250 DIGESTION. 



reason to doubt that they resemble very nearly those discharged 

 by the glands of like structure in the small intestine. 



The difficulty of determining the function of any single set 

 of the intestinal glands seems indeed almost insuperable, so 

 many fluids being discharged together into the intestine ; for 

 all acting, probably, at once, produce a combined effect upon 

 the food, so that it is almost impossible to discern the share of 

 any one of them in digestion. 



Ileo-ccecal valve. The ileo-csecal valve is situate at the place 

 of junction of the small with the large intestine, and guards 

 against any reflux of the contents of the latter into the ileum. 

 It is composed of two semilunar folds of mucous membrane. 

 Each fold is formed by a doubling inwards of the mucous mem- 

 brane, and is strengthened on the outside by some of the circu- 

 lar muscular fibres of the intestine, which are contained be- 

 tween the outer surfaces of the two layers of which each fold 

 is composed. The inner surface of the folds is smooth ; the 

 mucous membrane of the ileum being continuous with that of 

 the csecum. That surface of each fold which looks towards the 

 small intestine is covered with villi, while that which looks to 

 the csecum has none. When the csecum is distended, the 

 margins of the folds are stretched, and thus are brought into 

 firm apposition one with the other. 



While the circular muscular fibres of the bowel at the junc- 

 tion of the ileum with the csecum are contained between the 

 outer opposed surfaces of the folds of mucous membrane which 

 form the valve, the longitudinal muscular fibres and the peri- 

 toneum of the small and large intestine respectively are con- 

 tinuous with each other, without dipping in to follow the cir- 

 cular fibres and the mucous membrane. In this manner, 

 therefore, the folding inwards of these two last-named structures 

 is preserved, while on the other hand, .by dividing the longi- 

 tudinal muscular fibres and the peritoneum, the valve can be 

 made to disappear, just as the constrictions between the sacculi 

 of the large intestine can be made to disappear by performing 

 a similar operation. 



The Pancreas, and its Secretion. 



The pancreas is situated within the curve formed by the 

 duodenum ; ami its main duct opens into that part of the in- 

 testine, either through a small opening or through a duct com- 

 mon to itself and to the liver. The pancreas, in its minute 

 anatomy, closely resembles the salivary glands ; and the fluid 

 elaborated by it appears almost identical with saliva. When 

 obtained pure, in all the different animals in which it has been 



