264 HAND-BOOK OF PHYSIOLOGY. 



some of the circular muscular fibres of the intestine, which are contained 

 between the cuter surfaces of the two layers of which each fold is composed. 

 While the circular muscular fibres, however, of the bowel at the junction 

 of the ileum with the caecum are contained between the outer opposed 

 surfaces of the folds of mucous membrane which form the valve, the 

 longitudinal muscular fibres and the peritoneum of the small and large 

 intestine respectively are continuous with each other, without dipping 

 in to follow the circular fibres and the mucous membrane. In this man- 

 ner, therefore, the folding inward of these two last-named structures is 

 preserved, while, on the other hand, by dividing the longitudinal muscu- 

 lar 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 inner surface 

 of the folds is smooth; the mucous membrane of the ileum being con- 

 tinuous with that of the caecum. That surface of each fold which looks 

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

 the caecum has none. When the caecum is distended, the margin of the 

 folds are stretched, and thus are brought into firm apposition one with 

 the other. 



DIGESTION IN THE INTESTINES. 



After the food has been duly acted upon by the stomach, such as has 

 not been absorbed passes into the duodenum, and is there subjected to 

 the action of the secretions of the pancreas and liver, which enter that 

 portion of the small intestine. Before considering the changes which 

 the food undergoes in consequence, attention should be directed to the 

 structure and secretion of these glands, and to the secretion (succus en- 

 tericus) which is poured out into the intestines from the glands lining 

 them. 



THE PANCBEAS, AND ITS SECKETION. 



The Pancreas is situated within the curve formed by the duodenum; 

 and its main duct opens into that part of the small intestine, through a 

 small opening, or through a duct common to it and to the liver, about 

 two and a half inches from the pylorus. 



Structure. In structure the pancreas bears some resemblance to the 

 salivary glands. Its capsule and septa, as well as the blood-vessels and 

 lymphatics, are similarly distributed. It is, however, looser and softer, 

 the lobes and lobules being less compactly arranged. The main duct 

 divides into branches (lobar ducts), one for each lobe, and these branches 

 subdivide into intralobular ducts, and these again by their division and 

 branching form the gland tissue proper. The intralobular ducts corre- 



