344 THE VERTEBRATE BODY 



absorption and distribution of products in the body. Lymph consists of 

 the liquid part of the blood which filters out of the small capillaries to 

 bathe the tissues of the body directly. In the intestine there are small 

 lymphatic vessels called lacteals that aid in the absorption of food. 

 There are other special lymphatic vessels in the body that collect the 

 lymph after it has been in contact with various body tissues to give up 

 its food and to absorb excretory wastes. Eventually, the lymph finds its 

 way back into the blood through veins in the neck region and again be- 

 comes a part of the blood. 



The lymph will contain leucocytes, but no erythrocytes or thrombo- 

 cytes. This is possible because the amoeboid nature of the leucocytes 

 enables them to squeeze through the tiny openings of the capillaries 

 along with the liquid part of the blood. Since these are the corpuscles 

 that fight germs of infection they are valuable additions to the lymph, 

 for germs entering through the skin could spread over the body through 

 the lymph before they reached the blood if there were no leucocytes in 

 the lymph to combat them. 



In the frog there are two pairs of lymph hearts, an anterior pair and 

 a posterior pair, that pump the lymph back into the veins. These are 

 not present in man, however, and we are dependent on muscular con- 

 tractions to keep this important fluid moving until it is forced back into 

 the large veins in the neck region. Numerous valves in the lymph 

 vessels allow the fluid to move only in the right direction. As the body 

 muscles contract, the lymph is squeezed along. This is another reason 

 why physical exercise is necessary for a healthy human body. In man 

 lymph vessels enter masses of cells called lymph nodes and the lymph 

 is thoroughly filtered as it winds around through tiny spaces in the node. 

 This is very important because bacteria are often picked up by lymph 

 vessels, and these bacteria are soon destroyed in the lymph nodes. 

 When we have an infection in a finger and bacteria are very numerous, 

 the nodes on the inner surface of the upper part of the arm may enlarge 

 and produce a tender spot in this region. The tonsils are lymph nodes 

 that project into the pharynx and help guard us from infections. As 

 most of us know only too well, these tonsils may become so badly dam- 

 aged in their battle with disease that we must regretfully part with them. 



The Human Blood Types 



We should not leave a discussion of the circulatory system of man 

 without mentioning the blood types. These are of great significance in 

 blood transfusions, settling cases of disputed parentage, and identifica- 

 tion in criminal investigation. There are four major blood types whicb 



