VESSELS OF THE SMALL INTESTINE. 



213 



as forming a sphincter muscle for the veins which penetrate it. No valves 

 occur until the veins enter the tunica muscularis; there they appear, and 

 continue into the collecting veins 'in the mesentery. They are absent from 

 the large branches of the portal vein which receives the blood from the 

 intestines. 



Lymphatic vessels. The intestinal lymphatics [lacteals] appear as 

 central vessels within the villi (Fig. 239, B). Each villus usually contains 

 one, which ends in a blind dilatation near its tip; sometimes there are two 

 or three which form terminal loops. In some stages of digestion the dis- 



Villus. 



Intestinal glands. 



Submucosa. 



Longitudinal 

 layer 



of the muscularis. 



FIG. 241. TRANSVERSE SECTION OF AGGREGATE NODULES OF THE SMALL INTESTINE OP A CAT. 

 The crests of four nodules were not within the plane of the section. X 10. 



tention of these lymphatics is very great and their endothelium is easily 

 seen in sections. When collapsed they are hard to distinguish from the 

 surrounding reticulum. Small lateral branches and a spiral prolongation 

 of the central lymphatic have been found by injection, but these may be 

 tissue spaces. The lymphatics branch freely in the submucosa and have 

 numerous valves. They cross the muscle layers, spreading in the inter- 

 muscular tissue and the serosa, and pass through the mesentery to the 

 thoracic duct. 



Lymphoid tissue. The lymphoid tissue of the intestine occurs pri- 



