CIRCULATION OF THE BLOOD. 



45 



The Distribution of the Arteries and Veins. The 



organs of the body receive a supply of blood in propor- 

 tion to their size and activity. The artery supplying the 

 blood and the vein which returns it usually lie side by side 

 (see Fig. 16). The larger arteries are usually deep-seated 

 and in protected places. 



Demonstration of the Action of the Heart. The heart may be 

 mounted as shown in Fig. 17, and its action illustrated by compressing 

 the ventricles with both hands. Instead of the apparatus here shown 

 two retort stands may be used, though not so convenient. 



Capillaries 

 of the Lungs 



HI Capillaries 

 of the Body 



Fig. 17. Demonstration of the Action of the Heart (Heart Diagrammatic). 



The Action of the Heart. --The heart consists of 

 muscle fibers so arranged that they form a thick-walled 

 bag, which stands expanded when the muscles relax. But 

 when the fibers shorten the whole heart contracts, and the 



