THE HEART MUSCLE 



137 



vent its return. As soon as the blood is forced out, the 

 muscle relaxes and allows another supply of blood to 

 flow in and fill the chambers. This contraction and 

 relaxation are repeated one or more times a second 

 throughout life and cause the heart beat, which can be 

 readily felt on the left side of the front of the chest. 



Muscle cells. The heart of a human being when con- 

 tracted, is about the size of his clenched fist; when dis- 

 tended with blood, it is nearly twice as large. When it 

 contracts, the pressure which it ordinarily exerts is suffi- 

 cient to push the blood to a height of about eight feet. 

 At times of severe effort, as in heavy lifting, it may even 

 be much greater. The heart has therefore to be built 

 in such a way that it is 

 strong enough to generate 

 this amount of power, and 

 firm enough to prevent 

 any of the blood on which 

 it contracts from tearing 

 a way through its walls. 

 For these reasons, its walls 

 are built of short power- 

 ful muscle cells, which are 

 striated like those of the 

 voluntary muscles, but dif- 

 fer markedly from them in 

 that they have squared 

 ends, each of which is ce- 

 mented fast to the ends of the adjoining cells, and lateral 

 branches similarly cemented to other cells. This method 

 of building affords a wall with an interlacing mus- 

 cular meshwork which cannot be split apart by the 

 blood in its attempt to escape as the heart contracts 

 upon it. 



Connecting 

 branch 



Cement 



substance 

 Nucleus 



FIG. 79. Muscle cells of heart. 



