254 HAND-BOOK OF PHYSIOLOGY. 



ments adduced in favor of this theory are open to many objections,, and 

 afford only a negative support to the conclusions they are intended to 

 prove. Again, the pancreatic secretion acts best on proteids in an alka- 



FIG. 181. Auerbach's nerve-plexus in small intestine. The plexus consists of fibrillated substance, 

 and is made up of trabeculae of various thicknesses. Nucleus-like elements and ganglion-cells are im- 

 bedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Klein.) 



line medium; but it has no digestive action on the living intestine. It 

 must be confessed that no entirely satisfactory theory has been yet stated. 



THE INTESTINES. 



The Intestinal Canal is divided into two chief portions, named from 

 their differences in diameter, the (I.) small and (II.) large intestine (Fig. 

 165). These are continuous with each other, and communicate by means 

 of an opening guarded by a valve, the ileo-ccecal valve, which allows the 

 passage of the products of digestion from the small into the large bowel, 

 but not, under ordinary circumstances, in the opposite direction. 



/. The Small Intestine. The Small Intestine, the average length of 

 which in an adult is about twenty feet, has been divided, for convenience 

 of description, into three portions, viz., the duodenum, which extends for 

 eight or ten inches beyond the pylorus; the jejunum, which forms two- 

 fifths, and the ileum, which forms three-fifths of the rest of the canal. 



Structure. The small intestine, like the stomach, is constructed of 

 four principal coats, viz., the serous, muscular, submucous, and mucous. 



(1) The serous coat, formed by the visceral layer of the peritoneum, 

 and has the structure of serous membranes in general. 



(2) The muscular coats consist of an internal circular and an external 

 longitudinal layer: the former is usually considerably the thicker. Both 



