312 THE HUMAN BODY 



the apex of the ventricles is said to be wholly free from them. 

 Nerve-fibers, communicating with these cells, penetrate all parts 

 of the cardiac musculature. It has not been possible by histologic 

 means to show that these fibers are dendrites and axons such as 

 occur in the general nervous system, and many histologists and 

 physiologists believe that they form a continuous network or 

 plexus involving all parts of the heart and so constituted that a 

 stimulus applied at any point spreads over the whole organ. Ac- 

 cording to this view the nervous mechanism of the heart does not 

 show the irreversibility of conduction which is a cardinal feature 

 of the general nervous system. Some support for this idea is had 

 in the fact that certain other viscera, notably the stomach and in- 

 testines, have within their walls nerve plexuses showing similar 

 physiological properties. 



Physiological Peculiarities of the Heart. The most striking of 

 these is its automatic rhythmicity. The heart may be removed com- 

 pletely from the Body without its regular beating being at all in- 

 terfered with. In cold-blooded animals such as frogs or turtles 

 this activity outside the Body may continue for hours. While we 

 refer to this activity as automatic we do not mean by the word 

 anything more than the fact just stated, that the heart continues 

 to beat independently of the rest of the Body. The rhythmic na- 

 ture of the heart's activity is as characteristic as its automaticity. 

 The regular succession of contractions and relaxations is its normal 

 response to continuous or rapidly recurring stimulation. In this 

 respect it differs strikingly from skeletal muscle, which remains 

 strongly contracted throughout the period of such stimulation un- 

 less fatigue sets in to release it. 



Another peculiarity of heart-muscle, and one that probably ex- 

 plains in part its rhythmic property, is that its contractions are 

 always maximal. By this is meant that whenever heart-muscle 

 contracts it always does so to the full extent of its ability at the 

 time. In this respect we may compare its energy liberation with 

 the discharge of a gun. When the trigger is pulled all the powder 

 in the cartridge is exploded ; similarly whenever the heart contracts 

 it uses up all the energy available at the time. Because of this 

 it is necessary that the contraction be followed by a relaxation 

 during which an accumulation of energy may prepare for the next 

 contraction. 



