STOMACH AND INTESTINE. 



351 



sence occurs in the agminate follicles, or 

 " Peyer's patches." Here they are absent 

 over the several follicles which together form 

 each patch ; and become short, (a, Jig. 21*2, 

 p. 358.) blunt, irregular, or even confluent, 

 where they occupy their interstices. 



We have seen that each of the valvulae 

 conniventes is a doubled fold of membrane, 

 separated by a layer of areolar tissue. While 

 the minute intestinal tube may almost be re- 

 garded as a mere membranous lamina, which 

 is involuted so as to surround a cylindrical 

 cavity, and is packed in a sparing fibrous invest- 

 ment. But the villus constitutes, as it were, a 

 solid process of the mucous membrane. In 

 accordance with this structure, it consists of 

 an epithelium, a basement membrane, a stroma 

 or basis of fibrous tissue, unstriped muscle, 

 and numerous blood-vessels. And in ad- 

 dition to these constituents, which may be 

 found under various modifications throughout 

 the whole intestinal mucous membrane, the 

 interior of each villus encloses one or more 

 branches of the lacteal vessels which con- 

 tain the chyle. 



The epithelium of the villi (a, fig. 259. and 

 a, figs. 26-i, 265, 266.) consists of a single layer 

 of cylindrical cells, which, as regards size, 

 shape, and general appearance, closely 

 resemble those seen on the ridges between 

 the tubes of the stomach. They are, how- 

 ever, even more delicate in their structure, 

 as well as more conical in their shape. 



fig- 



And their contents are, even during fasting, 

 somesvhat darker and more granular. The 

 nucleus, which occupies the same situation 

 in both these varieties of cylindrical epi- 

 thelium, contains a single bright spot, or 

 nucleolus : in rare instances, this appears to 

 be double. 



The basement -membrane (at b, Jig. 260.) 

 docs not require any special mention. As 

 in the gastric ridges, it is very closely at- 

 tached to the subjacent structures, espe- 

 cially to the vessels. But its continuity 

 with the similar structure forming the in- 

 testinal tubes sufficiently indicates that it 

 is really a distinct membrane. And it is 

 often demonstrated to be such by the action 

 of water ; which, after transuding it from the 

 outer surface, raises the membrane, in the shape 

 of a delicate transparent bulla, from the general 

 mass of the villus beneath. 



The blood-vessels of the villi are extremely 

 numerous. 



Small arteries, (aa,fig. 260.) of about T oW tn 

 of an inch in diameter, pass between the in- 

 testinal tubes. The base of each villus re- 

 ceives one, two, or more of these, according 

 to its size. They now pass upwards in 

 the substance of the process, at some dis 

 tance from its surface; and rapidly diminish 

 by giving off numerous capillaries, into which 

 their own trunks entirely merge at about the 

 middle of the height of the villus. 



The ultimate capillaries themselves (c e, 



260. 



v a 



Vessels of two villi, injected. Magnified 100 diameters. 



a a, arteries entering the basis of each villus near its centre; vv, veins seen in the same sitnntion ; c, 

 capillaries lying immediately beneath the limitary membrane ; d, tortuous capillaries occupying the 

 free extremity of one villus; b, limitary or basement membrane of the villus, denuded of its epithelium 



fg. 260.) are, on an average, about rd of the usually such, that the length of its meshes is 



above diameter. They constitute a net-work, five or six times their width. The capillaries 



which lies directly under the basement mem- are distinguished by their being apt to ex- 



brane ; and covers the whole villus so thickly, hibit a wavy and tortuous course (d, fig. 



as to give it a vivid red colour in injected 260.) which often [causes their real length 



specimens. The shape and complexity of this greatly to exceed that of the villus itself, 



network is liable to great variety ; but is This character is especially marked at the 



