84 THE LYMPHATIC SYSTEM. 



of the lymph-vessels. The nutritive fluids, after exuding 

 from the blood-capillaries and bathing the tissues, accumulate 

 in the interstices between the cells and in the spaces in the 

 tissues. These spaces open directly into the smallest radicles 

 or capillaries of the lymphatic vessels ; or the fluids may 

 enter the vessels by osmosis through their thin walls. The 

 lacteals form another important class of lymphatic origins ; as 

 elsewhere described, they are small blind tubes occupying the 

 axis of the villi of the small intestine, and emptying into the 

 larger lymphatic vessels in the submucosa. They are really 

 lymphatic radicles or capillaries, and their special function is 

 to absorb into the circulation the fatty and other food-mate- 

 rials digested and elaborated in the alimentary canal. The 

 lymph-stomata, minute openings guarded by small spheroidal 

 cells, occurring at the points of junction of the endothelium- 

 cells of serous membranes, also form a sort of origin or open- 

 ing into lymph- vessels. 



Lymphatic vessels : The lymph-vessels form a series of 

 channels ramifying through the entire body. The fluids from 

 the lymph-spaces and lacteals are taken up by the small 

 lymphatic branches, these combine to form larger channels, 

 and finally they are all united into two vessels, the thoracic 

 duct and the smaller right lymphatic duct, which empty into 

 the blood at the junction of the jugular and subclavian veins 

 on each side. 



The smallest lymphatic vessels, the so-called lymph-capil- 

 laries, including the lacteals, are delicate protoplasmic tubes 

 made up of a single layer of polygonal endothelium-cells. 

 As these vessels unite and become somewhat larger, their 

 endothelial lining, now made up of more elongated cells, 

 becomes surrounded with a certain amount of connective 

 tissue. The smaller lymphatic channels are very delicate, 

 inconspicuous structures difficult to distinguish. 



The larger lymph- vessels have thicker walls, and in structure 

 somewhat resemble the veins. Their walls are composed of 

 three coats. The inner coat is lined with elongated endothelium- 

 cells, beneath which is a thin layer of connective and elastic tis- 

 sue. The middle coat consists mainly of elastic tissue and in- 

 voluntary muscle-cells arranged transversely. The external coat, 

 or adventitia, is an investment of fibrous and areolar tissue. 



