THE CIRCULATION. 



109 



larger than a man's fist, resides that sleepless force by 

 which, during the whole of life, the current of the blood 

 is kept in motion. 

 It is placed in the 

 middle and front 

 part of the chest, 

 inclining to the left 

 side. The heart- 

 beat may be felt and 

 heard between the 

 fifth and sixth ribs, 

 near the breast-bone. 

 Theshapeof Ihehenrl 

 is conical, with the 

 apex or point down- 

 ward and in front. 

 The base, which is 

 upward, is attached 

 so as to hold it se- 

 curely in its place, 

 while the apex is 

 freely moveable. In 



Order that loSS of FIG. 28. THE HEART AND LARGE VESSELS. 



power from friction S ht Vent ' icle ' % A^" 16 ' 

 may be obviated, the F ' Pulmonar y Artcr *- 



heart is enclosed between two layers of serous membrane, 

 which forms a kind of sac. This membrane is as smooth 

 as satin, and itself secretes a fluid in sufficient quantities 

 to keep it at all times well lubricated. The lining mem- 

 brane of the heart, likewise, is extremely delicate and 

 smooth. 



19. The Cavities of the Heart. The heart is hol- 

 low, and so partitioned as to contain four chambers or 

 cavities; two at the base, known as the auricles, from a 



19. Formation of the heart ? Right and left heart ? 



