THE LYMPHATIC NODES 227 



The internal is of elastic fibers arranged lengthwise, 

 covered with a layer of endothelial cells; the middle 

 coat consists of white fibrous tissue arranged longi- 

 tudinally, with non-striated muscle and elastic fibers 

 arranged transversely; the external coat is practically 

 the same as the middle coat, except that the muscle 

 fibers are arranged longitudinally. 



Lymph vessels possess valves, which are so close 

 together and so numerous as to appear as beads 

 upon the course of the vessels. They face toward 

 the larger vessels, are arranged in pairs, and are 

 formed from a reduplication of the vessel wall rein- 

 forced by white fibrous tissue from the middle coat. 



The Lymph Nodes. They are small, solid, gland- 

 ular bodies found along the course of the lymphatic 

 vessels. They vary in size from a microscopic 

 mass of lymphoid tissue to an olive. Their color 

 when cut is pinkish, except in the bronchial nodes, 

 which are black, due to the absorption of foreign 

 particles of dust from the mucous membranes of 

 the respiratory tract; the nodes around the liver 

 (hepatic) are yellowish, due to the absorption of bile 

 pigments. The splenic lymph nodes are brown. 



As the lymphatic vessels from the lymph spaces 

 approach a node they divide into numerous small 

 vessels called afferent vessels. The latter pierce the 

 capsule of the gland; devoid of their outer coat, they 

 enter the sinus beneath the capsule, termed the 

 subcapsular sinus, which communicates with a central 

 sinus. The lymph is "filtered within this node and is 

 collected by small vessels which unite beneath the 

 capsule and pierce it as a single efferent vessel. Upon 

 leaving the lymph node the efferent vessel is invested 

 by an external coat derived from the gland capsule. 

 The lymph continues to pass on until another set of 

 nodes are reached, when the same arrangement takes 

 place, and so on until the large lymphatic vessels 

 are reached, whereupon, through the thoracic ducts 



