A STUDY OF THE CIRCULATION OF BLOOD 151 



probably serve as filters in which disease germs and other 

 foreign bodies are removed from the 

 lymph and destroyed. 



The Lacteals. One portion of the 

 lymphatic system has been already 

 referred to in our study of the villi 

 of the small intestines (see p. 95). 

 The lacteals were described as small 

 tubes in the center of each villus 

 (Fig. 35). They are, however, larger 

 than the lymphatic in other parts of 

 the body. The fat which is absorbed 

 by these lacteals is poured with the 

 rest of the lymph into the thoracic 

 duct, and by this indirect course 

 reaches the blood that enters the 

 right auricle. Unlike the other mi- FlG 59 _ A Lymphatic 

 trients, therefore, the fat does not Node, showing Lym- 



pass through the liver on its way to P hati ? s ( w j th Valves) 



entering and leaving the 

 the heart. Node. 



5. HYGIENE OF THE CIRCULATORY SYSTEM 



Effect of Heat and Cold on the Arteries. It is a fact of 

 common experience that, when the hands or other parts of 

 the body are plunged into hot water, they assume a bright 

 red color. We know, too, that when first exposed to a cold 

 temperature, the surface of the body becomes pallid. These 

 changes in color are due to the action of the muscles in the 

 walls of the arteries. Heat causes the muscles to relax, 

 .thus allowing a greater quantity of blood to flow through 

 the tissue; cold, on the other hand, stimulates the arteries 

 to contraction. If, however, the cold temperature is not too 

 great, the walls of the arteries soon relax, and one feels a 

 warm glow all over the body. These facts will be referred 

 to again in the discussion of bathing (p. 240). 



