144 THE LYMPHATIC SYSTEM 



tiated into the same three coats which are found in the veins. 

 Except for the fact that they contain lymph instead of blood, 

 these vessels closely resemble the small veins, and like the latter 

 vessels they possess frequent valves. 



The tunica intima of the lymphatic vessel consists of an endo- 

 thelial lining with a thin delicate fibro-elastic membrane. The 



FIG. 143. LYMPHATIC CAPILLARY FROM THE SPERMATIC CORD OF A DOG, SHOWING 



NERVE ENDINGS. 



o, nerve fibres. Methylen blue. Highly magnified. (After Kytmanof.) 



tunica media is thin and contains circular smooth muscle fibres. 

 The adventitia is the thickest coat of the lymphatic vessel. It 

 consists of fibrous connective tissue and longitudinally disposed 

 bundles of smooth muscle fibres. 



The wall of the lymphatic vessels is supplied with small blood- 

 vessels and nerves, in the same manner as the veins. The nerves 

 form a plexus in the adventitia from which branches are distributed 

 to the media and intima. Kytmanof * 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 tissue; the most 

 abundant of these are the perivascular lymphatics. 



* Anat. Anz., 1901. 



