918 



THE ORGANS OF DIGESTION. 



are surrounded and enclosed by a delicate reticulum which forms the matrix of 

 the villus, and in the meshes of which are found large flattened cells with an oval 

 nucleus, and, in smaller numbers, lymph-corpuscles. These latter are to be 

 distinguished from the larger cells of the villus by their behavior with reagents, 

 by their size, and by the shape of their nucleus, which is spherical. Transitional 

 forms, however, of all kinds are met with between the lymph-corpuscles and the 

 proper cells of the villus. Nerve-fibres are contained within the villi ; they form 

 ramifications throughout the reticulum. 



The lacteah are in some cases double, and in some animals multiple. Situated 

 in the axis of the villi, they commence by dilated csecal extremities near to, but 

 not quite at, the summit of the villus. The walls are composed of a single layer 

 of endothelial cells, the interstitial substance between the cells being continuous 

 with the reticulum of the matrix. 



The muscular fibres are derived from the muscularis mucosge, and are arranged 

 in bundles around the lacteal vessel, extending from the base to the summit of the 

 villus, and giving off laterally, individual muscle-cells, which are enclosed by the 

 reticulum, and by it are attached to the basement membrane. 



The blood-vessels form a plexus between the lacteal and the basement mem- 

 brane, and are enclosed in the reticular tissue ; in the interstices of the capillary 

 plexus, which they form, are contained the cells of the villus. 



These structures are surrounded by the basement membrane, which is made 

 up of a stratum of endothelial cells, and upon which is placed a layer of columnar 

 epithelium. The reticulum of the matrix is continuous through the basement 

 membrane (that is, through the interstitial substance between the individual 

 endothelial cells) with the interstitial cement substance of the columnar cells on 

 the surface of the villus. Thus we are enabled to trace a direct continuity between 

 the interior of the lacteal and the surface of the villus by means of the reticular 

 tissue, and it is along this path that the chyle passes in the 

 O a process of absorption by the villi. That is to say, it passes 



^y^ first of all into the columnar epithelial cells, and, escaping 



cVi^ from them, is carried into the reticulum of the villus, and 



I'i^&S thence into the central lacteal. 



The simple follicles, or crypts of Lieberkiihn (Figs. 499, 

 oOO), are found in considerable numbers over every part 

 of the mucous membrane of the small intestine. They 

 consist of minute tubular depressions of the mucous mem- 

 brane, arranged perpendicularly to the surface, upon 

 which they open by small circular apertures. They may 

 be seen with the aid of a lens, their orifices appearing as 

 minute dots scattered between the villi. Their walls are 

 thin, consisting of a basement-membrane lined by columnar 

 epithelium, and covered on their exterior by capillary 

 vessels. 



The duodenal or Brunner's glands are limited to the duo- 

 denum and commencement of the 

 jejunum. They are small, flat- 

 tened,granular bodies embedded 

 in the submucotis areolar tissue, 

 and open upon the surface of 

 the mucous membrane by mi- 

 nute excretory ducts. They 

 are most numerous and largest 

 near the pylorus. They are 

 small compound acino-tubular 

 glands, and much resemble the 



and Noble Smith.) 



FIG. 499. Longitudinal 



the mucous membrane of the 



