

THE VALVUL^E CONNIVENTES. 513 



neum, is thin and elastic, to permit of various degrees of distension ; 

 whilst the smoothness and moisture of its free surface, facilitate the 

 changes of form and position of the intestinal convolutions upon each 

 other, and upon adjacent parts. The muscular coat, TW, consists, as 

 elsewhere, of an external layer of longitudinal, and an internal layer 

 of circular fibres. The longitudinal layer is thinner than the circular 

 layer, and is most distinct along the free border of the intestine ; the 

 circular fibres are arranged more closely together. The areolar or 

 submucous coat, , is loosely connected with the muscular coat, but 

 more firmly with the mucous membrane, which it supports. Thin cres- 

 centic extensions of this areolar coat project transversely, at intervals, 

 into the interior of nearly every part of the small intestine, and, cov- 

 ered, on both sides and at their edges, by the closely adherent mucous 

 membrane, constitute the so-called valvulce cortniventes, Fig. 91, v. 

 These valves may be displayed by opening the intestine, and immersing 

 it in water. In a portion of intestine inflated, dried, and laid open 

 longitudinally, they are seen as transverse crescentic folds or ridges, 

 wider in the middle, and tapering at either end. Each extends about 

 half or two-thirds around the interior of the tube ; the longest are 

 about two inches in length, and one-third of an inch wide at their 

 broadest part, but most of them are smaller ; the larger and smaller 

 ones alternate ; unlike the rugae of the stomach, they are permanent, 

 and not obliterated by distension ; they do not contain any of the cir- 

 cular muscular fibres, as the pyloric valve does. They begin in the 

 duodenum, about one inch below the pylorus ; in the lower part of the 

 duodenum, they are very large, and succeed each other closely ; about 

 the middle of the jejunum, they begin to get smaller and wider apart ; 

 in the lower half of the ileum, they become less distinct, and in the 

 lowest part of that intestine, they are altogether wanting. The mucous 

 membrane of the small intestine, which also covers the valvulse conni- 

 ventes, is specially characterized by being everywhere closely beset 

 with an immense number of minute thread-like processes, called villi ; 

 when immersed in water, these stand up and produce a flocculent ap- 

 pearance, resembling the pile of velvet; hence this mucous membrane 

 has been termed villous. It also contains the intestinal glands, to 

 be presently described, and other glands to be noticed, with the lac- 

 teals, in the section on Absorption. The nerves of the small intes- 

 tine are derived immediately from the sympathetic system: on their 

 finest branches in the submucous areolar tissue, are found multitudes 

 of the microscopic ganglia, elsewhere noticed (p. 259) ; others exist 

 between the circular and longitudinal muscular layers (Meissner, 

 Auerbach). 



The movements of the small intestine, depending on the contraction 

 of its longitudinal and circular fibres, afford the most perfect example 

 of vermicular or peristaltic movements. They consist, in the healthy 

 state, of slow, successive, wavelike contractions, chiefly of the circular 

 fibres, from the upper to the lower part of the intestine. They are 

 noticeable in very emaciated persons during life, but are powerfully 

 excited by exposure of the intestines to the air, especially when the 

 abdominal aorta has been tied ; they continue for a short time after 



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