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THE LYMPHATIC SYSTEM 



THE LYMPH NODES. 



The lymphoid structures so far studied are associated with the 

 lymph capillaries, i. e., they are situated near the beginning of 

 lymph vessels. A more elaborate organ, the lymph node, found 

 widely distributed in mammals, is an oval or bean-shaped gland 

 interpolated along the course of larger lymph vessels. Lymph 

 enters the node by one or more afferent lymph vessels and leaves 

 by one or more efferent lymphatics. Lymph nodes vary in size 

 from a few millimeters in length to 2 centimeters or more. They 

 are also indefinite in numlier, since they may degenerate or new 

 nodes may form in the loose fibroelastic connective tissue of certain 



Fig. 73. — Mesenteric lymph node of the gray squirrel, showing dense cortical 

 region and lighter medulla. The connective tissue of the capsule extends into the 

 medulla at the hilum and forms trabeculae in the cortex. 



regions. Lymph nodes are prominent in the mesentery near the 

 junction of the small with the large intestine and also in the con- 

 nective tissue in the region of the axilla and the groin. (Fig. 73.) 



Each node is invested with a capsule of fibroelastic connective 

 tissue in which smooth muscle cells are usually scattered. The 

 capsular tissue is more evident at the hilum, or concave depression, 

 where the efferent lymph vessel, an artery, vein, and nerves are 

 connected with the node. In examinations of stained preparations 

 of mammalian nodes, two regions, an outer cortex and an inner 

 medulla, usually stand out clearly. (Fig. 74.) 



Cortex. — Licomplete partitions, or trabeculse, of fibroelastic con- 

 nective tissue extend from the capsule into the interior, thus dividing 

 the cortex roughly into compartments. In these compartments are 



