1180 DIGESTIVE SYSTEM 



when this has attained a length of about 6.5 mm., the fold which separates the cloaca and hind, 

 gut from the aUantois deepens, and folds develop from the lateral walls of the cloaca which meet 

 and gradually separate the cloaca into a dorsal portion, which forms the rectum, and a ventral 

 portion which forms the uro-genital sinus. This uro-rectal septum extends in its further growth 

 until the cloacal membrane is reached, separating it into a ventral portion known as the uro- 

 genital membrane, and a dorsal portion known as the anal membrane. The anal membrane 

 ruptures comparatively late in development, establishing thus a communication between the 

 hind-gut (rectum) and the exterior. The mesoderm develops around the lower end of the 

 rectum, so that the ectoderm becomes slightly invaginated and lines the portion of the anal 

 canal below the valves. A want of rupture of the anal membrane constitutes an arrest of devel- 

 opment knowTi as atresia of the anus. 



Folds of the mucosa representing the rectal columns, valves and sinuses appear in embryos 

 during the third month, and are well developed during the latter half of the foetal period 

 (Johnson). 



Variations. — The large intestine is exceedingly variable in its structure and relations, 

 especially with reference to the peritoneum — so much so that it has been found more convenient 

 to include a consideration of the variations along with the preceding description of the individual 

 parts. The content of faeces (and gas) is as a rule relatively greatest in the caecum, decreasing 

 in ascending and transverse colons. The descending colon is usually empty, or nearly so, the 

 sigmoid colon and rectum somewhat variable. The rectal ampulla is usually more dilated in 

 women. 



Comparative. — The morphology of both small and large intestines will be briefly considered 

 here. As previously mentioned, the primitive form of intestine is a comparatively straight 

 tube extending from stomach to anus, and connected by a primitive mesentery to the mid- 

 dorsal hne of the body cavity. There is in many of the lower forms no clear division into small 

 and large intestine, though the rectal region is usually more dilated, and opens into a cloaca. 

 Diverticula often occur in the region between large and small intestine. In many fishes, 

 numerous "caeca" occur just below the pylorus, and in others an extensive spiral valve projects 

 into the lumen of the intestine. The absorptive and digestive surface of the mucosa is further 

 increased by the formation of various kinds of folds, and (beginning in amphibia) of villi. 

 Lymphoid tissue is typically present in the mucosa, often localized in definite masses. Solitary 

 nodules appear in amphibia, and Peyer's patches in birds. Tubular mucous glands occur in 

 the lower forms, but Brunner's glands and crypts of Lieberkuehn with Paneth cells apparently 

 only in mammals. A ccEcum is usually present from the reptiles upward (double in birds), 

 and often forms an important organ of digestion. The bile and pancreatic ducts open constantly 

 a short distance below the pylorus. The small intestine is always longer than the large, but 

 there is extreme variation in length among the various species. The four tunics — mucosa, 

 submucosa, muscularis and serosa — are typical for vertebrates, the muscularis consisting of 

 inner circular and outer longitudinal smooth muscle fibres. 



Among rnammals, the divisions of the intestine correspond in general to those found in 

 the human species, but there is exceedingly great variation in the relative development of the 

 various parts. In general, the length, size and complexity of structure is relatively greatest 

 in the herbivora (whose food is more difficult of digestion), least in the carnivora, and intermediate 

 in the omnivora. Even in the same species, the structure of the intestine may be appreciably 

 modified according to habitual diet. The large intestine varies, but is always shorter and wider 

 than the small intestine. In mammals the rectum only is said to be homologous with the large 

 intestine of lower vertebrates. The caecum is rarely absent and is enormously developed in 

 herbivora. It often contains large amounts of lymphoid tissue, which, in pig and ox forms a 

 so-called 'intestinal tonsil.' The vermiform process (found typically developed in man and 

 higher anthropoids) apparently represents a retrogressive evolutionary change in the caecal 

 apex, although this interpretation is denied by some (Berry), who interpret the appendix as a 

 progressive, functional lymphoid organ. 



THE LIVER 



The liver [hepar] is the largest gland in the body. Its secretion, the bile [bills ; 

 fel], is poured into the duodenum through the common bile duct. In addition it 

 has important functions as a 'ductless gland' in connection with the nitrogenous 

 and carbohydrate metabolism. In form it is a variable somewhat irregular mass, 

 roughly comparable to a modified hemisphere occupying the upper right portion of 

 the abdominal cavity (figs. 899, 914). It presents a convex, rounded upper or 

 parietal aspect, which is in contact with the diaphragm and adjacent body walls, 

 and a lower, flattened viscer.al surface, in contact with the abdominal viscera. 

 When viewed from the front, it is somewhat triangular in outline, occupying the 

 right hypochondriac, the(q)igastricand (slightly) the left hypochondriac regions. 



Physical characters. — In weight, the liver averages about 1500 gm. (83 lbs.), 

 init it is exceedingly variables, commonly ranging from lOOOgm. to 2000 gm. Its 

 relative weight is also variable, averaging about 2.5 per cent, of the body in the 

 adult male (somewliat higher iiutlu; fcsmale). li?^ specific gravity averages 1.056, 

 so that the average weight of 1500 gm. would correspond to a volume of 1420 cc. 

 Its dimensions are also quite variable. Its greatest depth (antero-posterior) 

 averages about 15 cm., and its greatest height (vertical) is about the same. Its 



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