animal treated. Leech and Bailey (37) have shown that the lactation period i s shortened i f thyroid active substances are used for galacto-poietic effects during the f i r s t to third lactations. i v . Adrenal cortex and milk secretion ; It has been established that the integrity of the adrenal cortex i s essential for the maintenance of lactation. Lactation diminishes i n rats when adrenals are removed. Flux (21) has shown that i f the ovaries are removed with the adrenals i n rats, the milk secretion i s diminished s t i l l further indicating that the ovaries can to a limited extent replace the adrenal steroids. Cowie (10) has shown that administration of cortisone and desoxycorticosterone restored lactation i n adrenalectomized rats. - 13 -v» Insulin and milk secretion; In studies of metabolism of mammary tissues from laboratory animals, insulin has been shown i n vitro to potentiate net fatty acid synthesis from small molecules when the tissue has been under the influence of the hormones of the lactogenic or galactopoietic complex of the anterior pituitary. In his review Cowie (7) points out that mammary tissue from the ewe, however, has given no evidence of any insulin response. The role of insulin i n milk synthesis i n the ruminants may be a minor one as their blood sugar level i s relatively low. v i . Parathyroid and milk secretion; Not much i s known about the role of parathyroid on lactation. Cowie and Folley (8) have indicated that when the parathyroids are removed i n rats, a marked impairment i n lactation results. - m -B. Specific The assumption put forth by Marshall (kO) that the udder could not possibly hold a l l the milk obtained at one milking, and that there i s a rapid secretion during milking after stimulation of the udder by nursing, suckling, etc. was proven to be incorrect by the work of Gaines (29). I t i s well recognized to date that the capacity of the udder i s more than enough to contain a l l the milk that i s obtained on milking and milk i s actually secreted and held i n the udder before milking i s commenced. There are two sites for storage of this milk secretion i n the udder: i . the alveoli and small ductules; these two structures have been shown to hold most of the stored milk; and i i . the gland and teat cisterns and the large ducts; holding part of i t . The ratio between these two amounts vary between species, and individuals of the same species, depending upon the anatomical variations of their udders and other factors. The milk lying i n the gland and teat sinuses can be removed at any time by hand milking or machine milking without stimulating the actual milk ejection procedure. This i s not true of the milk stored i n the alveoli and ductules since i t i s held up by capillary forces i n the fine ducts and capillaries of the secretory tissue. In the l a t t e r case, to obtain milk a complex physiological phenomenon of milk ejection must be triggered. - 15 -These two processes of milk removal have been termed: i , the passive withdrawal; and i i . milk ejection Milk ejection and intra-mammary pressure Milk ejection i s the term used to describe the process of milk l e t down that follows the reflex stimulation of the mammary glands at the time of milking. The mammary gland tissue has e l a s t i c i t y and thus i s capable of exhibiting variations i n the intra-mammary pressure at different times of milk secretion and milk ejection. Theories of milk ejection Different theories have been presented to explain the process of milk ejection. i . Erection theory:- Hammond (32) put forth a theory of milk ejection called the erection theory. According to this theory, during milk ejection an erection, comparable to that occurring i n the penis, was presumed to be involved. Manip-ulation of the udder and teats at the time of milking was believed to be the stimulus which contracted smooth muscle fibers i n the udder, thereby, occluding the veins which resulted i n an engorgement of the udder with blood. The pressure of engorgement was supposed to force the milk from the alveoli and finer ducts into the larger ducts and cisterns. But this theory no longer seems tenable. Peeters et a l . (50) experimenting on perfused udder showed that venous engorge-ment does not produce milk ejection. - 16 -i i . Neuro-hormonal theory;- Ott and Scott (U5) had observed that injection of posterior pituitary extract i n lactating goats caused an immediate flow of milk. They considered that this was due to an increase i n milk secretion as a result of posterior pituitary extract injection. Later, Gaines (29) put forth quite a new concept of milk secretion and milk ejection. These two processes, he distinctly distinguished from each other. He observed that the milking stimulus i n lactating animals brought about a reflex contraction of the mammary gland. This caused milk ejection, a process by which the milk stored i n mammary gland tissues became available on milking. These investigations by Gaines established the basis for neuro-hormonal mechanism of milk ejection. Ely and Petersen (20) carried out a series of experiments on lactating dairy cows to study the phenomenon of milk ejection. They put forth the neuro-hormonal theory of milk ejection which i s so well known today. According to this theory, milk ejection i s a reflex action brought about by stimulation of secretion of posterior hypophysis by the acts of nursing, suckling, etc. at the time of milking. The secretion of posterior hypophysis i s responsible for rise of pressure i n the mammary glands which i s noticeable on milk ejection. - 17 -Tgetgel (6i4.) i s one of the pioneer workers who carried out a comprehensive study of the udder pressures i n cows. He found that there i s a gradual increase i n the udder pressure between one milking and the next. V/hen milk ejection i s stimulated at milking time, there i s a very rapid increase i n the udder pressure which i n the course of a minute may double. The range of increase i s influenced by the udder conformation and milk y i e l d . If the cow i s not milked after stimulating milk ejection, the rise i n glandular pressure may persist from 1 0 to 6 0 minutes, but eventually i t f a l l s to the value slightly higher than would have been attained, had the cow not been stimulated. During the milking process the intra-mammary pressure i s produced by the height of the accumulated milk i n the gland plus the pressure exerted by the milk expulsion mechanism. Tgetgel has given results of his experiment (Table I.) showing the effect of stimulation on intra-mammary pressure i n a number of cows. The variations i n their i n i t i a l pressures and pressures subsequent to stimulation as also betxreen their milk yields are well marked indications of influences of udder size, conformations, and productivity. T A B L E I . S h o w i n g t h e e f f e c t o f u d d e r s t i m u l a t i o n o n i n t r a - m a m m a r y p r e s s u r e (6U) P r e s s u r e i n m . m . o f H g . y i e l d C o w I n c r e a s e l b s . B e f o r e A f t e r M a j a 25.5 61.5 26.0 17.0 B r o n d a 27.3 51.5 2U.2 16 .U N o r a 37.0 59.5 22.5 16.3 P e e r l a 28.U iiU.7 16.3 16.0 B e l l i n a 27.6 1*3.1 15.5 12.6 M u r e t t a 2U.9 37.6 12.7 13.1 P i n c h a 22.0 37.0 15.0 12.3 T a s s a 22.1 35.6 13.5 11.1 B e r n i n a 2iu6 27.6 3.0 7.1 B r u n a 15.1 21.9 6.8 6.8 - 19 -Prior to Tgetgel's work Gaines (29) studied the intra-mammary pressure changes i n the lactating goat. In these investigations he used p i t u i t r i n (extract of hypophysis cerebri) to study i t s effect on milk ejection. He recorded the pressure graphs by inserting a cannula i n the teat of a goat and connecting this cannula with a chloroform manometer recording on the revolving drum of a kymograph. He pointed out that the pituitary extract caused contraction of the mammary musculature so forcing out the milk. This reflex contraction i s brought about by nursing, milking, etc. with resultant milk ejection, the reflex having a period of delay between 35 to 60 seconds. Gaines showed that a l l the milk obtained was present as such i n the udder at the beginning of milking and i s "actively ejected by a reflex contraction of the gland musculature under the stimulus of milking"'. He also observed that the effect of injecting posterior pituitary extract varies with the stage of activity of the gland and such effect of pituitary extract injection noticed i n the lactating goats i s apparently absent i n the non-lactating goats. Furthermore, during this work, Gaines emphasized that the act of' milk ejection i s quite different from that of milk secretion. By this work Gaines refuted the theory of Ott and Scott (u5) that the posterior pituitary gland extract stimulated milk secretion. This conclusion Ott and Scott derived on noticing an immediate flow of milk i n lactating goats following an injection of posterior pituitary extract. Gaines and Sanmann (30) presented further evidence i n support of the view that the udder of a cow contains at the beginning of milking not only a l l the milk that can be obtained during milking but a quantity - 20 -i n excess of t h i s . While reviewing the work on the effect of the posterior pituitary extract on milk secretion. Turner and Slaughter (66) suggested that the milk ejection reflex might act through the pituitary gland and that "one of the normal functions of the pituitary gland, so closely connected with the nervous system, i s to regulate, the discharging phase of milk secretion". In Germany, Kryzwanek and Bruggemaan (35) and (36) working on the dairy cow recorded measurements of the changes i n internal udder pressure following milk ejection and subsequently during milking. The results showed that the magnitude of the variations was affected by the milk yield, udder capacity and udder conformation of the cows though the trends of the changes followed a typical pattern. The interval between stimulation of ejection and the time when maximum pressure was reached, varied from fifteen seconds to over two minutes and i f the udder was unmilked the pressure declined slowly to near i t s original value i n 10 to 1|0 minutes. At the end of milking, i f a l l the strippings were removed, the internal udder pressure f e l l to zero for a short time because the residual milk i n the udder would exert no pressure at the teat. It was i n 19ill that Ely and Petersen (20) put forward their theory of milk ejection which i s now well known and accepted. They suggested that milk ejection i s a neuro-hormonal mechanism. Many sources of afferent stimuli as nursing, suckling, etc. stimulated the posterior pituitary through the central nervous system to secrete a hormone (oxytocin) which increases the glandular pressure and squeezes out the - 21 -milk from the alveoli and smaller ducts whereas different stimuli of a different nature as fright cause the secretion of adrenalin which prevents the ejection of milk. Ely and Petersen utilized the earlier works of Ott and Scott (U5) and Gaines (29) which indicated rise in intra-mammary pressure at the time of milk ejection whether the milk was removed from the udder after that or not. Ely and Petersen did a series of experiments on Jersey cows using oxytocin, vassopressin and adrenaline. Denervation of one side (half) of the udder was performed and data obtained which showed that: i . denervation brought no effect on the rate of milk ejection, i i . Oxytocin and vassopressin, either of them, caused the gland to drain more completely though oxytocin was comparatively more effective in this regard, iv. Oxytocin is largely responsible for intra-glandular pressure increase exciting milk ejection. The amount of oxytocin and intra-glandular pressure vary directly, v. They claimed a new theory of holding up and letting down of milk as a result of these experiments. This left no doubt that milk ejection is a neuro-hormonal mechanism. In a subsequent paper, Smith and Petersen (63) remarked that there was no measurable increase in mammary pressure during the f i r s t hour after milking. Then the pressure gradually increased until the next milking period. The increase in pressure that occurs several hours after milking but prior to expulsion at a succeeding milking, is probably due to the height of the milk that has accumulated in the larger milk ducts and cisterns. - 22 -Andersson (l) working i n Stockholm i n a series of experiments on sheep and goats, recorded milk ejection from the unanaesthetized animals following stimulation of the region of the supra optic nuclei. The response was not affected by denervation of one side of the udder or by complete sacral anaesthesia and that jugular blood withdrawn from stimulated animals produced milk ejection i f injected into other animals. More confirmation of the authenticity of the neuro-hormonal mechanism of milk ejection came from Cross and Harris (15). They investigated the effect of e l e c t r i c a l stimulation of the supraoptico-hypophysial tract (connected anatomically to neurohypophysis) i n rabbit preparations. Such stimulation resulted i n milk ejection. The response was reported to be identical to that obtained after an injection of an appropriate dose of anterior pituitary extracts. Cross (11, 12, 13 and ll+) at Cambridge i n Great Britain has continued to work on this problem and has made valuable contribution to the study of neuro-hormonal mechanism of milk ejection i n rabbits. Peeters et a l . (1+9) studied milk ejection i n bovines. They collected blood from four cows, the udders then washed and teats manipulated without mil king and further samples of blood withdrawn. Cows were then k i l l e d and both halves of each udder perfused with different samples of blood respectively. The milk secretion from different halves were measured. They concluded that a factor aiding milk ejection appears i n the blood during preparation of the cow for milking. Their further experiments subsequent to this proved more elaborately that the posterior pituitary provided the hormone for milk ejection at which time the intra-mammary pressure rises. - 23 -In 1°5U> Harris (33) gave a review including an account of the recent work on the mediating role of the hypothalamus i n the milk ejection reflex. He suggested referring also to his earlier investigations that milk ejection could be described as a neuro-hormonal reflex on the grounds that the afferent pathway i s nervous In character and the efferent, hormonal. The nature of the contractile tissue of the mamma that plays an important role i n milk ejection mechanism has been engaging the attention of a number of workers. Gaines and Sanmann (30) referred to this tissue as "gland musculature". Hammond (32) also referred to the p o s s i b i l i t y of contractile c e l l s being present around the alveoli but none of these earli e r workers could explain the nature of these contractile c e l l s and they took i t mostly as the non-striated smooth muscle fibers. Richardson (55 and $6) did extensive work on the mammary tissues of the goat to study their histology to find out the nature of the contractile tissue responsible for contraction during milk ejection. He found that the presence of the smooth muscle fibers i n the glandular tissue i s too scarce to bring about this contraction. Instead he discovered the presence of myoepithelium i n the mamma (alveoli and ductules) which he believed i s responsible for alveolar contraction during milk ejection thus causing a rise i n the intra-mammary pressure. In this way the milk contained i n alveoli and small ductules i s forced into the larger ducts and gland cistern during milk ejection. Richardson demonstrated the presence i n large numbers of myoepithelia cells on the stromal surface of the alveoli. The longitudinal or sp i r a l arrangement of the cells on the duct walls probably serves to shorten and widen the duct lumen so that milk i s readily transferred from the alveoli to the gland cistern. L i n z e l l (38) - 2U -working on other species confirmed these observations of Richardson about the presence of myoepithelium i n the mammary tissue. Pharmacological Studies On Milk Ejection Peeters and his associates (U8), (k9), and ($0) i n Belgium have investigated various aspects of this subject i n cows. They studied the cistern pressure relationship i n the intact and isolated bovine udder. The injection of three units of oxytocin caused an increase i n the cistern pressure. In experiments on the effect of various pharmacologic-a l l y active substances on milk ejection i n the isolated udder Peeters et a l . found that acetyl choline caused increased blood flow and parti a l ejection and that this action was inhibited by atropine. The effects of various pharmacologically active substances on milk ejection and blood flow i n the perfused mammary gland have been given by Silver (59) i n the form of a table. Braude and Mitchell (U) have shown that a complete inhibition of milk ejection i n response to oxytocin may be produced by the previous injection of adrenalin. Whether such an effect i s the result of vaso constriction i n the gland or i t represents a direct action on the contractile mechanism rendering i t refractory to oxytocin i s s t i l l a matter of speculation. Earlier work of Ely and Petersen (20) on the cow has indicated another possible way in which the liberation of this hormone i n response to sympathetic stimulation may affect the ejection of milk. They showed that the i n i t i a t i o n of milk ejection reflex by the milking stimulus could be completely prevented by fright or high doses of adrenalin but that an ejection response could always be el i c i t e d i n such an animal by the subsequent injection of - 2$ -of oxytocin. These results indicate that i n addition to i t s peripheral action adrenalin may have a central inhibitory action preventing the release of oxytocin though such a po s s i b i l i t y was not considered by Ely and Petersen. Whittlestone (69) tested the activity of a number of posterior pituitary preparations against a standard oxytocin preparation about one hour after normal suckling i n sows and came to the conclusion that purified vasopressin has milk ejecting activity equal to one-fifth of that of the equivalent number of oxytocic units but he stresses that further work i s required to confirm t h i s . Subsequent to this work, Whittlestone (70) made a study of the effect of adrenalin on the normal milk ejection response of the lactating sow to oxytocin for a limited range of concentrations. His results indicate that even the injected oxytocin f a i l s to produce milk ejection for a short time when given along with adrenaline In New Zealand, Whittlestone (69, 70, 71, 72, 73, 7k, and 75) has carried out extensive work on the subject of milk ejection i n various domestic animals. For most of his work on this subject he used the sow because of the advantages i t presents on account of anatomy of i t s udder and the temperament of the animal. Whittlestone 1s investigation (69) concludes from his work with sows that the milk ejection hormone not only causes a contraction of the myoepithelium around the alveoli thus increasing the pressure i n them but also causes an opening of the small ducts due to the tightening of the longitudinal myoepithelial structures which surrounds these ducts. Whittlestone (71), (72), and (73)' made a further study of intra-mammary pressure changes i n the - 26 -lactating sow. His studies concerned with the effects of oxytocin, vasopressin and acetylcholine on the intra-mammary pressure changes and the effects of l e v e l of the dose of oxytocin and the rate of injection. He also studied the effect of adrenaline on the milk ejection response of the sow. In addition to his work on the sow, Whittlestone (7u) studied the intra-mammary pressure changes i n the lactating cow during the process of milking. With an e l e c t r i c a l pressure recording system, he recorded simultaneously the intra-mammary pressure changes i n two quarters, one milked and the other unmilked. The unmilked quarter showed a very slow f a l l i n pressure while as expected the milked quarter showed a steady pressure f a l l . Restimulation sometime after the beginning of the experiment resulted i n a rise and f a l l i n pressure within the milked quarters but no change when the quarter was not milked. In another communication, Whittlestone (75) has presented the results of a study of intra-mammary pressure changes i n a lactating ewe. The pattern of the pressure curve recorded i n the case of the ewe i s shown to be different from both the sow and the cow. - 2? -III. MATERIALS AND METHODS A, Materials 1. Experimental Animals; Seventeen lactating cows of the Division of Animal Science dairy unit were used as the experimental animals for the experiments described below. These animals represented different breeds, ages, stage of lactations, milk yields, etc. Complete information about each of these cows is presented in Table II. 2. Hormone Products; "Oxytoxin", a patent product of the Austin Labs. Ltd., Canada, was used. Each ml. of the preparation contains 20 U.S.P. units of purified oxytocic principle in an aqueous vehicle. 3. The Apparatus; All measurements of intra-mammary pressure changes were made using the apparatus for recording pressure changes manufactured by the Sanborn Co., U.S.A. It consists of:-(a) Sanborn Physiologic Pressure Transducer - Model U67-Aj (b) Sanborn Strain Gage Amplifier - Model lUO-Bj (c) Sanborn Industrial Recorder - Model 127. A mercury manometer was permanently fixed with the above apparatus for calibration of pressure recording. U. Functional Description of the apparatust Physical Trans- Signal Voltage Stfalp Cm^* Recorder Finished load ducer Excitation Voltage Amplifier Recording Simplified Diagram of Recording System Using a Strain Gage Amplifier TABLE II, Detailed Information about the experimental animals used. Name Breed Date of Birth Lactation No. Date of Calving 1. Apple Holstein July 8, 1956 2 August 11, 1959 2. Barbara Holstein March 15, 195"? 1 June 25, 1959 3. Belle Ayrshire February 29, 1952 5 April 7, 1959 iw Blossom Ayrshire August 2, 1952 5 December 30, 1958 5. Bonnie Holstein January 13, 1957 1 February 20, 1959 6. Cathie Ayrshire February 6, 1953 k November 6, 1958 7. Dorothy Ayrshire February 2 3 , 195U 3 January 16, 1959 8. Dorrane Ayrshire February 2k, 195U 3 February 7, 1959 9. Fayne Holstein August 5, 1955 2 January 5, 1959 10. Fleta Holstein July 6, 1955 2 January 5, 1959 11. Grace Ayrshire July 17, 1957 1 August 23, 1959 12. Grade Holstein May 12, 1956 2 July 9, 1959 13. Greta Ayrshire August 18, 1957 1 August 13, 1959 Hi. Miranda Holstein June 17, 195U k November 12, 1959 15 . Paula Holstein January 10, 195U 3 November 10, 1958 16. Roberta Ayrshire March 21, 19U7 9 April 28, 1959 17. View Holstein January 2k, 1956 1 February 2, 1959 - 29 -The Strain Gage Amplifier supplies a 2500 cycle excitation voltage to the transducer. The transducer returns a signal voltage to the Sanborn Strain Gage Amplifier, Mien the physical variable applied to the transducer i s zero, the signal voltage i s zero, When a physical load i s present, the signal voltage has a magnitude and phasing which represent the magnitude and direction of the physical load. The Strain Gage Amplifier interprets this signal voltage i n terms of the physical load, and moves the galvanometer stylus up or down on the recording paper to show the magnitude and direction of the load on the transducer. B. Methods: 1, Standardization of Apparatus; The transducer i s connected to the e l e c t r i c a l system with a cable eight feet i n length and a five pin connector. The transducer i s provided with a pair of Pj, Luer Locks. By one of these i t i s connected to the f l u i d system (a 12 oz. bottle containing physiological saline fixed with the stand at a lev e l higher than that of the transducer) and by the other ?i Luer Lock, the transducer i s connected to about 3 f t . long polyethylene tubing (diameter 2/5 cm.) with a cannula at i t s free end. Preparing the Transducer: Before attaching the valves, the transducer i s thoroughly but gently flushed with gas free saline and i n the same way the catheter i s flushed to ensure that the hydraulic system i s free 30 from bubbles and ready for use. Theory of Transducer Levelling: Physiological pressure measurements are made with respect to an arbitrary zero l e v e l . This i s done by fastening the transducer at a convenient position and then to balance the transducer e l e c t r i c a l l y with the catheter t i p beneath the surface of a jar of saline. The pressure at the transducer due to the weight of the l i q u i d i n the tube (between the transducer and the surface of the saline) i s balanced out e l e c t r i c a l l y . This makes the transducer output appear as though the Zero Level lin e on the transducer body were level with the surface of the saline. Calibration: The Strain Gage Amplifier requires calibration so that the operator can interpret the recording i n terms of the variable being measured. This variable here i s pressure i n mm. Hg. To make this as simple and reliable as possible, "a basic sensitivity" figure i s used to express the stylus deflection i n terms of the 1 oad applied to the transducer. This "basic sensitivity" i s defined as the number of load units which gives one cm. of stylus deflection when the Attenuator i s at XL. By using this "basic sensitivity", the variable being measured i s read directly from the record, by multiplying the cm. of stylus deflection by the "basic sensitivity" and the Attenuator setting. The actual calibration of the system i s performed by applying a static load to the transducer. For this purpose, the mercury - 31 manometer i s used. The following steps are taken for calibration: 1. A basic sensitivity i s selected as mentioned above. 2. A known static load eg. UO mm. of Hg. i s applied. 3. The calibration deflection Is calculated. Ii. And without changing the static load, the Gain control for the calculated stylus deflection Is adjusted. Attenuation: Attenuation selects the sensitivity of the recording i n terms of mm. of Hg. per ten divisions of stylus deflection. For this purpose the attenuator i s provided with the front panel data of the Strain Gage Amplifier. The sensitivity for different positions i s given below: Position Sensitivity c li6?A Transducer XI 1 mm. Hg. per 10 divisions X2 2 mm. Hg. per 10 divisions S mm. Hg. per 10 divisions XLO 10 mm, Hg. per 10 divisions X20 20 mm. Hg. per 10 divisions XSO $0 mm, Hg. per 10 divisions X100 100 mm. Hg. per 10 divisions - 32 -2. Procedure of recording: The complete recording apparatus i s set up before recording i n close proximity to the experimental animal. The transducer with the stand i s fixed at the lev e l of the mammary quarter concerned. The Strain Gage Amplifier i s turned on so that i t can warm up and the trans-ducer i s connected to the TRANS socket. After warming up the Strain Gage Amplifier for 30 minutes, the resistance and capacitance unbalance of the transducer c i r c u i t and cabling i s balanced outj balancing and calibration must be done before f i l l i n g the pressure head with f l u i d . By bringing the stylus to a certain base line on the recording paper by adjustment with the Zero Switch, the Zero Line i s established when also the calibration and attenuation i s checked and suitably adjusted. Physiological saline i s passed through the cannula, by opening the stop cock of the transducer, to ensure that the cannula i s f i l l e d with saline upto i t s t i p . The recorder i s now switched on. The t i p of the cannula i s brought close to the t i p of the teat and Zero point marked with the MARKER. The cannula i s now smoothly passed into the teat canal where the milk of the mammary gland cistern comes i n contact with the saline i n the cannula and thus the intra-mammary pressure i s recorded. The cannula i s l e f t i n the teat as long as required by the experiment, using the MARKER on the apparatus for designating the points of interest. The point, at which the cannula i s taken out, i s also marked to f a c i l i t a t e correct and accurate reading and interpretation of the results. - 33 -3. Hormone administration; During experiments with oxytocin hormone, injections of "Oxytoxin" were administered per sub-cutaneous route i n the neck region of the animal. 4. Milk y i e l d : Milk yield was recorded i n some experiments that required a study of the milk output of the animal. 5. Precautions: Certain precautions are required i n the operation of recording the intra-mammary pressure for accurate results. These are mentioned below:-i . The animal should be standing even on a level floor; i i . The animal must be kept i n a calm and unexcited state during the experiment; i i i . The level of the transducer has to be corrected each time that the animal or i t s s t a l l i s changed from one experiment to another; i v . To warm up the Strain Gage Amplifier for 30 minutes before operation of the apparatus; v. To check and verify the calibration and attenuation shortly before the experiment commences; v i . To ensure that the hydraulic system of the transducer i s free from a i r bubbles; v i i . To flow the Physiological saline through the cannula f i l l i n g i t to the t i p each time that the cannula i s inserted into a teat. v i i i . The Zero Base line must be marked for every individual recording of the intra-mammary pressure from different quarters of the udder. - 3U -As a safeguard against infection of the udder, the cannulae used are to be properly s t e r i l i z e d and other antiseptic conditions maintained. - 35 -RESULTS AND DISCUSSION The importance of the subject of milk secretion cannot be over-emphasized. The study of this complex problem has received attention for many years, but s t i l l complete understanding of the phenomenon of milk secretion and milk ejection i s lacking. Milk secretion as a separate study from milk ejection was emphasized by Ely and Petersen (20). They postulated that milk ejection i s a neuro-hormonal mechanism brought about by the stimulation of secretion of posterior pituitary, resulting into rise of the intra-mammary pressure at the time of milking. Earlier Tgetgel (6U) had recorded variations i n intra-mammary pressure i n lactating cow before and after milking. His results (Table II) show well marked increase i n the intra-mammary pressure as a result of udder stimulation at the time of milking. In Germany Kryzwanek and Bruggemaan (35 and 36) recorded measurements of the changes i n the intra-mammary pressure i n the dairy cow following milk ejection and subsequently during milking. These workers discussed the possible effect of various factors such as milk yield, udder capacity and udder conformation on the intra-mammary pressure of the cow. Whittlestone (69 to 75) made a valuable contribution towards an understanding of the phenomenon of milk ejection i n various animals. He also suggested improvements on the type of equipment and technique to record the intra-mammary pressure changes. Whittlestone's study (7U) on the problem of milk ejection i n the cow i s not comprehensive. It leaves many aspects of the problem for further investigation. A complete understanding of the problem of milk ejection i n lactating animals i s very essential since with this i s associated the hormonal - 36 -efficiency, milk yield and health of the animal. The intra-mammary pressure values are reliable indications of the efficiency of the milk ejection reflex in a lactating animal. The present study deals with recording of the intra-mammary pressure changes during different phases of milk ejection in lactating cows. Various experiments were planned to study different aspects of the problem. As large a number of cows as seventeen was used during these series of experiments. This study is only a preliminary work for the academic and scientific understanding of the mechanism of milk ejection. Table 111 The intra-mammary pressure changes before and after complete milking* Intra-mammary pressure (mm. Hg.) Cow Belle Dorrane Cathie Dorothy Roberta Blossom Milking Time RF RH LF LH RF RH LF LH RF RH LF LH RF RH LF LH RF RH LF LH RF RH LF LH # I n i t i a l AM PM LU 18 21 33 12 11 10 9 11 13 lU 25 9 10 10 13 9 18 9 18 9 l5"~8" 17 12 15 20 26 9 9 9 11 16 21 20 26 12 15 in 17 13 12 19 25 9 9 13 lC After Stimulation A M PM 29 x 18 2U 22 30 22 32 23 32 23 32 20 2U" 19 x 12 26 11 29 11 25 12 25* 21 26 22 28 19 2j 19 217 22 27 23 27 . to* 2% 217 2F 28 26 35 25 21 18" 31 2H After Milking AM P M 7 8 9 8 13 12 9 11 x 7 8 7 U x 5 7 3 7 "5 ~8* 7 6 2 6 ~5 7 ~6" 9 3 3 5 U 6 6 8 8 ~ 5 ~ ~ c 5 ~ c J Milk y i e l d i n l b s . AM PM 18 15" 17 13 9.6 7 22 17 25.2 19 18 13 * - Thursday, July 3 0 t h . 1 9 5 9 AM - denotes morning milking at 5 . 3 0 A.M. PM - denotes afternoon milking at U . 3 0 P.M. x - unreliable results. # - Quarters of the udder - right front, right hind, l e f t front and l e f t hind. Table IV The intra-mammary pressure changes before and after complete milking**-Intra-mammary pressure (mm. Hg.) Cow -Dorrane •Cathie Dorothy Blossom Belle Roberta # LH LF RH RF Milking Time LH LF RH RF LH LF RH RF IH LF RH RF LH LF RH RF LH LF RH RF I n i t i a l AM PM 28 10 23 28 22 18 2T 20 + + + + 10 6" 9 o* 28 22 29 27 11 11 19 18" 17 13 16 13 HT 12 25 IB" 18 1U 16 13 2u" lo" 22 % 25 22 29 25 20 12 IB" i f After Stimulation AM PM 30 30 22 29 23 29 3l 23 15 12 28 13 27 13 29 12 33 27 33 27 30 20 x 29 33 25 31 26 33 % 30 23 38 28 36 29 32 2H 33 23 35 32 33 26 27 2U 29 2% After Milking AM PM 8 7 9 6 x 1 W B~ 6 k 5 5 E 9 T x 5 x 5 6 ~7 ~E ~ ~Z 9 9 8 12 "8" 7 7 7 6 8 5 9 " 5 x l 3 Milk Yield i n lbs. AM PM 16 % 7 31 m 16 IT 22 17 24 m \ * - Wednesday, August 5th, 1959 AM - Denotes morning milking at 5*30 A.M. PM - Denotes afternoon milking at 4 .3 0 P.M. x Un reliable results. # - Quarters of the udder - l e f t hind, l e f t front, right hind, right front. + - Not recorded. - 39 -A. Fhyslological Studies 1. The intra-mammary pressure (IMP) changes before and after complete  milking. In this experiment, changes In the intra-mammary pressure (IMP) were recorded before milking, after stimulation of the udder by hand and after complete milking (Tables III and IV) on two different days using six cows. The i n i t i a l IMP readings are indications of the pressure that existed within the udder just before milking and without any stimulation of the udder. After the i n i t i a l pressure recording, the udder was stimulated by hand manipulation and the IMP recorded. At this stage the animal was completely milked and the IMP recorded subsequently. The milk yield was also recorded. This process of recording the IMP was repeated at the afternoon milking of both days. The results are tabulated i n the order of the animals milked and the teats cannulated to record the IMP i . e . the right front (RF), the right hind (RH), the l e f t front (LF), and the l e f t hind (LH) quarter respectively (Table III) and the LH, the LF, the RH and the RF quarter respectively (Table IV). The order of recording the IMP from different quarters has a special significance especially i n the case of the i n i t i a l IMP values which should be recorded with the least manipulation of the udder. However, the effect of some stimulation of the udder caused by cannulation cannot be prevented completely. This effect i s noticeable as one studies the values of the i n i t i a l IMP recorded going from the f i r s t quarter to the l a s t . Due to the p o s s i b i l i t y of stimulating milk ejection during the process of cannulation, the results recorded as i n i t i a l IMP are not completely devoid of milk ejection effects. This - ko -important factor cannot be overlooked and, therefore, the i n i t i a l IMP figures recorded cannot be considered as the true IMP values at the time of milking prior to any stimulation of the udder. The IMP values recorded after stimulation of the udder are without any such side effects and, therefore, are regarded as the accurate or true values. Similarly, the IMP measurements recorded after milking the animal represent the true IMP values except where results were considered unreliable due to the movements of the animal or some other obstruction i n the recording procedure. On studying the results of this experiment (Tables III and IV), a typical pattern of the IMP changes i s noticed. It i s observed that i n general the i n i t i a l IMP rises instantaneously after stimulation of the udder and after complete milking of the animal, the IMP f a l l s even below the i n i t i a l IMP levels. The results also show that the i n i t i a l IMP values i n different quarters of different cows ranged between 9 mm. Hg to 29 mm. Hg pressure except i n a few cases (Table I I I : animal - Cathie, the LF quarter at afternoon milking, 8 mm. Hg. Table IV: animal - Cathie, the LF and the RF quarters at afternoon milking, 6 mm. Hg. Table III: animal - Belle, the LH quarter at morning milking, 33 mm. Hg,) Milk ejection was brought about spontaneously by mechanical stimulation of the udder. The IMP values recorded after milk ejection are much higher than the i n i t i a l IMP values and range between 18 mm. Hg and 36 mm. Hg with the exception of $ cases out of U8 where the IMP values recorded ranged between 11 mm. Hg 1J> mm. Hg. In many cases i t i s observed that the IMP value of a quarter after stimulation of the udder i s nearly double that of the i n i t i a l IMP value of the same quarter and sometimes even more than double. This - Ill -sudden rise of the IMP after udder stimulation at the time of milking was shown earlier by Tgetgel (6i|) and Kryzwanek and Bruggemaan (3i>and 3 6 ) . This sudden rise of the IMP i s due to the mechanism of milk ejection which i s brought about by quick response from the udder stimulation, Ely and Petersen (20) explained the milk ejection process by propounding the neuro-hormonal theory of milk ejection. The data presented i n Tables III and 17 also show the IMP values recorded immediately after milking, A very sudden f a l l i s noticed i n each quarter and the pressure dropped as low as 3 mm, Hg to 9 mm, Hg, The only exceptions were - (Table III: animal - Belle, the RF, the RH and the LH quarter at afternoon milking, 11 mm. Hg to 13 mm, Hg: Table IV: animal - Blossom, RF quarter at morning milking, 12 mm. Hg.) These results show that the IMP immediately after milking the animal i s at i t s lowest but starts to build up since the previous milking u n t i l i t reaches a certain high l i m i t at the actual milking process. The i n i t i a l IMP values when recorded are not influenced by the effect of udder stimulation, represent the udder pressure owing to the amount of milk present i n the individual mammary gland cistern as reported by Ely and Petersen (20). At milk ejection as a result of the udder stimulation, the IMP suddenly rises reaching i t s maximum limits and f a l l s to i t s minimum limits after milking as observed by Tgetgel (6I1) and Ely and Petersen (20). The present results tend to prove this current hypothesis by Ely and Petersen. A noted feature of the results i s the comparison of the IMP values of hind quarters to those of the front quarters after stimulation of the - U2 -udder. It i s noticed that without exception a l l the IMP readings from both the hind quarters, l e f t and right, are higher than the IMP readings from both the front quarters, l e f t and right, respectively. These differences are noticed from the IMP values recorded at both the morning and the afternoon milking. The range of variations between the IMP values of the front and hind quarters i s quite wide but i n general the differences are only of a few mm. Hg pressure. It i s a well established fact that milk y i e l d from the hind quarters i s more than that from the front quarters. I f this fact i s related to the present findings of higher IMP figures of the hind quarters, i t may be considered that the IMP values are influenced by the amount of milk yielded. Another point of interest that these results present i s the comparison of IMP values recorded after stimulation of the udder at morning milking with those recorded at afternoon milking. In general, the morning figures are higher than those recorded i n the afternoon. At the same time the present results reveal that without an exception the milk yield of a l l the individual cows under the experiment at morning milking Is higher than that of the afternoon milking. This may be due to the longer interval (13 hours) before morning milking than for afternoon milking (11 hours). These observations tend to support the view that the milk yield has a profound influence on the level of the IMP. To what extent the direct relationship of milk y i e l d and the IMP observed from the present study can be considered valid, i s a question that cannot be answered conclusively from this experiment. This requires further investigation using much larger number of cows. - 43 -However, the milk yield cannot be considered to be the only factor responsible for the IMP levels in various quarters of the udder. Other factors such as udder conformation, stage of lactation, breed and age of the animal and other individual variations cannot be overlooked. Table V. The effect of increasing time intervals on the intra -mammary pressure.* Time Intra-mammary pressure (mm. Hg.) — I Cow Roberta RF LF RH LH Belle RF LF RH ffi # I n i t i a l At 6.20 A.M.(immediately after milking) 6 6 5 5 9 8 5 6 I n i t i a l 2 hours after milking 8 10 7 8 10 8 7 8 I n i t i a l ii hours after miling 8 11 8 1^  11 9 9 8 I n i t i a l 6 hours after milking 10 13 11 12 11 8 10 8 I n i t i a l 8 hours after milking 13 16 16 16 15 15 2k 21 I n i t i a l 10 hours after milking 15 17 18 20 15 16 22 23 After Milking At U.20 P.M. (immediately after milking). 3 ii U 6 8 5 6 6 Milk yield i n lbs. " Morning milking Afternoon mikling 2k 19 22 17 # - Wednesday, August 5th, 1959. # - Quarters of the udder - right front, l e f t front, right hind and l e f t hind. Table VI The effect of increasing time intervals on the intra-mammary pressure.* Time In^ra-mammary pressure (mm. Hg.) Cow Fleta Grade Miranda • LF LH RH RF Millc Yield i n lbs LF IH RH RF Milk Yield i n lbs LF III RH RF Milk Yield i n lbs I n i t i a l At 5»30 AM (morning milking time 12 15 25 21 - 21 33 29 23 -Showed reflex intra-mammary so not recorde rise of pressure d. After Stimulation 5.30 AM. 29 37 37 27 26 38 35 26 mm 35 36 UO 35 After Milking 5.30 AM. x x 6 6 16 6 7 6 5 15.5 6 6 5 6 35 I n i t i a l 3 hours after milking 8 8 8 6 7 7 6 5 6 7 6 6 I n i t i a l 6 hours after milking 9 10 7 6 8 9 7 6 15 11 10 9 I n i t i a l 9 hours after milking 21 25 22 19 13 lit 12 16 26 32 39 31 I n i t i a l 12 hours after milking 18 20 23 19 m, 22 32 30 21 28 3U 33 35 I n i t i a l 15 hours after milking (8.30 PM). 18 25 36 25 21 U5 3U 2k 37 UU U5 38 I n i t i a l 18 hours after milking (11.30 PM). 26 39 UU 37 Left hind quarter was showing signs 15 UO U6 U7 U3 After Stimulation 11.30 PM. 28 U5 46 36 of inflammation so cow was U5 50 50 U6 After Milking 11.30 P.M. 5 6 U 5 13 milked at this stage - 6 6 7 7 38 * - Monday, January, 11th i 9 6 0 . # - Quarters of the udder - l e f t front, x - Unreliable results. l e f t hind, right hind and right front. - U6 -2 . The effect of increasing time Intervals on the IMP: This experiment was performed at two different occasions using different animals each time. It was reported by Tgetgel (6k) and Kryzwanek and Bruggermaan (35> and 36) that there i s a gradual increase i n the IMP between one milking and the next milking. Korkman (3k) demonstrated that the internal udder pressure rises rapidly i n the f i r s t hour after milking, the rise being due to the hydrostatic pressure of the milk which drains from the secretory tissue to the udder and teat sinuses. After this i n i t i a l period the rise i n pressure i s due to the pressure of the milk which i s being secreted. At f i r s t the increase i n pressure i s small but the rate accelerates with increasing interval since milking. The present experiment was undertaken to study the manner i n which, after milking, the increasing time interval affects the IMP of the cow. The results shown (Table V) are from two cows, Roberta and Belle. These cows were milked at the morning milking time after which the IMP from each of their quarters was recorded. With increase of the time interval a progressive increase i n the IMP i s noticed i n a l l cases except the few odd cases showing an insignificant deflection from the general pattern. A sharp drop found i n the IMP values after the afternoon milking (Table V) i s i n accordance with the characteristic pattern discussed under experiment 1, of this work. Data presented i n Table VI were recorded from three cows. A noted feature of this experiment i s that the IMP was recorded after the morning milking and subsequently the i n i t i a l IMP was recorded after every three hours and the cows were not milked t i l l the i n i t i a l IMP appeared to be reaching a plateau. The data (Table VI) show a very progressive increase - u? -i n the i n i t i a l IMP since the morning milking u n t i l the animals were milked after an unusually long interval of 18 hours. The literature presents very l i t t l e evidence of this type of experiment where the i n i t i a l IMP was recorded over an interval as long as 18 hours without milking the animals. The object of this experiment was to mark the time after which the i n i t i a l IMP reached a plateau and also to study i t s effect on the milk yie l d . Certain d i f f i c u l t i e s were quite probable i n the way of such an attempt. Repeated cannulation of the teats was obviously disturbing the animals during the experiment. Apart from that the animals showed restlessness on not being milked at the usual afternoon milking time. The large quantity of milk i n the unmilked udder was responsible for the higher i n i t i a l IMP than usual. An increasing stress on the unmilked udders and the animals was noticeable especially i n one cow, Grade, which showed signs of inflammation of the udder and was, therefore, milked only 15 hours after the morning milking. In the case of the other two animals also, the experiment was not continued beyond the interval of 18 hours when they were milked i n view of the imminent adverse effects. It i s noticed from the results (Table VI) that up u n t i l 5:30 P.M., the IMP values recorded, evidently do not indicate any effect of udder stimulation. But for the values recorded at 8:30 P.M. and 11:30 P.M., some effect of udder stimulation can be suspected. However, the IMP values recorded after the udder stimulation are s l i g h t l y higher than the i n i t i a l IMP values recorded at this time. If these i n i t i a l IMP values were with-out any effect of the udder stimulation, then i t may be concluded that i n an udder, unmilked over a long interval, the i n i t i a l IMP keeps on - 1*6 -increasing and may even surpass the usual after stimulation IMP levels which are recorded at the regular milking time. This contention i s supported by the present data (Table VI) which show that the i n i t i a l IMP values recorded at and after 8:30 P.M. are higher than the after stimulation IMP figures of the morning milking. However, this cannot be accepted conclusively because of the p o s s i b i l i t y of some mechanical or reflex stimulation effects influencing the i n i t i a l IMP values. The milk yield after the longer interval (Table VI) was higher than that at the previous milking. The increase, however, i s not significant i n view of the length of the interval. fable V l l The effect of par t i a l milking on the intra-mammary pressure.* Milking time >Intra-mammary pressure (mm. Hg.). Cow Roberta LH LF RH RF Belle -LH LF RH RF I n i t i a l AM PM 25 22 29 2 5 20 12 IB" 15" 18 lk 16 13 2 1 7 15 22 15 After Stimulation AM PM 35 32 33 26 27 2L7 29 2k 38 28 36 29 32 217 33 23 After p a r t i a l milking AM PM 21 19 16 19 19 20 12 18" 25 20 27 23 20 20 19 IB" After complete milking AM PM 5 5 5 6 " 5 x 1 - 3 6 7 5 9 "5 "5 "5 1 Milk y i e l d i n lbsv. After partial milking AM PM 12 ~To| 10 9 After complete milking AM PM 2k ~^9h 22 * - Wednesday, August 5th, 1959 AM - Denotes morning milking # - Quarters of the udder - l e f t PM - Denotes afternoon milking hind, l e f t front, right hind, x - Unreliable result, right front. Table V l l l The effect of partial milking on the intra-mammary pressure.-* Intra - mammary pressure (] urn. Hg.). Cow Grace Apple Barbaba Fleta # LF LH RH RF LF LH RH RF Yield i n lbs LF.LH 1ST RF Milk Yield i n lbs LF LH RH RF Minjc Yield i n lbs Milk Yield i n lbs After Stimulation 24 31 31 21 - 35 34 39 32 - 33 31 36 28 - 29 35 31 25 -After PartaL Milking No. 1 19 29 26 18 4.5 27 27 29 25 7 2i+ 2h 25 21 ii.5 29 31 26 19 ii.8 After Partial Milking No. 11 1U 15 15 11 5 19 19 17 10 7 17 15 Hi 12 5.7 19 15 U 9 ii After Complete Milking 7 8 1+7 3.8 6 7 6 x U 7 7 6 ii 5.5 6 9 5 5 3.5 Total Milk Yield 13.3 18 15.7 12.3 * - Wednesday, January 6th, i960 - 3.30 P.M. # - Quarters of the udder - l e f t front, l e f t hind, right hind, right front, x - Unreliable result. - 51 -3' The Effect of p a r t i a l milking on the IMP; I t has been demonstrated by various workers that on milk ejection the IMP suddenly rises to i t s highest levels and i t f a l l s back to i t s lowest values after milking. The pattern of the f a l l i n the IMP during milking was investigated i n more detail by Kryzwanek and Bruggemaan (35 and 36). They measured the changes i n the internal udder pressure following milk ejection and subsequently during milking at different levels of emptying the udder. They showed that the f a l l i n the IMP after milk ejection depends upon the amount of milk removed and reaches i t s lowest when the udder i s completely emptied. The present experiment was conducted to study the effects on the IMP when the udder i s emptied p a r t i a l l y and then completely. The results obtained from this study are presented i n Tables VII and VIII. In one case (Table VII), two cows were used for this experiment and the data recorded at one morning milking and also at the same afternoon milking. At these milking times, the i n i t i a l IMP and the IMP after stimulation were recorded. Subsequently the animals were milked, removing only approximately half the quantity of the total anticipitated milk yield, and the IMP recorded. Finally they were completely milked and the IMP recorded again. The milk y i e l d figures of each milking are also given (Table VII). It i s observed from these results that i n some cases the i n i t i a l IMP figures are close to the IMP values recorded after stimulation. This points out to the p o s s i b i l i t y of some effect of stimulation of the udder on the i n i t i a l IMP during the recording process. However, the IMP figures after stimulation are correct and without any side effects. After p a r t i a l milking, a sharp drop i n the IMP i s observed. Complete milking of - 52 -of the animal brings about a further drop i n the IMP which reaches i t s minimum l e v e l (Table VII). These results present a typical pattern showing how markedly the amount of milk removed from the udder affects the IMP. The data presented i n Table VIII are from four other cows. Their IMP was recorded after the udder stimulation at the afternoon milking. The animals were milked to remove a part of their anticipated milk y i e l d (quantities of milk removed at each milking are shown i n Table VIII) and the IMP recorded; subsequently another portion of milk was removed and the IMP recorded again. Finally the animals were completely milked and the IMP values recorded. These repeated p a r t i a l rrdlkings were performed one after the other without any interval of time except when the IMP was being recorded. The results presented here (Table VIII) show a very regular f a l l i n the IMP brought about by every removal of milk u n t i l the IMP reaches i t s lowest limits on complete emptying of the udder. The present study reveals the manner i n which the maximum IMP recorded after milk ejection f a l l s on pa r t i a l and complete milking. According to the present results (Tables VII and VIII) a direct relation-ship appears to exist between the amount of milk removed and the drop i n the IMP, These results are found to be i n close conformity with the findings of Kryawanek and Bruggemaan (35 and 36). Table IX The effect of complete milking of the front quarters on the intra-mammary pressure of the unmilked hind quarters.* Intra-mammary pressure (mm. Hg.) Fayne Fayne X Cow RF LF RH LH RF LF RH LH # I n i t i a l 16 19 26 2k 15 15 13 13 After stimulation Not well Recorded 2k 2k 20 25 After Milking both front Quarters k k 33 36 k 5 2k 27 After complete milking of a l l quarters 7 6 13 15 5 5 8 7 Milk Yield 12.5 lbs 10 lbs. 0* - Monday, July 6 th , 1959 - 3.30 P.M. * - Thursday, July 23rd, 1959 - 3.30 P.M. # - Quarters of the udder - right front, l e f t front, right hind and l e f t hind. - 5U -lu The effect of complete milking of the front quarters on the IMP  of the unmilked hind quarters. The anatomy of the bovine udder has been described by Sisson and Grossman (60) and Turner (65). The udder of the cow comprises four quarters, each representing a mammary gland. A l l the four mammary glands are anatomically separate from one another and no direct communication exists between any two quarters. Whittlestone (7U) recorded simultaneously the IMP changes i n two quarters of the cow, one milked and the other unmilked. He demonstrated that there i s a very steady pressure f a l l i n the milked quarter which has no influence on the unmilked quarter which shows a very slow f a l l i n the IMP. L i t t l e work has been done on this particular subject and thus conformation of Whittles tone's work i s not available from the literature. Therefore, the present work was undertaken to study this problem. The present experiment was conducted on one cow at two different occasions (Table IX) to study the effects of completely milking the front quarters on the IMP of the hind quarters. The data collected (Table IX) show that as usual the i n i t i a l IMP values appear to have been influenced to some extent by some unavoidable stimulation of the udder. The IMP figures recorded after stimulation are satisfactory and follow the usual pattern. On one occasion (July 6 , 1959) the IMP after stimulation could not be recorded satisfactorily owing to the movements of the animal. However, i n this case, a study of the IMP figures after complete milking of the front quarters shows that the rise i n the IMP following udder stimulation was of the normal l e v e l . The IMP readings after milking both the front quarters indicate a sharp drop i n the IMP of the milked front quarters. On the other hand, the IMP of - 55-the unmilked hind quarters stands as high as after stimulation and i s not affected by the marked drop i n the IMP of the milked front quarters. On milking the hind quarters, the characteristic f a l l i n their pressure i s noticed (Table IX). The results of this experiment (Table IX) point out that the IMP of the front and hind quarters i s independent of each other as can be expected from a study of anatomy of the udder. These results confirm Whittles tone's observations (7li) reported earlier. Table X The effect of milking each individual quarter on the intra-mammary pressure of the other quarters.* Intra-mammary pressure (mm. Hg.) Cow Apple Fleta Greta LF LH RH RF Milk Yield i n lbs LF LH RH RF Milk Yield i n lbs LF m RH R|f Milk Yield i n lbs. 30 UO U l 29 - 31 U l 37 25 - 36 U5 U6 35 -After Milking the l e f t front quarter 19 U l U3 30 1.8 11 38 35 25 2.2 x U2 39 3U 3.6 After Milking the 18 1U 27 x u.u 10 8 28 22 1.3 13 12 39 33 U.7 After Milking the 16 10 16 23 3.8 8 7 7 20 3.3 12 12 1U 31 3.7 After Milking the right front quarter 12 11 9 x 2.0 8 9 7 7 2.5 12 12 13 x 3.5 After Stripping a l l 6 5 x x 1.3 7 8 U 3 2.7 x 5 5 U 1.0 Total Milk Yield 13.3 12.0 16.5 * - Thursday, January 7th, i960 - U P.M. x - Unreliable results. # - Quarters of the udder, l e f t front, l e f t hind, right hind, right front. - *7 -$. The effect of milking each individual quarter cm the IMP of the other quarters; In the previous experiment (No. U.) of this work i t was demonstrated that the milking of both the front quarters together has no effect on the IMP of the unmilked hind quarters. Further investigation of this problem was undertaken under the present experiment i n order to study the effect of milking individual quarters on the IMP of the other unmilked quarters. Three animals were used for this experiment. During this experiment, the animals were hand milked; each quarter was milked i n quick succession except when recording the IMP. At the milking time, the udder was stimulated mechanically and the IMP of each quarter recorded. Subsequently one quarter (l e f t front) was milked and the IMP of this and the other three unmilked quarters recorded. Similarly, the other three quarters (left hind, right hind and right front) were milked singly, one after the other. After milking of each individual quarter, the IMP of a l l the four, milked and unmilked quarters was recorded. Finally a l l the four quarters were stripped out simultaneously and their IMP recorded. At each milking the milk yield was noted. The results obtained (Table X) present a characteristic pattern of the IMP as influenced by milking each quarter. After milking the f i r s t quarter ( l e f t front) a sharp drop i n i t s IMP i s noticed. This, however, has no significant effect on the IMP values of the other three unmilked quarters which stand about as high as recorded after stimulation. The same trend i n the f a l l of the IMP i s observed on milking the second (l e f t hind) quarter. Milking of this quarter brings about a f a l l i n i t s IMP without any effect on the other quarters. Some decrease noticed i n the IMP of the two unmilked quarters (Table X) at this stage i s only a result - 58 -of keeping them unmilked. This inference i s derived from the observations of Tgetgel (64) and whittlestone (74). They pointed out that the IMP of the quarters which are not milked subsequent to stimulation gradually subsides to the point that would have been reached had no stimulus been applied. The third (right hind) quarter on milking registers a sharp drop i n i t s IMP. This has no effect on the IMP of the other quarters, A similar drop i n the IMP of the fourth (right front) quarter i s observed on milking. Finally a l l the four quarters were stripped simultaneously and their IMP recorded. This resulted into a further drop of their IMP values which reached their minimum level (Table X), This can be explained as the effect of completely emptying the udder by removing the accumulated strippings. This experiment demonstrates that although eaoh quarter of the udder registers a marked ri s e i n i t s IMP subsequent to the udder stimulation at the time of milking, the level i n the IMP of an individual quarter i s not affected by milking any of the other quarters. The simultaneous rise of the IMP of a l l the four quarters after stimulation i s i n accordance with the neuro-hormonal theory of milk ejection put forth by Ely and Petersen (20). Pharmacological Studies, It was reported by Gaines (29) that injection of oxytocin causes milk ejection i n a lactating animal, Folley (21a) showed a rise i n the IMP i n an unanaesthetized goat after the intravenous injection of oxytocin. Up to a certain l i m i t , the rise i n the IMP was influenced by the l e v e l of the hormone (oxytocin) injected. Silver (59) i n his review of the effects of various pharmacologically effective substances on the perfused - 59 -udder, pointed out that oxytocin brings about milk ejection and thus raises the IMP, Whittlestone's studies (71 and 73) on the sow demonstrated the effect of oxytocin i n causing a rise i n the IMP. In the case of the cow, however, the study of the effects of oxytocin injection i s more complex i n view of the spontaneous release of oxytocin from the cow's pituitary gland even on sl i g h t stimulation of the udder. This makes i t d i f f i c u l t to record the unmixed effect of oxytocin injection. Donker (17a) injected various doses of oxytocin into a cow which had become unable to eject milk normally. He used the intravenous route for injection to determine the dose levels effective i n causing milk ejection and found 10 I.U. of oxytocin to be more effective. Donker's results are based on the milk y i e l d and he did not study the IMP i n his experiment. The role of oxytocin i n the milk ejection mechanism and the IMP was established beyond doubt by Ely and Petersen (20). The present work was undertaken to study the direct effect of oxytocin administration before and after milking, on the IMP of the lactating cow. Table XI The effect of the administration of oxytocin prior to milking on the Intra-^nammary pressure.  30/11/59 Monday 18/12/59 Friday 18/12/59 Friday 21/12/59 Monday 21/12/59 Monday 22/12/59 Tuesday Cow Bonnie Bonnie View View Barbara View LF IH RH RF LF LH RH RF LF IH RH RF LF LH RH RF LF LH RH RF LF IH RH RF # After stimulation 2k 28 23 20 12 19 16 13 25 27 28 22 31 36 36 30 30 32 35 27 21 28 28 22 Five Minutes After injection of oxytocin 27 33 30 26 22 25 23 20 29 36 37 28 3k k3 k2 31 38 k2 kS 32 30 36 28 32 After Milking 2 8 8 3 8 10 x 8 Not recorded 6 5 k 3 6 11 6 5 5 P 8 k Milk Yield i n Lbs 1(% 9 13 134 17 15 SS - ij.0 U.S.P. units of oxytocin were injected sub-cutaneously at the neck region after stimulation. x - Unreliable result. # - Quarters of the udder, l e f t front, l e f t hind, right hind and right front. - 61 -1. The effect of the admlnlstration of oxytocin prior to milking on  the IMP: Three cows were used for this experiment on different occasions (Table XI). Milk ejection was brought about by stimulating the udder mechanically and the IMP was recorded after stimulation. At this stage a dose of 10x3^0x1^ * equivalent of UO U.S.P. units of oxytocin was injected sub-cutaneously at the neck of the animal. After an interval of fi v e minutes from the time of injection, the IMP was recorded again. (This l e v e l of the oxytocin dose and of the time interval to record the IMP after the injection was found to be more effective on trying different dose and time interval levels). A study of the data of this experiment (Table XI) reveals a definite trend towards an increase i n the IMP as a result of the oxytocin injection. This rise i n the IMP levels, though small, i s significant and i s evidence of the effect of oxytocin on the IMP. The low values of increase i n the IMP may be due to various factors. F i r s t l y ; prior to the administration of oxytocin, the IMP recorded was at i t s maximum level due to the milk ejection brought about by udder stimulation. This limits the scope for further rise i n the IMP subsequent to the injection of oxytocin since the capacity of the alveoli of the udder to contract and raise the IMP i s limited. Secondly; inspite of a l l the care taken, the animal resented the prick of injection and was apparently disturbed. This might have resulted i n the release of adrenaline from the animal system. It has been shown by Braude and MLtchel (U) and Yokoyama (76) that the presence of fc'Qxytcxin1 - a purified-oxytocic-principle product of Austin Laboratories Limited, Guelph, Canada. - 6 2 -adrenaline counteracts the effect of oxytocin and inhibits milk ejection. In view of these facts even the small increase i n the IMP that was registered each time following the administration of oxytocin after natural milk ejection, leads to significant conclusions. It indicates that l e v e l of the IMP brought about by natural milk ejection can be raised by supplementing the supply of oxytocin to the animal system. In view of the important role that the oxytocin hormone plays i n milk ejection and milk production, the present findings emphasize the need for further investigation of this problem. Table X l l ^ e effect of the aciministration of oxytocin subsequent to milking on the IntraHnammary pressure.  -22/10/59 Thursday - 5/H/59 Thursday • 9/11/59 Monday 12/11/59 Thursday IntraHnammary pressure (mm. Hg.) Cow View View View . View LF LH RH RF LF LH RH RF LF LH RH RF LF LH RH RF # After Stimulation 2h 30 31 2k 29 30 39 35 17 21 20 15 22 28 28 23 After Milking 6 6 - 9 6 11 11 10 x 2 5 9 ii 13 13 8 9 XX Five minutes After injection of oxytocin 9 9 15 10 13 11 12 12 5 6 5 k Ik Hi 1U 11 Milk Yield in lbs. 15 16% 1 & H i i XX x # - 6h -2. The effect of the administration of oxytocin subsequent to milking  on the IMP: In the previous experiment (No. 1.) oxytocin was injected into the cow after stimulating the milk ejection process, but before milking the animal. At that stage the IMP assumes i t s highest level and, therefore, the increase i n the IMP marked as a result of the oxytocin administration was not spectacular. The present experiment was undertaken to study the effect of oxytocin on the empty udder after complete milking, a stage when the IMP was at the minimum l e v e l . One cow was employed on four milking occasions for the execution of this experiment. The data presented i n Table XII show the IMP values after stimulation, after milking and five minutes' after injecting 30 U.S.P. units of oxytocin. The injection was given sub-cutaneously at the neck region. A study of the data (Table XII) showing the effect of oxytocin on the IMP indicates, i n general, a well marked trend. A sl i g h t increase i n the IMP values i s noticed as a result of the oxytocin administration i n a l l quarters except one (right hind quarter on November 9» 1959). In this case the IMP recorded after milking was 9 mm. Hg whilst after injection of oxytocin i t dropped to $ mm. Hg. The higher value of 9 mm. Hg appears to have been affected by a movement of the cow. However, a general slight increase i n the IMP as an effect of oxytocin administration i s i n s i g n i f i -cant i f compared with the usual rise i n the IMP recorded after stimulation as shown i n the previous experiments of this work. An explanation for the failure of oxytocin to produce a significant rise i n the IMP after milking, -65-may be found from the observations of Ott and Scott (U5)« They studied the effects of oxytocin on the udder of the non-lactating goat and found that the oxytocin has no effect on the IMP of the non-lactating animals. This may be due to the absence of the secretory glandular tissue, responsible for milk ejection, from the non-lactating udder. It must be pointed out here that during the present experiment the udder was completely stripped before injecting the oxytocin. Therefore, the behaviour of the completely emptied lactating udder to the administration of oxytocin may be expected to be similar to that of the non-lactating udder. Further support for the present findings comes from Whittlestone's work. He (7k) t r i e d to stimulate milk ejection i n the cow after milking the animal. This resulted i n the second let-down and rise of the IMP which, however, was far below the IMP increase recorded on stimulation before milking. Although this meager rise of the IMP may be attributed to the smaller quantity of the pituitary extract released on second stimulation, the state of emptiness of the udder i s also a factor of no less importance. The results of this experiment (Table XII) indicate that apart from the a v a i l a b i l i t y of oxytocin, the degree of fullness of the lactating udder also plays an important role i n the IMP levels. - 66 -V. GENERAL DISCUSSION The present work deals with the recording of the intramammary pressure (IMP) changes i n different phases of lactation. It i s observed that there i s a typical pattern of the IMP changes which show well marked fluctuations on milk ejection and after milking. The i n i t i a l IMP recorded before the udder stimulation increases suddenly on milk ejection and then f a l l s to the minimum level' after' milking. These results support the earlier work of Gaines (29), Tgetgel (6U), Kryzwanek and Bruggemaan (35 and 36) and Whittlestone (7u). In the performance of this experiment various important observations were made which require close attention and further investigation. La the cow, milk ejection i s caused very suddenly on stimulation of the udder and i n some cases i t has become a conditioned reflex. The mere sight of the operator and especially cannulation of a teat i s liable to cause milk ejection with the result that the recording of the correct values of the IMP becomes very d i f f i c u l t . This i s evident from an observation of the i n i t i a l IMP values recorded during the present work. This situation i s peculiar only i n the case of the cow since i n other species eg. the sow, the goat and the ewe, milk ejection i s not stimulated as quickly except when oxytocin i s injected. At the same time when the cow i s excited and emotionally disturbed or i r r i t a t e d , milk ejection i s interfered with and inhibited. This inhibition has been attributed to the release of the hormone adrenaline. This observation has been recorded by Braude and Mitchell (U), Cross (11) and Whittlestone (70). Therefore, the recording of the normal and true values of the IMP i n the cow Is d i f f i c u l t - 67 -i f not impossible. Tgetgel (6k) appeared to overcome this problem by leaving the cannula i n the teat between two milkings. This i t s e l f causes constant i r r i t a t i o n and inconvenience to the animal which may react unfavourably and thus unnoticeably affect the results. The effect of this reflex action to stimulate or Inhibit milk ejection to some degree was noticed on the IMP readings i n some cases during the present study. From the present observations i t i s found that after milking the IMP begins to rise and continues to increase with the increasing interval of time, u n t i l the subsequent milking. This supports the earlier work of Tgetgel (6k) and Kryzwanek and Bruggemaan (35 and 3 6 ) . However, there i s need for further investigation to establish the rate at which the IMP increases from hour to hour between milkings and also to study the effect of the increased IMP on the rate of secretion of milk. The latt e r aspect i s very important since with this i s associated the problem of the frequency of milking. It has been reported by Gasnier et a l . (31) and Hammond (32) that the milk yield i s increased by more frequent milkings. The explanation offered i s that the rate of secretion of milk declines with the increase i n the IMP and that the milk secretion becomes negligible when the IMP rises beyond a certain l i m i t . On the other hand, Turner (67) has thrown doubt on whether this i s the reason for any increases i n yi e l d that occur with thrice daily milking. However, some effect of the IMP on the rate of milk secretion cannot be denied. Studies at the University of Missouri (68) and the work of Petersen (51) demonstrated that the more frequent and the more complete the milkings, the lower the average intra-alveolar pressure and the greater the rate of milk secretion; or - 68 -what i s the same, the longer the interval between milkings, the less the milk production per unit time. The work at the University of Missouri (68) also points out a f a l l i n the fat percentage of milk with the increase i n the IMP, As reported above, the present study deals only with the effects of the increasing time intervals on the IMP, The impact of the increasing IMP on the composition and the rate of secretion of milk requires a separate study. It has been demonstrated by this study that the IMP of each quarter i s independent of that of the other. Furthermore, i t has been shown that the decrease i n the IMP of each quarter after milk ejection depends upon the degree to which that quarter i s emptied. These findings support earlier work of Whittlestone (7U)» Ely and Petersen (20), Cross and Harris (1E>) and Whittlestone (69) had reported that oxytocin stimulates milk ejection. Two experiments were performed to study the effects of the oxytocin on the IMP, The results obtained show that there i s an increase i n the IMP when oxytocin i s injected after stimulation, but before milking and also when i t i s injected after milking. However, the increase i n the IMP i n both these cases i s not spectacular. In the f i r s t case this may be attributed to the maximum level of the IMP already attained after stimulation prior to the oxytocic injection. In the second case the absence of a marked ri s e i n the IMP following the oxytocin injection may be due to the complete evacuation of the udder before the injection. Gaines (29) had earlier reported his failure to stimulate milk ejection i n the non-lactating goat with the injection of the posterior pituitary extract. From the pharmacological studies of this work i t i s observed that there i s a certain optimum li m i t of the IMP i n an animal beyond which the IMP - 69 -cannot rise even with the administration of the milk ejection hormone, the oxytocin. In completely milked mammary glands the oxytocin i s almost ineffective to stimulate milk ejection. The presence of both the secretory glandular tissue and of milk i n the alveoli of the mammary glands i s essential for the stimulation of milk ejection. As would appear from the present work, the importance of the study of the IMP i n general and i n the dairy cow i n particular, cannot be overemphasized. Dairy industry i s an indispensable need of the human population. Efficient lactation i s an important factor on which depends the success of this industry so that i t can stand competition from other more mechanized f i e l d s . The dairy man i s to meet the expensive labour problem especially when faced with the task of milking the animals at inconvenient hours. This makes i t imperative to find out as to how the animal can be milked at longer intervals than at present without suffering a loss of milk or causing a strain on the health of the animal. Therefore, an understanding of the problem of milk ejection and the IMP with which i s connected the efficiency of lactation, becomes of paramount importance. Besides, any improvement on the present method of milking i s not possible without a sound knowledge of the changes that take place within the mammary glands at different phases of lactation. Unfortunately the present information about the IMP changes i s very limited. Therefore, more fundamental work i s required to understand a l l the aspects of this phenomenon. Only then any practical application of the results can be made. In view of the influence of various factors such as the breed, age size, temperament and endocrinological state of the animal, conformation and capacity of i t s udder, the stage of lactation and the milk yield - 70 etc., the study presents d i f f i c u l t i e s which cannot be avoided. However, a thorough understanding of a l l the physiological aspects of the IMP changes i s absolutely necessary. Only then a s t a t i s t i c a l study can be undertaken to support any recommendations that may be made to introduce improved methods of milk production. It i s hoped that the present study would be a contribution of some value i n this direction. - 71 vi:., BIBLIOGRAPHY 1. Andersson, B., Some observations on the neuro-hormonal regulation of milk ejection. Acta Physiol. Scand., 23, 1-7, 1951. 2. Azimov, G.J. and Krouze, N.K. The lactogenic preparations from the anterior pituitary and the increase of milk yield i n cows. J. Dairy Sci., 20, 289-306, 1937 3. 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