338 



A YEAR IN SCIENCE 



in a large vessel than there is in a small one, conse- 

 quently more blood enters it. 



The heart structure. The heart is a conical shaped 

 organ about the size of a man's fist. It lies in the chest 

 cavity just above the diaphragm and directly back of 

 the lower two-thirds of the breast bone. The apex is 

 below and slightly to the left of the breast bone, where 



its beat can easily 

 be felt between the 

 ribs. The heart is 

 enclosed in a sac 

 called the pericar- 

 dium. Between this 

 and the heart there is 

 a fluid which lessens 

 the friction caused by 

 the beating of the 

 heart. 



Since the heart is 

 a pump, we naturally 

 expect that it is made 

 of muscle. Inter- 

 nally the heart is 



Fig. 158. External view of the heart, divided into f O U r 



cavities; the upper two called the auricles, the lower 

 two the ventricles. The ventricles are larger than 

 the auricles and have much thicker walls. There 

 are no openings between the left and the right sides 

 of the heart, but there is an opening between the left 



