398 COMPARATIVE ANATOMY 



greatly in size, and in the translucent mesentery are visible to the naked 

 eye when distended with the milky lymph absorbed from the intestine 

 after a meal. (Fig. 301) 



The lymphatic system includes capillaries, collecting vessels, and 

 lymph nodes. Like blood capillaries, lymph capillaries exchange dissolved 

 substances with the surrounding tissues. They form complicated 

 plexuses, of which few parts of the body are devoid. 



From the lymph capillaries, the lymph passes into lymph vessels or 

 ducts by which it is conveyed to the innominate veins. The largest 

 of the lymph trunks is the thoracic duct, which extends along the backbone 

 and enters the left innominate vein. The wall of a lymph vessel resembles 

 that of a vein, having three coats, intima, media, and adventitia. The 

 lymph vessels, however, have more valves than the veins to insure flow 

 in one direction only. 



On its way through the lymph vessels, lymph before it enters the vein 

 passes through one or more lymph nodes consisting chiefly of adenoid 

 tissue. Lymph vessels enter as vasa afferentia into the cortex of a node, 

 where they form lymph sinuses surrounding lymph nodules. Within the 

 lymph node, the lymph vessels pass from the outer cortical sinuses into 

 the medulla of the node, where they again form sinuses surrounding 

 medullary cords of adenoid tissue. The vasa efferentia which convey 

 lymph away from a node are relatively large vessels. Each lymph node 

 has on one side an indentation or hilum where arteries enter and veins and 

 efferent lymphatic vessels emerge. Lymph nodes have a double function. 

 They are centers of proliferation of lymphocytes, and they serve as lymph 

 filters. The gland cells have the capacity, it is believed, to ingest bacteria 

 and to neutralize the action of foreign substances in the lymph. Fig. 324. 



The thoracic duct extends from the second lumbar vertebra along the 

 spinal column to the point where the jugular and subclavian veins unite to 

 form the left innominate vein. Here the lymph is restored to the veins 

 in the region of least pressure. Into the thoracic duct flows lymph from 

 the lower part of the body as well as the lymph brought by the left jugular 

 and subclavian lymph vessels. At its lower extremity the thoracic duct 

 expands into a cistema chyli or receptaculum. (Fig. 302) 



