The Circulatory System 183 



likewise aid, as does the negative pressure of the thoracic cavity. Pos- 

 sibly the rapid flow of blood past the entrance of the lymphatics into 

 the main veins aspirates to some degree. The flow, however is very 

 slow, averaging only 4 mm. per second in the largest duct. 



Among some of the lower vertebrates, the frog for example, there 

 are actually lymph hearts located along some of the main lymphatics. 

 These pump the lymph forward. 



In most mammals, the lymph from the entire abdominal region, legs, 

 left side of head, and chest enters the veins through the left thoracic duct 

 which is at the junction of the jugulars and the subclavian where they 

 unite to form the left innominate. The lymph from the right side of the 

 head, neck, and chest enters through the right lymphatic duct at the 

 origin of the right innominate (Fig. 49). 



In its route through the body, the lymph vessels pass through 

 lymph nodes where they break up into many small channels. The lymph 

 nodes are scattered throughout the body along the courses of the lymph 

 channels. Concentrations of them occur in the neck, under the arms, 

 in the mesenteries, and in the groin. They are definite organs whose 

 tissue is formed of a spongelike framework with many free cells in 

 the meshes. One of the very important types of cells present in this 

 tissue are the macrophages which are able to engulf and frequently 

 destroy bacteria and other foreign substances. Another important func- 

 tion of these nodes is the production of lymphocytes. 



All the lymph passes through the various nodes, and is thus 

 filtered before returning to the veins. In cases of infections, vast 

 quantities of bacteria are carried by the lymph to the nodes. In this 

 way, infections are often localized and kept from spreading throughout 

 the body. In severe infections the nodes may become swollen and 

 tender. As they are more abundant at the joints, an infection in a finger 

 may result in a swelling under the arm. Even foreign bodies such as 

 dust are filtered out by these lymph nodes. City dwellers and coal 

 miners usually have such accumulations of these foreign particles that 

 the nodes in the region of the lungs become black. 



The intestinal region is well supplied with lymph vessels which 

 absorb the digested fats. They are quite milky in appearance and bear 

 the descriptive name of lacteals. 



Once the lymph returns to the blood stream, it is again an essential 

 part of the blood from which it was originally derived. This process of 

 tissue fluid formation and lymph return is a continuous one, essential 

 for the maintenance of homeostasis in the organism. 



