1174 



SPLANCHNOLOGY 



from this point, down to the middle of the ileum, they diminish considerably in size. In the 

 lower part of the ileum they almost entirely disappear; hence the comparative thinness of this 

 portion of the intestine, as compared with the duodenum and jejunum. The circular folds 

 retard the passage of the food along the intestines, and afford an increased surface for absorption. 



....Capillary network 



FIG. 1063. Vertical section of a human aggregated lymphatic nodule, injected through its lymphatic canals, a, 

 Villi with their chyle passages. 6. Intestinal glands, c. Muscularis mucosse. d. Cupola or apex of solitary nodule. 

 . Mesial zone of nodule. /. Base of nodule, g. Points of exit of the lacteals from the villi, and entrance into the true 

 mucous membrane, h. Retiform arrangement of the lymphatics in the mesial zone. i. Course of the latter at the 

 base of the nodule, k. Confluence of the lymphatics opening into the vessels of the submucous tissue. I. Follicular 

 tissue of the latter. 



The intestinal villi (villi intestinales) are highly vascular processes, projecting from the mucous 

 membrane of the small intestine throughout its whole extent, and giving to its surface a velvety 



appearance. They are largest and most numer- 

 ous in the duodenum and jejunum, and become 

 fewer and smaller in the ileum. 



Structure of the villi (Figs. 1059, 1060). The 

 essential parts of a villus are: the lacteal vessel, 

 the bloodvessels, the epithelium, the basement 

 -membrane, and the muscular tissue of the mu- 

 cosa, all being supported and held together by 

 retiform lymphoid tissue. 



The lacteals are in some cases double, and in 

 some animals multiple, but usually there is a 

 single vessel. Situated in the axis of the villus, 

 each commences by dilated cecal extremities 

 near to, but not quite at, the summit of the 

 villus. The walls are composed of a single layer 

 of endothelial cells. 



The muscular fibers are derived from the mus- 

 cularis mucosa:, and are arranged in longitudinal 

 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 and to the lacteal. 



The bloodvessels (Fig. 1C61) form a plexus 

 under the basement membrane, and are enclosed 

 in the reticular tissue. 



These structures are surrounded by the 6ose- 

 , ment membrane, which is made up of a stratum 



of endothelial cells, and upon this is placed a layer of columnar epithelium, the characteristics of 

 which have been described. The retiform tissue forms a net-work (Fig. lOdO) in the meshes 

 of which a number of leucocytes are found. 



The intestinal glands (glandulce intestinales [Lieberkuhni]; crypts of Lieberkuhn) (Fig. 10G2) are 



Large ^..j 

 circular'' , 

 vessel 



Fid. 1064. Transverse section through the equatorial 

 plane of three aggregated lymphatic nodules from the 

 rabbit. 



