532 THE OUTGO OF ENERGY 



as the effect of a constant stimulus (see pag 

 533). 



The apical half of the normal ventricle con- 

 tains no nerve cells. Consequently its failure to 

 contract after its separation from the remainder 

 of the heart would indicate that the adult heart 

 muscle is incapable of spontaneous rhythmical 

 contraction. It has been shown, however, that the 

 " apex " of the mammalian heart will beat after 

 its complete removal from the remainder of the 

 heart, provided the circulation in the extirpated 

 piece is maintained by supplying it with blood. 



Rhythmic Contractility of Heart Muscle. Fur- 

 ther evidence of the rhythmic contractility of 

 the heart muscle is found in the bulbus arteriosus. 



Place very small pieces of the bulbus arteri- 

 osus in normal saline solution under the 

 microscope. 



They will contract rhythmically. 



Histological examination shows that nerve 

 cells seldom occur in the bulbus. It is scarcely 

 credible that they are present in each of the small 

 pieces seen contracting under the microscope. 



Constant Stimulus may cause Periodic Contrac- 

 tion. In a frog with ventricular apex isolated 

 by Bernstein's ligature, compress one or both 

 aortae, thus raising the pressure in the ventricle. 



