260 HANDBOOK OF PHYSIOLOGY. 



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. 164). 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. 



I. The Small Intestine. The Small Intestine, the average length 

 of which in an adult is about twenty feet, has been divided, for conveni- 

 ence 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 exter- 

 nal longitudinal layer; the former is usually considerably the thicker. 

 Both alike consist of bundles of unstriped muscular tissue supported by 

 connective tissue. They are well provided with lymphatic vessels, which 

 form a set distinct from those of the mucous membrane. 



Between the two muscular coats is a nerve-plexus (Auerbaoh/s plexus, 

 plexus myentericus) (Fig. 189), similar in structure to Meissner's (in the 

 submucous tissue), but with more numerous ganglia. This plexus regu- 

 lates the peristaltic movements of the muscular coats of the intestines. 



(3.) Between the mucous and muscular coats is the submucous coat, 

 which consists of connective tissue, in which numerous blood-vessels 

 and lymphatics ramify. A fine plexus, consisting mainly of non-medul- 

 lated nerve-fibres, Meissner's plexus, with ganglion cells at its nodes, 

 occurs in the submucous tissue from the stomach to the anus. From 

 the position of this plexus and the distribution of its branches, it seems 

 highly probable that it is the local centre for regulating the calibre of 

 the blood-vessels supplying the intestinal mucous membrane, and pre- 

 siding over the processes of secretion and absorption. 



(4.) The mucous membrane is the most important coat in relation to 

 the function of digestion. The following structures, which enter into 

 its composition, may now be successively described: the valvulcv conni- 

 ventes; the villi; and the glands. The general structure of the mucous 

 membrane of the intestines resembles that of the stomach (p. 245), and, 

 like it, is lined on its inner surface by columnar epithelium. Adenoid 

 tissue (Fig. 190, c and d) enters largely into its construction; and on its 



