256 



HAND-BOOK OF PHYSIOLOGY. 



the jejunum; then, gradually diminishing in size and number, they cease 

 near the middle of the ileum. They are formed by a doubling inward of 

 the mucous membrane; the crescentic, nearly circular, folds thus formed 

 being arranged transversely to the axis of the intestine, and each indi- 

 vidual fold seldom extending around more than ^ or f of the bowel's cir- 

 cumference. Unlike the rugae in the oesophagus and stomach, they do 

 not disappear on distension of the canal. Only an imperfect notion of 

 their natural position and function can be* obtained by looking at them 

 after the intestine has been laid open in the usual manner. To under- 



X.Tfl, 



FIG. 183. 



FIG. 184. 



FIG. 183. Vertical section Arough portion of small intestine of dog. v, two villi showing e, epithe- 

 i; (/, goblet cells. The free surface is seen to be formed by the "striated basilar border, " while 



FIG. 

 lium ; 



inside the villus the adenoid tissue and unstriped muscle-cells are seen ; If, Lieberkuhn's follicles : m 

 m, muscularis mucosee, sending up fibres between the follicles into the villi; sm, submucous tissue; 

 containing (gm), ganglion cells of Meissner's plexus. (Schofield.) 



FIG. 184. Piece of small intestine (previously distended and hardened by alcohol) laid open to 

 show the normal position of the valvulea conniventes. 



stand them aright, a piece of gut should be distended either with air or 

 alcohol, and not opened until the tissues have become hardened. On 

 then making a section it will be seen that, instead of disappearing, they 

 stand out at right angles to the general surface of the mucous membrane 

 (Fig. 184). Their functions are probably less Besides (1) offering a 

 largely increased surface for secretion and absorption, they probably (2) 

 prevent the too rapid passage of the very liquid products of gastric diges- 

 tion, immediately after their escape from the stomach, and (3), by their 

 projection, and consequent interference with a uniform and untroubled 

 current of the intestinal contents, "probably assist in the more p'erfect 

 mingling of the latter with the secretions poured out to act on them. 



