72 ACTION OF THE HEART. 



HOW THE BLOOD IS PUMPED. 



The Action of the Prog's Heart. Before reading the 

 description of the action of the heart, it will pay to see the 

 action of the frog's heart. 



Kill a frog as directed on page 13, and destroy its brain and 

 spinal cord. Carefully open the body cavity, and lay bare the 

 heart without injuring it. Its beats will be seen to be com- 

 posed of three parts : 



1. The contraction of the auricle. 



2. The contraction of the ventricle. 



3. These actions are followed by a pause, after which No. 

 1 and No. 2 are repeated. 



The Rate of the Heart-Beat. The heart beats about 

 seventy-two times a minute in men. In women, about eighty. 

 At birth the rate is from one hundred and thirty to one hun- 

 dred 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 in- 

 creased by muscular activity, food, external heat, internal 

 heat (fever), pain, mental excitement. Music accelerates the 

 pulse-rate. The pulse-rate varies during the twenty-four 

 hours, being lowest during the night, and highest about 

 eleven A.M. Certain diseases increase the frequency of the 

 pulse. Some drugs quicken the pulse-rate, and others di- 

 minish ^ it. 



The Action of the Heart. The heart consists of muscle 

 fibers so arranged that they form a thick-walled bag, which 

 stands expanded when the muscles relax. But when the 

 fibers shorten, the whole heart contracts, and the cavity is 

 much reduced in size, if not entirely obliterated, and the blood 

 is forced out. 



The complete action of the heart consists of three parts, 



