CHAPTER VIII. 

 THE HEART AND CIRCULATION. 



The Heart. Shielded within the chest are, as has been 

 said, the heart and lungs. The heart lies on the left side 

 behind the sternum and the cartilages of the fourth to 

 seventh ribs in a closed, conical, membranous sac, the 



FIG. 39. The heart. (Stoney.) 



pericardium, which is attached by its base to the central 

 tendon of the diaphragm, and whose point extends up 

 between the pleurae of the lungs. This sac has an exter- 

 nal fibrous layer and an internal serous layer that is re- 

 flected back over the heart itself, forming a closed sac, 



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