DIGESTION. 195 



of the small intestine, which are very numerous, fere derived mainly 

 from the superior mesenteric arteryj| After penetratiriglhe intestinal 

 walls the smaller vessels ramify in the submucous coat and send 

 branches to the muscle and mucous coats, supplying all their struc- 

 tures with blood. After circulating through the capillary vessels the 

 blood is returned by small veins, which subsequently unite to form 

 the superior mesenteric vein, which, uniting with the splenic and gas- 

 tric veins, forms the portal vein. The nerves are derived from the 

 lower part of the solar plexus. The branches follow the blood-vessels 

 and become associated with two plexuses, one (Auerbach's) lying 

 jbetween the muscle coats, the other (Meissnef^)"lying in the sub- 

 mucous coat. The lymphatics, which originate in the mucous and 

 muscle coats, are very abundant. They unite to form those vessels 

 seen in the mesentery and empty into the thoracic duct. 



Intestinal Glands. The gland apparatus of the intestine by 

 which the intestinal juice is secreted consists of the duodenal (Brun- 

 _ner's) and the intestinal (Lieberkuhn's) glands. 



The duodenal glands are situated beneath the mucous membrane 

 and open by a short wide duct on its free surface. They are racemose 

 glands lined by nucleated epithelium. The secretion of these glands 

 is clear, slightly viscid, and alkaline. Its chemic composition and 

 function are unknown. 



The intestinal glands or follicles are distributed throughout the 

 entire mucous membrane in enormous numbers. They are formed 

 mainly by an inversion of the mucous membrane and hence open on 

 its free surface. Each tubule consists of a thin basement membrane 

 lined by a layer of spheric epithelial cells, some of which undergo 

 distention by mucin and become converted into mucous or goblet 

 cells. The epithelial secreting cells consist of granular protoplasm 

 containing a well-defined nucleus. The intestinal follicles constitute 

 the apparatus which secretes the chief portion of the intestinal juice. 



Intestinal Juice. Owing to its admixture with other secretions 

 and to the profound disturbance of the digestive function, caused by 

 the establishment of intestinal fistulae, this fluid has rarely been ob- 

 tained in a state of purity or in quantities sufficient for accurate 

 analyses or for experimental purposes. Its physiologic properties 

 and functions are therefore imperfectly known. Various attempts 

 have been made by physiologists, by the employment of different 

 methods, to obtain this secretion. The method usually employed 

 is that of Thiry and Vella. This consists in dividing the intestine at 

 two places, about eight or ten inches apart, restoring the continuity 

 of the intestine, and then uniting the two ends of the resected portion 

 to the edges of two openings in the abdominal walls. The resected 

 portion, being supplied with blood-vessels and nerves, maintains its 

 nutrition and secretes a more or less normal juice. 



