282 STUDIES IN ADVANCED PHYSIOLOGY. 



usually closed, -and opens only from time to time to allow 

 properly digested food to pass. The small intestine is 

 much coiled, and has a length of about twenty feet. The 

 coils, however, do not lie superimposed one on the other in 

 the abdominal cavity, but are all suspended in mesenteric 

 slings from the back bone. This suspension not only keeps 

 the folds from resting upon each other, but also prevents 

 them from being relatively displaced. The first twelve 

 inches of the intestine are called the duodenum (which 

 means twelve) , because in this portion of the intestine but 

 little active digestion is going on, the mixture of the foods 

 with the pancreatic juice and the bile taking place here. 



By far the larger portion of the remainder is called the 

 jejunum ) while the final third is spoken of as the ileiim. 

 This division is quite arbitrary, and the commencement of 

 the ileum is roughly stated to be that point where the disin- 

 tegration of food has begun to reach the putrefactive stage. 



The structure of the intestinal walls does not vary very 

 much from the typical four coats. On the outside is the 

 serous coat, a reduplication of the peritoneum, and called 

 the mesentery. In these folds of the mesentery the loops 

 of the intestines are of course supported, and through these 

 folds blood-vessels, nerves and lymphatics reach them. 

 The muscular coat consists of an outer longitudinal and an 

 inner circular, between which are the nerve plexus of Auer- 

 bach and numerous blood-vessels. The muscular coat is 

 followed by the submucous, in which there are numerous 

 blood - vessels and lymphatics and the nerve plexus of 

 Meissner. The submucous is followed in turn by the mu- 

 cous coat, the principal coat of the intestine. This mucous 

 coat shows a number of peculiarities. In the first place, it 

 is even in a fairly distended intestine thrown into numer- 

 ous transverse folds called valvulcz conniventes, the pur- 

 pose of which is to afford a greater secreting surface, and 

 at the same time to form lateral pouches in which the food 

 will be detained and so better acted upon by the digestive 

 juices. Close examination of the mucous membrane shows 



