XII. CIRCULATION. 

 Anatomt of the Heart and Blood Vessels. 



Enough has been said in our study of the blood to make 

 clear that its function is the distribution of food and 

 oxygen to the tissue cells, and the removal of waste from 

 these cells. In other words, its purpose in the body is 

 distribution. Furthermore, it is' clear that in order that 

 distribution be made to all parts of the body, the blood 

 must penetrate to all these parts, and its flow to and 

 from these points must be continuous. Such conditions 

 are satisfied by inclosing the blood in tubes of varying 

 size and structure, so arranged as to enter all parts of the 

 body, and connected in such a way that a pump placed 

 in one part of the system will force the blood in a constant 

 stream from the pump to these places and back to the 

 pump again. In our body this closed system of blood 

 vessels is spoken of collectively as the circulatory system, 

 while the pump which keeps the blood in motion is called 

 the heart. The various kinds of tubes are called veins, 

 arteries, and capillaries, according to the direction of blood 

 flow. Let us first examine the structure and workings of 

 the heart, or pump. 



Anatomy of the heart. — (See Ex. XL VII.) This organ 

 is a hollow, muscular sac about the size of a large pear, 

 lying in the chest cavity just above the diaphragm, and 

 immediately behind the lower two thirds of the breast- 

 bone (see Fig. 65). It is shaped somewhat like a cone 



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