346 



STOMACH AND INTESTINE. 



This tunic everywhere consists of the ordi- 

 nary elements of a mucous membrane: 

 namely, a basement membrane, an epithelium, 

 and a layer of areolar tissue that contains an 

 admixture of the muscular fibre-cells. But, 

 instead of forming a simple, flat expanse, it 

 undergoes numerous modifications; which, un- 

 der the names of valvuke conniventes, intestinal 

 tubes, villi, agminate follicles, solitary follicles, 

 and racemose glands, will especially claim our 

 notice. 



Vahulce conniventes. Almost all the small 

 intestine is complicated by the presence of 

 transverse folds of mucous membrane ; which 

 project from its inner surface into its cavity. 

 These projections, which were known to many 

 of the earlier anatomists, were named by 

 Kerkring the valvulcs conniventes; apparently 

 from his thinking that they delayed the intes- 

 tinal contents, but still as it were, connived 

 at their passage. They begin in the second 

 portion of the duodenum, and only cease in 

 the lower fifth or sixth of the small intes- 

 tine. They are best shown by moderate dis- 

 tention of the tube with alcohol; which slowly 

 hardens them, so that they retain their shape, 

 even after a portion of the wall of the bowel 

 has been removed to display its interior. 



Fig. 255. 



Small intestine distended and hardened by alcohol, and 

 laid open to sliow the valvulce conniventes occupying 

 its interior. (From a preparation in the Museum 

 of King's College.} 



Extreme distention greatly diminishes their 

 size, but never effaces them altogether. And 

 such a permanent character sufficiently distin- 

 guishes these folds from those temporary 

 creasings which are seen generally throughout 

 the stomach and intestine, and which are 

 sometimes spoken of as preceding them in 

 the first part of the duodenum. At first they 

 are very small and scattered, rise little above 

 the general mucous surface, extend but a 

 short distance across the tube, and break up 

 at their extremities into st'll more minute 

 creases, which often pass obliquely to join 

 those next them, above and below. In the 

 lower part of the duodenum, they gradually 

 acquire a number and size, which are retained 



throughout the whole of the jejunum. But 

 from the beginning of the ileum, they again 

 diminish; first in frequency, and latterly in 

 length and depth. And in the lower third of 

 this segment, they generally disappear alto- 

 gether. 



Each of these folds consists of a duplica- 

 ture of mucous membrane, enclosing a process 

 of the loose areolar tissue which everywhere 

 separates the mucous from the muscular coat. 

 Opposite the attached border of the valvula, 

 this layer is somewhat thicker ; but does not 

 appear to contain more than its ordinary 

 small quantity of fibre-cells. The process 

 which it gives off to each of the folds con- 

 tains vessels, nerves, and lacteals. The relation 

 of this tissue to thevalvula is well shown by the 

 result of its inflation ; which produces a kind 

 of artificial emphysema, that completely oblite- 

 rates the whole projection. When the cavity 

 of the intestine is forcibly distended, the 

 valvulce are placed vertically to the general 

 surface. But in the ordinary state of the 

 bowel, they are easily moved by any ex- 

 ternal force ; so that their free margin is 

 generally directed obliquely upwards or down- 

 wards. Their direction is nearly transverse 

 to the axis of the tube. Their variable extent 

 around the wall of the bowel forms one-half, 

 two-thirds, or even three-fourths of a circle. 

 Their greatest projection occupies the middle 

 of their length, where they are often from 

 one-fourth to half an inch deep. But towards 

 either extremity, they gradually sink into the 

 general mucous surface. In doing this, the 

 valves usually swerve a little from their hither- 

 to transverse and parallel course ; so that each 

 joins by one or both ends with the fold imme- 

 diately before or behind it. And sometimes 

 a bifurcation of the tapering fold unites its 

 extremity to two of its neighbours. 



The office of these permanent folds has 

 been a matter of considerable speculation. It 

 is evident that they increase the extent of the 

 mucous surface to at least twice or thrice 

 what it would be in a simple hollow cylinder 

 of equal size. It is equally obvious, that their 

 transverse position is peculiarly calculated to 

 render this enlarged surface an effective one. 

 For they are at right angles to the direction 

 of peristalsis, and therefore to the general 

 course of the intestinal contents. Such an 

 arrangement of the mucous membrane, taken 

 in connection with the great mobility of these 

 folds, must not only insure a thorough ad- 

 mixture of the various constituents of the 

 chyme, but, by delaying its direct passage 

 onwards, bring every portion of it into con- 

 tact with the greatest possible extent of the 

 active intestinal surface. 



Intestinal tubes. The structure of the 

 remaining constituents of the intestinal mu- 

 cous membrane can only be seen distinctly 

 by the aid of the microscope. Amongst 

 these minute organs, the intestinal tubes 

 or, as they are commonly called, the folli- 

 cles of Lieberkuehn are the first to de- 

 mand our notice. For, with slight modifica- 

 tions, they occupy the whole of the small and 



