256 OF FOOD AND THE DIGESTIVE PROCESS. 



small and the large. In the small intestine, which constitutes in Man 

 about five-sixths of the whole, the surface of the mucous membrane is 

 greatly extended by the valvulce conniventes, which are folds or dupli- 

 catures, often several lines in breadth, not entirely surrounding the 

 intestine, but extending for about one-half, or three-fourths of its cir- 

 cumference. These are wanting at the lower part of the ileum. The 

 whole surface of the mucous membrane of the small intestine, below 

 the entrance of the biliary ducts, is thickly covered with villi, or little 

 root-like projections, in which* the proper absorbent vessels originate. 

 No proper valvulse conniventes exist in the large intestine ; the only 

 extensions of the mucous membrane being crescentic folds at the edges 

 of the sacculi or pouch- like dilatations in its walls ; and the villi are 

 comparatively few in number, gradually disappearing towards the termi- 

 nation of the intestine. 



449. The mucous membrane of the alimentary canal, through its 

 whole course, is studded with the orifices of numerous scattered glands, 

 which lie in its thickness, or immediately beneath it. The simplest of 

 these are the follicles of Lieberkiihn, which are small pouches, formed 

 by an inflexion of the mucous surface,- analogous to the follicles of other 

 mucous membranes and apparently destined for the elaboration of the 



protective secretion ( 237, see Fig. 31 , b). These 

 follicles in the small intestine, are very simple in 

 their character, and not very deep ; and their 

 apertures, which are small, are situated for the 

 most part around the bases of the villi. In the 

 large intestine they are more prolonged, especially 

 towards the extremity of the rectum, where they 

 form a distinct layer, the component tubes of 

 which are visible to the naked eye ; they probably 

 hite ^ Y ^ the peculiarly thick and tenacious mucus of 

 that part. These mucous follicles become parti- 

 cularly evident when the membrane is inflamed ; for they then secrete 

 an opaque whitish matter, which is absent in the healthy 'state, and 

 which distinguishes their orifices (Fig. 75). A modified kind of these 

 follicles, rather more complex in structure, is found abundantly in the 

 stomach ; where it is concerned in the secretion of the gastric fluid 

 ( 469). 



450. The coats of the intestine contain other glandulse, of which some 

 appear destined, like the preceding, to elaborate fluids of use in the 

 system ; whilst others serve rather to draw off from the blood certain 

 products of decomposition, which are to be excreted from it. The 

 former are known as the glands of Brunner, and the latter as those of 

 Peyer, after the names of their respective discoverers. The glands of 

 Brunner are situated in the duodenum, and lie, not in the mucous but 

 in the sub-mucous tissue. Though their size is only about that of a 

 hemp-seed, they are of very complex structure, consisting of several 

 hundred follicles, clustered round the ramifications of an excretory duct, 

 so as^to resemble the salivary glands (Fig. 79); and each pours its 

 secretion through a single orifice into the intestinal tube. Although 

 nothing is certainly known of the properties of the fluid secreted by these 



