40 PHYSIOLOGY. 



eighty. At birth the rate is from one hundred and thirty 

 to one hundred and forty, and gradually decreases till 

 about the age of twenty, when the average of seventy-two 

 is reached. This rate holds till old age, when it increases. 

 The rate is increased by muscular activity, food, external 

 heat, internal heat (fever), pain, and mental excitement. 

 Music accelerates the pulse rate. The pulse rate varies 

 during the twenty-four hours, being lowest during the 

 night, and highest about r I A.M. Certain diseases increase 

 the frequency of the pulse. Some drugs quicken the pulse 

 rate, and others diminish it. 



Fig. 14 A. The Heart, from the front. 



The Position of the Heart. The base of the heart is 

 in the center of the chest, just back of the breast bone, 

 but the apex points downward and to the left. 



The Covering of the Heart. The heart is inclosed in 

 a loosely fitting membranous bag, the pericardium. Within 

 the pericardium and around the heart is a small quantity 

 of liquid, called the pericardial fluid. 



