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AN INTRODUCTION TO ZOOLOGY 



column, to pass through the diaphragm. A short distance inside 

 the abdominal cavity it opens by an aperture termed the cardia 

 into the stomach on the left side of its anterior wall. Its walls are 

 fairly thick, containing non-striate muscle fibres and it is lined by 

 a mucous membrane. The stomach itself is a wide dilated sac-like 

 structure, rounded and much larger at the left or cardiac end and 

 narrower and smaller at the right or pyloric end. It is asymmetri- 

 cally situated, its main mass lying to the left of the middle line. 

 It terminates on the right in a constriction, the pylorus, where 

 it is narrowed down to a small opening leading on into the intestine. 

 This passage is guarded by a ridge of the mucous membrane and a 

 circle of muscles, constituting the pyloric sphincter, which enables 

 the stomach to be shut off from the intestine during the preliminary 

 stages of digestion. By means of its 

 muscle layers the stomach is able to keep 

 the food in it churned up until it is ready 

 to be passed on. The first part of the 

 intestine is the duodenum, and this takes 

 the form of a long U-shaped loop, held 

 together by an omentum and running 

 almost the whole length of the abdominal 

 cavity on the right side. 



The duodenum passes on into a very 

 similar shaped tube, the small intestine or 

 ileum, about seven or eight feet long, and 

 thrown into a series of folds held together 

 by omenta. On its walls appear, here and 

 there, small granular areas slightly darker 

 in colour than the remainder. These are 

 known as Peyer's patches, and while, like 

 the tonsils, they consist of lymphoid 

 nodules, their exact significance has not 

 yet been ascertained. The walls of the 

 small intestine will be found to be thrown 



up into a number of small, closely set, blunt papillae the villi, 

 giving it the appearance of the " pile " of a carpet. Each is 

 supplied with blood-vessels and also lymphatics, and by their 

 means a large part of the absorption is carried on ; the blood- 

 vessels taking up the soluble food and the lymphatics the 

 emulsified fat. The ileum terminates in a swollen portion, the 

 sacculus rotundas, whose walls are similar in structure to the 

 patches, and this opens into the next part of the intestine a short 

 distance beyond the proximal end of the latter. This part of the 

 gut, the large intestine or colon, is about one and a half feet long, and 



FIG. 103. Two intestinal 

 villi. Magnified i oo di- 

 ameters. -From Quain. 



a., b., and c.. lacteals ; d., 

 blood-vessels. 



