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Some observationson the histological structure of the digestive tract of the common fowl, Gallus Domesticus Bolton, Lloyd Lawrence 1924

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Some Observations On The HISTOLOGICAL STRUCTURE OF THE DIGESTIVE TRACT of the COMMOl FOWL, GALLUS DOMES TIG US BY LLOYD LAWRMCE BOLTOH A Thesis Submitted f o r the Degree of MASTER OF ARTS i n the Department of loology THE OTI7ERSITY OF BRITISH COLUMBIA A p r i l , 1924 CONTESTS Pag© Foreword 1. Technique 3* Introduction and General Description 4. The Oesophagus ( f i r s t Part) 8. The Crop 9. The Oesophagus (SmtQisM. Part) 10. The Proventrieulus 10. The Gizzard 12. The Small Intestine 14. The large Intestine 1&. the Caecum 17. /Bibliography 19. Table of Abbreviations K - Kaupp, B.F., "Anatomy of the Domestic Fowl", 1918, W.B.Saunders Co., Philadelphia, Pa., D&K- Dahlgren & Kepner, "P r i n c i p l e s of Animal Histology", I908, The MacMillan Co., New York. L i s t of I l l u s t r a t i o n s Oesophagus, f i r s t part, section from the middle " " " cross-section near crop " " " cross-section of w a l l " n " cross-section of w a l l , showing glands and ducts. Oesophageal Gland Oesophageal Lymph Nodule Longitudinal section at ^taction of crop and oesophagus (part one) Cross-section of crop Muscularis Mucosae of crop, showing i t s two layers Oesophagus, second part, cross-section Longitudinal section at junction of oesophagus (second part) and Proventrieulus Gross-section through region where oesophagus (second part) joins Proventrieulus Gross-section of Proventrieulus, half-way between middle and anterior end Cross-section through middle of Proventrieulus Mucous Epithelium ( s u p e r f i c i a l glands) of Proventrieulus Section of deep gland of Proventrieulus Q£Q-s*3-*seetidn of l i n i n g of gizzard Cross-sections of gizzard Surface of duodenum, showing form and arrangement of v i l l i Longitudinal section of duodenum Glands of duodenum from cross-section L i s t of I l l u s t r a t i o n s (Oont.) f i g . 22 from a tangential section of duodenum, cut near the base of the v i l l i , showing glands branching from base of p i t s 23 Gross-section of posterior end of small intestine 24 Cross-section of anterior part of large i n t e s t i n e 25 Cross-section of posterior part of large intestine 26 Gross-section of proximal part of caecum 27 Gross-section of enlarged d i s t a l part of caecum > Some Observations on the H i s t o l o g i c a l Structure of the Digestive Tract of the Common Fowl, Gallus Domesticus Foreword The "birds are a class of vertebrates, highly specialised f o r a mode of l i f e mueh d i f f e r e n t from that of mammals. The domestic fowl, while not so highly s p e c i a l i z e d as the birds of the a i r i n many respects, i s nevertheless no l e s s highly s p e c i a l -i s e d i n regard to i t s habits of feeding and the manner of handling the food material which i t has eaten. The problem of the structure of i t s digestive organs therefore presented i t s e l f to me as a most promising f i e l d for f r u i t f u l research, and indeed i t has proven so, though the task has been correspondingly d i f f i c u l t . The d i f f i c u l t i e s encountered have been due c h i e f l y to two causes: F i r s t l y , up to the present time h i s t o l o g i c a l i n v e s t i -gation has been confined for the most part to the mammals, and even more p a r t i c u l a r l y to man himself. Thus there was l i t t l e with whieh to compare the fowl. Many d i f f i c u l t i e s which aris e , and which I cannot explain, would no doubt prove very simple to understand, were the structure of the corresponding tissues of the phylogenetic ancestors of the birds known. In short, studying the structure of the higher forms of vertebrates before that of the lower ones,^*even some of the invertebrates, i s no doubt as much a matter of "st a r t i n g at the wrong end of the ladder" i n histology as i t proved to be i n anatomy. The second great cause of d i f f i c u l t y i s the fact that 2 l i t t l e or nothing i s known of the physiological processes involved i n digestion i n the b i r d s . Here again, research to date has been confined for the most part to the mammals, from the structure of the t r a c t of bi r d s , one can only conclude that i t s physiological processes must d i f f e r considerably from those of the mammals, and i t would seem that the only solution of the problem l i e s i n the working hand-in-hand of the h i s t o l o g i s t and the physiologist, when the findings of both w i l l prove of mutual benefit. In presenting t h i s thesis, therefore, i t i s done with the f e e l i n g that the work i s f a r from complete; indeed, I might say that only the surface has been scratched, yet at the same time I f e e l that i t i s a step, though but a small one, i n a great f i e l d of not only i n t e r e s t i n g but what should prove most f r u i t f u l and useful research. Before c l o s i n g t h i s note, I wish to express my sincerest thanks to Dr. 0.McLean Eraser, under whom t h i s work has been done, for the u n f a i l i n g interest he has shown, and for the assistance and suggestions which he has given most freely at a l l times. Also, I wish to thank Dr.A.H.Hutchinson, who has given many valuable suggestions, p a r t i c u l a r l y with regard to matters of technique, and Mr. I.A.Lloyd, of the Department of Poultry Husbandry of t h i s u n i v e r s i t y , who very generously supplied from h i s pens an unlimited amount of material on which to work. Technique: Material for t h i s work was obtained from the pens of the University of B.C., the birds being so f a r as could be determined, i n a normal, healthy condition. They, with one exception, were k i l l e d with chloroform or ether, and portions of the various tissues at once removed and placed i n either B.G. Fix a t i v e or Zenker's F l u i d . The material f i x e d i n Zenker's f l u i d I found much the better, both i n the matter of cutting the sections and i n staining them. The greatest d i f f i c u l t y i n the matter of technique was encountered i n the i n t e s t i n e . In a l l the e a r l i e r sections cut, the epithelium covering the v i l l i of the intestine was much broken down over t h e i r free ends, and drawn away from the underlying connective t i s s u e . This breaking down may have been due, to some s l i g h t extent, to mechanical i n j u r y i n the process of washing out the food material, but i n many cases i t was quite apparent i n sections of tissue i n which the food material had been allowed to remain undisturbed. Br. Fraser suggested the p o s s i b i l i t y of auto-digestion beginning immediately a f t e r death, and with t h i s i n mind a b i r d was anaesthetized, and while i n t h i s condition sections of the intestine were removed and plaeed i n Zenker's f l u i d . The sections from t h i s material showed a much more continuous epithelium over the v i l l i , and no doubt the d i f f i c u l t y would have been completely overcome had a f i x a t i v e such as Gilson's, which i s much quicker i n i t s action, been used. , The stains used mostly were methylene blue and eosin, or 4 haematoxylin and eosin. Special methods of preparation, either i n f i x a t i o n or i n s t a i n i n g , have not been used up to the present, j c h i e f l y on account of lack of time. Xylene was used throughout, as the clearing agent, and the sections were mounted i n Canada Balsam. Introduction: The tissues examined to date consist of sections from the ©esophagus to the large intestine only, no attempt having been made to work out the structure of the various regions of the buccal and cloacal c a v i t i e s . The structure of the t r a c t as a whole i s f a i r l y comparable to that of the t y p i c a l mammalian t r a c t , i n that i t i s composed of a raucnsa, made up of epithelium, tunica propria and museularis mucosae; a submucosa, which for the most part seems l i t t l e developed; and a museularis, composed of an inner c i r c u l a r and outer l o n g i t u d i n a l layer. Outside of t h i s i s usually a tunica adventitia, or tunica serosa. 4 note of the outstandings differences may be made ,here, and they w i l l be taken up i n greater d e t a i l i n the p a r t i c u l a r description of the various regions. The greatest modifications occur i n the f i r s t part of the t r a c t , that i s , the oesophagus and stomach, and these are no doubt due to the fact that the b i r d masticates i t s food l i t t l e , i f I any, i n the buccal cavity. The oesophagus i s w e l l supplied with glands throughout i t s entire length, with the exception of that part which i s 'enlarged to form the crop. The f i r s t part of the stomach, the proventricuius, i s mad© up mostly of glandular tissue. I t s walls are heavy and the glands are of two kinds, s u p e r f i c i a l , and deep. Glandular tissue comprises, I should say, f u l l y 80% of the structure. The second part, the gizzard, on the other hand, i s composed mostly of muscular t i s s u e . I t has a horny l i n i n g , however, which i s secreted by a layer of tubular glands l i n i n g i t s cavity. In connection w i t h the stomach, the presence of a great number of large lymph nodules i n the region of the junction of the oeso-phagus and proventrieulus, i s p a r t i c u l a r l y of note, as indeed i s the presence of numbers of them throughout the l i n i n g of the oesophagus i t s e l f . The intestine i s i n many respects s i m i l a r to that of the mammals, with the exception that I f i n d no i n d i c a t i o n of * any glands comparable to the submucous or Brurj^r's glands, t y p i c a l of the duodenum. Of t h i s more w i l l be mentioned under the description of the duodenum. A layer which has given me much trouble to place, i s the musculajpis mucosae, and t h i s has involved also the p l a c i n g of the tunica submucosa. D i r e c t l y beneath the tunica propria, i n the oesophagus, i s a layer of muscle, following the convolutions of the t&§, mucosa. I t i s quite t y p i c a l of that of the muscularts mucosae of mammals, and rests on a t y p i c a l sub-mucous layer. Towards the crop, however, t h i s layer becomes much heavier ( f i g . 2 ) , and does not follow nearly so c l o s e l y the convolutions of the mueosa, also the submucous layer becomes much reduced. In the second part of the oesophagus these features become s t i l l more marked, the mustular layer becoming nearly, i f not equally, as heavy as 6 the c i r c u l a r muscle layer, and i s separated from i t by a t h i n layer of connective tissue ( f i g . 1 0 ) . In addition to t h i s , a great number of small bundles of muscle are found scattered throughout the tunica propria. The structure i n the proventri-eulus i s s i m i l a r i n t h i s respect, the chief difference being that the scattered bundles are separated from the heavy, continuous layer, by the great mass of the deep, proventricular glands. Throughout the small i n t e s t i n e , t h i s inner muscular layer i s separated from the c i r c u l a r layer of the museularis by only a t h i n layer of connective t i s s u e , and here also, scattered f i b r e s are found i n the tunica propria. In the 3#rge int e s t i n e and caecum, the mucosa i s somewhat convoluted, and t h i s inner, muscular layer follows the convolutions. Beneath * i t the sub-mucous layer i s somewhat thicker than i n the small i n t e s t i n e , and i s s i m i l a r to the t y p i c a l structure of the sub-mucosa of mammals. In regions where the sub-mucosa i s reduced, the blood vessels, nerves & c t y p i c a l of that layer, evidently pass from the tunica serosa d i r e c t l y through the muscular coat into the tunica propria before branching to supply a l l parts of the mucosa. On account of the p o s i t i o n of t h i s inner muscular layer, and the scattered f i b r e s i n the second part of the oesophagus, the proventriculus and the small i n t e s t i n e , the question a r i s e s , does i t r e a l l y represent the museularis mucosae, i n which case the scattered bundles have merely wandered away from i t , and the submucosa i s represented by only a t h i n connective-tissue layer; or i s i t r e a l l y an inner 7 l o n g i M d i n a l muscle layer, i n which case the museularis mueosae i s represented i n these regions only "by the scattered bundles of muscle, the tunica propria and submucosa become confluent or i n d i s t i n c t l y separated by the muscle bundles and the deep glands of the proventriculus and tubular glands of the intestine become submucous i n p o s i t i o n . I t w i l l be seen that the i n t e r p r e t a t i o n of the t r a c t throughout a great part of i t s length, depends to a great degree on the i n t e r p r e t a t i o n of the museularis mucosae. While i t seems d i f f i c u l t to conceive of the submucosa being p r a c t i c a l l y absent, as such, throughout a great portion of the length of the t r a c t , the former i n t e r p r e t a t i o n seems by far the (most probable, as the layer whieh appears d e f i n i t e l y as a museularis mucosae i n the f i r s t part of the oesophagus, the colon and the caeca, appears to be i d e n t i c a l and continuous with that whieh appears as an inner, l o n g i t u d i n a l layer i n the other parts of the t r a c t * Therefore, i n what follows I s h a l l consider t h i s layer as being., or representing, the museularis mucosae, and interpret the structure of the t r a c t accordingly. In connection with the structure of the museularis mucosae i t s e l f , a few remarks must be made. In the oesophagus t h i s layer i s of t y p i c a l muscular tissue, but i n the proventriculus another type of tissue appears i n connection with i t , i n which the c e l l s are not packed nearly so closely together, and the c e l l s themselves are much l e s s compact, appearing i n many eases even more or les s vacuolated i n nature. In the proventriculus, both types of c e l l may be present within the same bundle. In the two intestines and the caeca, the looser form of tissue seems to predominate. I 8. fhe Oesophagus ( F i r s t Part) fhe tunica mucosa makes up the greater part of the thickness of the w a l l of the t r a c t . I t i s thrown into deep, longitudinal f o l d s , and consists of the three t y p i c a l layers, epithelium, tunica propria, and muscularis mucosae. fhe Epithelium i s a comparatively heavy layer, and of i s A t h e squamous s t r a t i f i e d type. I t s "basal or germinative side i s very i r r e g u l a r . From the epithelium arise the oesophageal glands ( f i g s . 4 and 5 ) , which are comparatively large, and embedded i n the tunica propria, being s i m i l a r to those described by^Dahlgren and Kepner (page 54). fhe duct i s comparatively short, i t s walls being .continuous with the epithelium. I t opens into a large lumen from which tubular branches are given off r a d i a l l y , fhe whole structure, with the exception of a small portion of the duct, i s l i n e d with secretory c e l l s , which seem to be e n t i r e l y mucous i n nature, fhe secretory epithelium of the tubular branches i s one c e l l layer i n thickness, and the w a l l s of adjacent tubules are supported and separated by a t h i n layer of the connective tissue of the tunica propria. In close association with a great proportion of the glands, are comparatively large lymph nodules ( f i g . 6 ) . A t h i n layer of connective tissue usually extends between the glandular and lymphoid tissue, though i n many cases t h i s i s so t h i n as to be almost imperceptible, even under a high magnification, or i t may even disappear completely, leaving the lymphoid tissue d i r e c t l y i n contact w i t h the bases of the gland c e l l s . In f a c t , I have found many cases i n which the tubules of the glands were l i t e r a l l y embedded i n the lymph nodule S t r a t i f i e d epithelium lumen of t r a c t Oesiphageal glands Submucosa Ci r c u l a r muscle Longitudinal muscle J. Tunica adventitia Jk. Fig I. F i r s t part of the oesophagus- cross section from middle portion (x 10.5) (Square shows position of f i g 4) Fug 2. F i r s t part of the oesophagus near the crop (x 1 0 . 7 ) Lumen of tr a c t S t r a t i f i e d e p i t h e l i u m Tunica propria Muscularis mucosae Oesophageal gland Submucosa C i r c u l a r muscle - L o n g i t u d i n a l muscle Tunica adventitia F i g 3. F i r s t part of oesophagus* cross section of w a l l (x 2 8 . 5 ) -lumen of t r a c t S t r a t i f i e d epithelium oesophageal gland Muscularis mucosae 1 Blood vessels i n sub-) / mucosa C i r c u l a r muscle long i t u d i n a l muscle Fig 4. F i r s t part of oesophagus- cross section of wall (x 30.5) showing three glands with ducts. (S uare denotes position of f i g 5») S t r a t i f i e d epithelium _ Lumen of duct Tunica propria Lumen of gland Blood vessel uscularis mucosae Submucosa Fig 5*. Oesophageal gland (x 8 8 . 5 ) Lymph nodule Tubule of gland Tunica propria Museularis mucosae Submucosa Fig. 6 . Oesophageal lymph nodule (x 114.5) 9 The Tunica Propria consists of a somewhat loose, connective-tissue layer, containing blood vessels, nerves and lymph nodules,, and supporting the glands. The Museularis Mucosae i s a well-developed layer of l o n g t i t u d i n a l muscle, following the p l i c a t i o n s of the tunica propria. I t becomes somewhat heavier near the crop ( f i g . 2 ) . The Submucosa i s a t h i n layer of connective t i s s u e , carrying the usual blood and lymph vessels and nerves. I t extends up into the fo l d s of the mucosa, and f i b r e s from i t may pass between the bundles of the museularis mucosae, to j o i n with the tunica propria. The Tunica Museularis i s composed of an inner c i r c u l a r , and ana: outer lo n g i t u d i n a l layer. The coat i s f a i r l y heavy, and the two layers are of approximately equal thickness. Outside the museularis i s the Tunica Adventitia, a t h i n l a y e r of connective tissue carrying the blood vessels, nersres &c. and i n many places extending into the museularis, carrying the ' blood-vessels and nerves with i t . The Prop The structure of the w a l l of the crop i s very s i m i l a r to that of the oesophagus. The epithelium i s s t r a t i f i e d , and i s p r a c t i c a l l y i d e n t i c a l withathat of the oesophagus. The t y p i c a l oesophageal glands extend a short distance into the crop, SJHA where i t joins with the f i r s t part of the oesophagus (fig.7) and appear again i n the region where i t narrows into the second part. In no other part of the crop have I found any ind i c a t i o n of glands. The tunica propria i s a t h i n connective-tissue layer. The muscular!  mucosae i s comparatively heavy, and t y p i c a l l y divided into two Lumen of trac$ Oesophageal epithelium Museularis mucosae Tunica propria C i r c u l a r layer ) M u s c u l a r i s Longitudinal l a y e r ' — Tunica adventitia Epithelium of crop Fi g 7. Longitudinal section at function of crop and f i r s t part of oesophagus (x 10 . 5 ) F i g 8 . Cross section of crop (x 3 3 . 5 ) (Square indicates position of f i g ?) S t r a t i f i e d epithelium Inner c i r c u l a r layer Outer l o n g i t u d i n a l layer Tunica propria Submucosa Blood vessel Fig 9 . Muscularis mucosae of crop showing i t s two layers (x 1 5 0 ) Muscularis mucosae S t r a t i f i e d epithelium Mucous gland Tunica propria C i r c u l a r muscle Connective tissue Longitudinal muscle FiglO. Cross s e c t i o n of second part of oesophagus (x 14.3) 10. layers, an inner c i r c u l a r and an outer layer ( f i g .9 ) varying from lo n g i t u d i n a l to oblique as i t surrounds the crop. The submucosa i s a comparatively t h i n layer of somewhat loose connective t i s s u e , carrying the usual blood-vessels, etc. In the tunica museularis the two usual layers are found, the outer l o n g i t u d i n a l being somewhat the heavier. The Oesophagus Second Part) The second part of the oesophagus ( f i g . 10) d i f f e r s s l i g h t l y from the f i r s t part i n one or two minor respects. Scattered throughout the tunica propria are many small bundles of muscle f i b r e s , which evidently hate branched o f f from the museularis mucosae. The museularis mucosae i t s e l f i s a much heavier coat, and as before mentioned, does not to any great extent follow the convolutions of the mucosa. The submucosa i s represented by a t h i n connective-tissue layer, separating the museularis mucosae from the tunica museularis. Of the two layers of the tunica museularis, the inner c i r c u l a r i s somewhat the heavier, the outer l o n g i t u d i n a l not being continuous, but consisting of a few i r r e g u l a r bundles. A t h i n connective-tissue layer, as before, forms a tuniea adventitia. The Proventriculus The most noticeable feature of the proventriculus i s i t s great glandular development. Of these there are two toinds, the " s u p e r f i c i a l " and the "deep" glands. The w a l l of the proventriculus i s cl o s e l y perforated by the ducts of the s u p e r f i c i a l glands. These are deep, simple or s l i g h t l y c o i l e d , tubular p i t s , l i n e d with columnar, mueus-secreting c e l l s ( fig. 1 5 ) . Longitudinal muscle C i r c u l a r musele Museularis mucosae . Mass of lymph nodules Deep gland Tunica propria End of oesophageal glands End of oesophageal epithelium S u p e r f i c i a l glands Fig I I . Longitudinal section at junction off oesophagus and proventriculus ( 1 6 . 5 ) Oesophageal epithelium Oesophageal gland Pig 12. } Mass of lymph nodules Deep gland Museularis mucosae Cir c u l a r muscle Longitudinal muscle Cross section through region where oesophagus joins proventriculus (x 10.2) 11 The deep glands are of the type termed complex (D.& K. page 54), that i s , they are primarily of the compound-alveolar type, but the lobules themselves are composed of numerous small, r a d i a t i n g tubules, the walls of which are l i n e d with cuboidal secretory c e l l s , which are granular and evidently serous i n nature ( f i g . 16). The duct i s much branched, the branching beginning very close to the proximal end of the duct. The lumen of the common duct i s l i n e d with pseudo s t r a t i f i e d epithelium, which i s continued to l i n e the lumen of the a l v e o l i . As i n the oesophageal glands, the secretory w a l l of the tubules i s one c e l l - l a y e r i n thickness, and adjacent tubules are supported and separated by a t h i n connective-tissue septum. Having described the two kinds of glands, an idea of the structure of the proventriculus as a whole may probably best be gained by describing sections from various regions: A section through the region of the junction of the oesophagus with the proventriculus ( f i g . 1 2 ) : The mucosa i s quite t y p i c a l of the oesophagus for some distance into the proventriculus. As the lumen of the tract enlarges, the convolutions become correspondingly greater. In t h i s region there i s an aggregation of lymph nodules ( f i g . 11) i n the tunica propria, which almost completely f i l l s the space between the oesophageal glands. The lumen i n t h i s region i s s t i l l l i n e d with s t r a t i f i e d epithelium, but the layer i s becoming thinner. Oesophageal glands are s t i l l present i n the tunica propria (fig.12), which also contains scattered muscle bundles. Outside t h i s the deep glands begin to appear. The museularis mucosae i s a heavy Deep gland Aggregation of lymph nodules S u p e r f i c i a l glands Tunica propria Oesophageal epithelium Lumen of deep gland Longitudinal muscle Muscularis mucosae Oireular muscle Fig 13. Cross section of proventrieulus midway between middle and anterior end (x 9 . 6 ) Longitudinal muscle Ci r c u l a r muscle Muscularis mucosae Duct of deep gland S u p e r f i c i a l gland Deep srland F i g 14. Cross section through middle of proventrieulus (x 8 . 3 ) Columnar epithelium Tubules i n cross section Connective tissue septum Cuboidal serous c e l l s Blood vessel Pig 1 6 . Section of deep gland of proventriculus (x 144) 12. regular layer, containing both, the compact and the looser tissue mentioned i n the general description. The submucosa i s a very t h i n layer of connective tissue. The muscularis consists of a well-developed c i r c u l a r coat and an outer poorly developed, incomplete, longitudinal coat, embedded i n a 3cose tunica serosal A section midway between the anterior and and the middle point ( f i g . 1 3 ) : In t h i s region the s t r a t i f i e d epithelium has nearly disappeared, as i t ends abruptly about here. The glands t y p i c a l of the oesophagus have also disappeared, and the s u p e r f i c i a l proventicular glands appear, lymph nodules are s t i l l present i n the tunica propria. The deep glands here make up a great proportion of the thickness of the w a l l of the t r a c t . The other layers are p r a c t i c a l l y the same as described i n the previous section: A section through the middle part of the proventrieulus ( f i g . 14): Here the glands of the proventrieulus reach t h e i r greatest development. The walls of the t r a c t are very heavy, due c h i e f l y to the great amount of space occupied by the deep glands. The Gizzard The structure of the gizzard i s t o t a l l y d i f f e r e n t from that of any other part of the t r a c t . As i s w e l l known, i t s chief function seems to be one of thorough mastication of the food material, and i t s two chief distinguishing features are i t s great muscular development and i t s horny l i n i n g . The muscles are so F i g 1 8 . Gross sections of g i z z a r l i A shows duodenum joining B i s through middle region. (Natural size) Intermediate layer of connective tissue Muscle tissue Connective tissue Glands secreting horny coat Gland layer horny l i n i n g F i g 17. Cross section of l i n i n g of gizzard (x 3 7 . 5 ) 13 large that t h e i r arrangement becomes an anatomical rather than a h i s t o l o g i c a l problem, and adequate descriptions of them may be found i n various anatomical t r e a t i s e s on the subject. The cavity of the gizzard i s t y p i c a l l y thrown into f o l d s and l i n e d by a heavy, horny c u t i c l e (fig.17) which i s secreted by a compact layer of comparatively deep, simple, tubular glands. I t i s evidently produced as a f l u i d or semi-f l u i d mass, and hardens on passing from the t i p of the gland, thus keeping the layer renewed as i t s exposed surface i s worn away by the food and stones which are constantly present i n the cavity of the gizzard (B.&K. page 288). In a l l the sections examined to the present time I have found no i n d i c a t i o n of any other kind of glands inthe gizzard, although others are mentioned as occurring on the anterior and posterior parts (K.page 147). Below the bases of the glands i s a layer of connective tissue(D.& E. 288). This t i s s u e , however, i s not t y p i c a l of any of the eommon connective tissues of mammals, although i t resembles to some extent that o r d i n a r i l y known as mueous tissue. I t i s very compact i n structure (fig.17) and i t i s d i f f i c u l t to make out any d e f i n i t e c e l l structure or f i b r e s i n i t . In fac t , i t appears more l i k e a homogeneous, dark-staining, gelatinous mass, with numerous spindle- and star-shaped openings, rather than the s i m i l a r shaped c e l l s t y p i c a l of mucous tissue . Below t h i s layer i s the muscular tissue of the gizzard, which i s separated into numerous s t r a t a by layers of connective tissue of the type just described, a l t e r n a t i n g with layers of muscle (fig. 1 7 ) . F i g 1 9 . Surface of the duodenum showing the form and arrangement of the v i l l i , (x 1 8 ) Arrow denotes lon g i t u d i n a l d i r e c t i o n of the tr a c t . Epithelium P i t Tunica propria I n t e s t i n a l gland at base of p i t Museularis mucosae Cir c u l a r muscle lo n g i t u d i n a l muscle Tunica serosa Pig 20. longitudinal section of the duodenum (x 5 3 ) .14. fhe Small Intestine Prom the gizzard, the small intestine i s given o f f at i t s anterior end ( f i g . 18), close to the entrance of the proventrieulus. I t i s not d i f f e r e n t i a t e d into three parts, as i s that of mammals, hut only into the duodenum and the free portion. H i s t o l o g i c a l l y , the small i n t e s t i n e i s nearly uniform throughout i t s length, fhe inner surface i s covered with tongue-shaped v i l l i ( f i g . l ? ) s i m i l a r to those of mammals, hut the pliclejav c i r c u l a r e s t y p i c a l of the mammalian jejunum and ileum are quite absent. fhe v i l l i are covered with a single layer of high columnar e p i t h e l i a l c e l l s (E. page 148), amongst which are numerous mucus-secreting, goblet c e l l s , fhe goblet c e l l s increase i n number toward the bases of the v i l l i , and also are much more numerous toward the posterior part of the i n t e s t i n e . At the base of the p i t s between the v i l l i the epithelium i s continued to form simple, unbranehed, s l i g h t l y c o i l e d , tubular or tubo-alveolar glands, embedded i n the tunica propria (figs.20 & 21). fhe tunica propria makes up the stroma of the mucosa. I t consists of loose connective tissue, the interspaces of which are more or l e s s f i l l e d with lymphocytes, u n t i l indeed the tissue might e a s i l y be taken f o r lymphoid t i s s u e , fhe tunica propria also c a r r i e s rather large blood vessels, which branch and come closely i n contact with the w a l l s of the v i l l i . The c a p i l l a r y system, however, does not seem to be nearly so much branched, nor the vessels themselves nearly so minute, as those of the i n t e s t i n a l mucosa of the mammals. I s mentioned i n the general remarks, muscle f i b r e s are also found scattered throughout the tunica propria. Fig 21. Glands of duodenum from cross section(x 1 3 5 ) m BBliS Glands fcganohjng^rom Tunica propria : j Cross section of v i l l u s Fig.22. From a-tangential section of duodenum across the base of the v i l l i (x. 45) 15. The museularjs mucosae i s a t h i n layer composed almost e n t i r e l y of the looser kind of tissue mentioned i n the general description. The f i b r e s run l o n g i t u d i n a l l y , and give o f f branches on the inner side to the tunica propria. The submucosa i s reduced to a very t h i n connective-tissue layer, and shows no i n d i c a t i o n whatever of glands. Mr. Kaupp, i n h i s book on the "Anatomy of the ^omestie Fowl", page 149, states,"Openings of simple i n t e s t i n a l tubular glands, the duodenal glands, or glands formerly known as Brunner's glands, are located between the v i l l i " . The only glands I f i n d i n the duodenum, or indeed anywhere i n the small i n t e s t i n e , are the tubular glands which are, as before described, p i t s contin-uous with the mucous epithelium of the v i l l i . These are t y p i c a l i n t e s t i n a l glands, but i n no way comparable to the duodenal or Brunner's glands of the submucosa of the mammalian duodenum, liven granting that my int e r p r e t a t i o n of the muscularis mucosa iias given i n the general desciption be wrong, and that i t i s represented only by the scattered muscle f i b r e s of the tunica propria, i n which case these tubular glands would be embedded i n the sub-mucosa, s t i l l I cannot see that they are at a l l homologous with the duodenal or Brunner's glands of mammals, for they are i n no wise comparable with them i n structure, being much simpler, and wholly mucous i n nature. — Epithelium — Tunica propria — Tubule of gland ~ Museularis mucosae — C i r c u l a r muscle — l o n g i t u d i n a l muscle 2 3 . Cross section od the posterior end of the small intestine ( 3 5 . 5 ) 16. The tunica museularis consists of the two usual layers, She inner c i r c u l a r layer i s heavy and i t s bundles are large. A quite noticeable feature i s the tendency of a t h i n layer of i t on the inner side to be s l i g h t l y separated from the remainder. The outer l o n g i t u d i n a l muscle layer i s usually very t h i n and incomplete, and more or l e s s embedded i n the loose tissue of the tunica serosa, though many sections show i t to form a complete layer. Towards i t s posterior end, the only remarkable feature of the small i n t e s t i n e seems to be that the v i l l i become somewhat ( f i g . 23) reduced i n size,/and the development of goblet c e l l s i n t h e i r epithelium i s c a r r i e d to a degree exceeded only i n the large i n t e s t i n e . The Large Intestine The mucosa of the large intestine i s very s i m i l a r i n structure to that of the small i n t e s t i n e , but i t shows some tendency to f a l l into l o n g i t u d i n a l f o l d s . The v i l l i are compara-t i v e l y much smaller than those of the small i n t e s t i n e . ^ke epithelium covers the v i l l i , and extends below the p i t s between them, to form tubular glands i n a manner s i m i l a r to that of the small i n t e s t i n e ( f i g . 24). The development of mucous secreting epithelium i s ca r r i e d to the greatest possible degree here. I t appears as being almost e n t i r e l y composed of goblet c e l l s . The tunica propria, as throughout the i n t e s t i n a l t r a c t , i s t y p i c a l l y a net-work of connective tissue f i b r e s , f i l l e d with lymphocytes, and also bearing scattered muscle f i b r e s and a small number of lymph nodules. The museularis mucosae i s a comparatively t h i n layer following the folds of the mucosa, and i s e n t i r e l y of the looser Fig 24. Gross section of anterior part of large i n t e s t i n t (x 44) Fig 25. Cross sectionof oosterior part of large intestine (x 23) 17. structure previously described. The submueosa i s a connective-tissue layer, s l i g h t l y heavier than i n the small i n t e s t i n e , and thickened beneaihthe infol d i n g s of the mucosa. I t c a r r i e s many blood and lymph vessels, and i s i n general quite t y p i c a l of the usual structure found i n the mammalian submucosa. She museularis i s a heavy layer, being at least equally as thick as the mueosa. Both the inner c i r c u l a r and the outer l o n g i t u d i n a l lap r s are developed much more extensively than i n the small i n t e s t i n e , and the l o n g i t u d i n a l lpyer forms, as a r u l e , a complete band. Toward the posterior end of the large i n t e s t i n e , the v i l l i become much shortened ( f i g . 2j>), the glands much fewer i n number, and the muscular layers become very heavy. The Caecum The paired caeca are about s i x inches long,and are given off where the small intestine joins the large. The proximal portion i s small in,diameter, and l i n e d with v i l l i . The mueosa i s deeply folded l o n g i t u d i n a l l y . Toward the d i s t a l end i t becomes enlarged and thinner walled, showing the l o n g i -tudinal folds very prominently when viewed from the inner surface. In the proximal portion ( f i g . 26) v i l l i are present and covered with columnar, mucous epithelium, which, as i n the rest of the i n t e s t i n e , i s continued into tubular glands, at the base of the p i t s between them. These glands are, however, not so numerous nor so large as i n the i n t e s t i n e . The goblet c e l l s are not nearly so p l e n t i f u l , a n d are confined c h i e f l y to the lower parts of the v i l l i and the p i t s of the glands. Fig26. Gross section of proximal part of caecum, showing longitudinal folds (x 48.5) Tunica propria Glandular p i t Muscularis mucosae Ci r c u l a r muscle longitudinal muscle "Fig 27. Cross section of enlarged d i s t a l part of caecum (x 6 0 ) 18. The tunica propria and mus c u l a r i s mucosae are quite s i m i l a r to that of the i n t e s t i n e . The submucosa i s very t h i n , except that part under-l y i n g the elevated folds of the mucosa, where i t has a quite t y p i c a l submucous structure. Lymph nodules may be present. The museularis layer i s w e l l developed, p a r t i c u l a r l y the inner c i r c u l a r layer, which alone i s regularly thicker than the whole mucosa. The longitudinal layer forms a f a i r l y complete band, but i s not nearly so heavy as the c i r c u l a r . The w a l l of the d i s t a l part of the caecum i s t h i n i n comparison with the diameter of the lumen ( f i g . 2 7 ) . The v i l l i become short, and p r a c t i c a l l y disappear as such, leaving only a small glandular p i t s , continued into ±fe& short tubular glands. The l o n g i t u d i n a l f o l d s stand out prominently i n the absence of the v i l l i . The goblet c e l l s of the epithelium are p r a c t i c a l l y confined to the glands and lower parts of the p i t s . The tunica propria i s s i m i l a r to that of the i n t e s t i n e . The museularis mucosae and submucosa are both t h i n , and the two layers of the museularis are also much reduced i n thickness. BIBLIOGRAPHY text; 1. Bailey, F.R., "A Text Book of Histology", 1?20. 2. Bohra, Bavidoff and Huber, "A Text Book of Histology", I916. 3. Bahlgren and Eepner, "Principles of Animal Histology", I908. 4. Jordan, H.I., "A Text Book of % s t o l o g y " , 1920. 5. Kaupp, B.F., "Anatomy of the domestic Fowl", I ? l 8 . 6. Lewis,and Stohr, "Text Book of Histology", 1913. 7. P i e r s o l , G.A., "Normal Histology", 1?20. 1 Technique: Guyer, M.F., "Animal Micrology", 1?17. Lee, A.B., "Micro tomist's Yade Mecum". 

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