LYMPHATIC VESSELS 229 



The tunica intima of the lymph vessel consists of an endothelial 

 lining with a thin delicate fibro-elastic memhrane. The tunica media is 

 thin and contains circular smooth muscle fibers. The adventitia is the 

 thickest coat of the lymph vessel. It consists of fibro-elastic connective 

 tissue and longitudinally disposed bundles of smooth muscle fibers. 



The wall of the lymph vessels is supplied with small blood-vessels and 

 nerves, in the same manner as the veins. The nerves form a plexus in 



FIG. 237. LYMPHATIC AND BLOOD VESSELS IN THE HILUM OF A HUMAN LYMPH-NODE. 

 a, lymph vessels; b, blood-vessels. Hematein and eosin. Photo. X 160. 



the adventitia from which branches are distributed to the media and 

 intima. Kytmanof (Anat. Anz., 1901) has traced the fine nerve fibrils 

 to the smallest lymphatic capillaries, where, as in the intima of the 

 larger vessels, they end in close relation to the endothelial cells. 



To summarize : the lymphatic capillaries arise by one of three 

 methods : 



1. As lymphatic plexuses in all connective tissues; the most abun- 

 dant of these are the perivaseular lymphatics. 



2. As dilated pouches having blind extremities, as in the villi of 

 the small intestine, where they are known as lacteals. 



