222 CIRCULATION. 



will facilitate our comprehension of this, to compare this 

 action with that of the ordinary voluntary muscles. 



In the first place, every one knows that the action of the 

 heart is involuntary. We can neither arrest, retard, nor 

 accelerate its pulsations by a direct effort of the will. In this 

 statement we of course except those examples of arrest by 

 the stoppage of respiration, or acceleration by violent exer- 

 cise, etc. In this respect the heart differs from certain mus- 

 cles, like the muscles of respiration, which act involuntarily, 

 it is true, but whose action may be temporarily arrested or 

 accelerated by a direct voluntary effort. 



The last-mentioned fact gives us the precise difference 

 between the heart and all other striped muscles. All of 

 them, in order to contract, must receive a stimulus, either 

 natural or artificial. The natural stimulus comes from the 

 nervous centres, and is conducted by the nerves. If the nerves 

 going to any of the respiratory muscles, for example, be 

 divided, the muscle is paralyzed, and will not contract with- 

 out some kind of irritation. Connection with the nervous 

 system does not seem necessary to the action of the heart, for 

 it will contract, especially in the cold-blooded animals, some 

 time after its removal from the body. 



When a muscle has been removed from the body, and is 

 subjected to a stimulus, such as galvanism, mechanical or 

 chemical irritation, it is thrown into contraction ; but if care- 

 fully protected from irritation, will remain quiescent. Con- 

 traction in this instance is evidently produced by the appli- 

 cation of the stimulus ; but the question arises, Why does the 

 muscle thus respond to stimulation ? This is a question which 

 it is impossible to answer satisfactorily, but one concerning 

 which our ideas, since the time of Haller, have assumed a 

 definite form. This great physiologist called the property 

 which causes the muscle thus to contract, irritability / which 

 is nothing more nor less than an unexplained property in- 

 herent in the muscle, and continuing as long as it retains its 

 absolute physical and chemical integrity. More than a hun- 



