CIRCULATION. 



83 



venous into arterial blood is effected, and it flows back to the 

 heart purified and ready for further circulation. 



6. The Heart. The great central organ of blood circu- 

 lation is a powerful pump made of muscular tissue. It is 

 placed near the middle of the chest, between the lungs, which, 

 in fact, almost sur- 

 round it, there being 

 only a small trian- 

 gular portion in front 

 uncovered. At this 

 point the action of 

 the heart can be 

 readily examined. It 

 is conical in form, 

 and is placed ob- 

 liquely, with the base 

 upward, the apex 

 pointing downwards 

 and forward, toward 

 the left side. The 

 size of the heart var- 

 ies in different per- 

 sons, and according 

 to age, but the size 

 of the individual does 



not modify it. It F IG - 26 The Heart: -4, the right ventricle ; fi, the left 

 * ventricle ; C, the right auricle ; D, the left auricle. 



is, however, usually 



smaller in the female. The size of one's fist is said to cor- 

 respond with that of the heart. ^It continues to grow for 

 some time after the full size of the body is attained, especially 

 in the male. The average measurement in the adult is about 

 five inches in length, three inches and a half from side to 

 side in the broadest part, and two inches and a half from 

 the front to the back surface. 



