THE VASCULAR SYSTEM 289 



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 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. 



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. 252) 



