THE HEART 



257 



contracts unless in response to the arrival of impulses by 

 way of the motor nerves. Smooth muscle shows an 

 automatic tendency and this rhythmic property is most 

 highly developed in the heart. A summary demonstra- 

 tion is obtained when the heart of a frog is removed from 

 the body of the animal. The organ continues to beat. 

 This makes it plain that it does not depend on the central 

 nervous system for the initiation of each beat and we must 

 remember that there is just this difference between the 

 heart action and the breathing: 

 the heart beats of itself while the 

 breathing muscles are thrown 

 into contraction by the brain. 



The heart of a mammal taken 

 from the body declines rapidly, 

 though its automatic nature is 

 evidenced by the execution of at 

 least a few contractions. Ex- 

 periment has shown that the 

 reason the heart of a frog beats 

 longer than that of a cat after 

 its isolation is chiefly that the 

 latter has a greater need of oxygen. If this want is 

 supplied by an artificial circulation of blood, or other- 

 wise, the cat's heart will survive for a long time. It is as 

 truly automatic as the cold-blooded heart. 



When the different regions of the heart are compared 

 it is found that the degree of automatic power is not 

 everywhere the same. The rhythmic tendency is more 

 marked in the auricles than in the ventricles. The 

 student must not be confused by the fact that the part 

 of the heart which is physically weaker has a stronger 

 inclination to activity. The ventricles are far more 

 massive than the auricles and do a heavier work but their 

 rhythm is believed to be dictated by these less robust 

 divisions of the heart. It will be desirable to cite certain 

 experiments bearing on this matter. 



17 



FIG. 64. Cardiac muscle 

 cells (or syncytium). 



