408 THE HUMAN BODY 



Like the stomach, the small intestine possesses four coats; a 

 serous, a muscular, a submucous, and a mucous. The serous coat 

 is formed by a duplicature of the peritoneum, but presents noth- 

 ing answering to the great omentum ; this double fold slinging the 

 intestine is named the mesentery. The muscular coat is composed 

 of plain muscular tissue arranged in two strata, an outer longitu- 

 dinal, and an inner transverse or circular. The submucous coat is 

 like that of the stomach ; consisting of loose areolar tissue, binding 

 together the mucous and muscular coats, and forming a bed in 

 which the blood and lymphatic vessels (which reach the intestine 

 in the fold of the mesentery) break up into minute branches be- 

 fore entering the mucous membrane. 



The Mucous Coat of the Small Intestine. This is pink, soft and 

 extremely vascular. It does not present temporary or effaceable 

 folds like those of the stomach, but is, throughout a great portion 

 of its length, raised up into permanent transverse folds in the form 

 of crescentic ridges, each of which runs transversely for a greater 

 or less way round the tube (Fig. 131). These folds are the valvulce 

 conniventes. They are first found about two inches from the 

 pylorus, and are most thickly set and largest in the upper half of 

 the jejunum, in the lower half of which they become gradually 

 less conspicuous; and they finally disappear altogether about the 

 middle of the ileum. The folds serve greatly to increase the sur- 

 face of the mucous membrane both for absorption and secretion, 

 and they also delay the food somewhat in its passage, since it must 

 collect in the hollows between them, and so be longer exposed to 

 the action of the digestive liquids. Examined closely with the eye 



FIG. 131. A portion of the small intestine opened to show the valvulce conniventes. 



or, better, with aid of a lens, the mucous membrane of the small 

 intestine is seen to be not smooth but shaggy, being covered every- 

 where (both over the valvula3 conniventes and between them) 



