56 CIRCULATION OF THE BLOOD. 



call it an explanation to say that it is dependent on vital properties. But if we are yet 

 ignorant of the actual cause of the rhythmical contraction of the heart, we are pretty 

 well acquainted with the influences which render its action regular, powerful, and 

 sufficient for the purposes of the economy. It will facilitate our comprehension of this, 

 to compare the action of the heart 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 respira- 

 tion or acceleration by violent exercise, etc. In this respect the heart differs from cer- 

 tain muscles, like the muscles of respiration, which act involuntarily, it is true, but the 

 action of which may be temporarily arrested or accelerated by a direct voluntary effort. 

 The last-mentioned fact gives us the difference between the heart and all other striated 

 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 without some kind of irritation. Connec- 

 tion 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 or mechanical or chemical irritation, it is thrown into contraction ; but, if 

 carefully protected from irritation, it will remain quiescent. Contraction in this instance 

 is evidently produced 'by the application 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 inherent in the muscle and continuing so long as it retains its absolute physical 

 and chemical integrity. More than a hundred years ago, Haller described certain tissues 

 of the body as possessing this " irritability," such as the muscles, stomach, bladder, etc., 

 and the different degrees of irritability with which each one was endowed. He ap- 

 plied this theory to the action of the heart, which he considered as the part endowed 

 with irritability to the highest degree. His theory of the action of the heart was that its 

 rhythmical contraction depended upon the irritability inherent in its muscular fibres. He 

 was far from denying the various influences which modified this action, but regarded its 

 actual power of contraction as independent. 



Experiments have shown that the heart will pulsate for a time when removed from 

 all connection with other parts of the organism. 1 In the cold-blooded animals, in which 

 the irritability of the tissues remains for some time after death, this is particularly marked. 

 It is not the blood in the cavities of the heart which causes it to contract, for it will 

 pulsate when its cavities have been emptied. It is not the contact of the air, for the 

 heart will pulsate in a vacuum. The heart does not receive its irritability from the 

 nervous system, for, when removed from the body, it has no connection with the nervous 

 system ; and it is not probable that it receives any influence from sympathetic ganglia 

 which have lately been discovered in its substance, for detached portions of the heart 

 will pulsate, and the contractions of the organ will continue in animals poisoned with 

 woorara, which is known to paralyze the motor system of nerves. 2 



1 Numerous instances of contractions of the heart in cold-blooded animals continuing for a very long time after 

 excision are on record. Dr. Dunglison, in his work on Physiology, mentions several instances in which the heart pul- 

 sated for from ten to twenty-four hours after removal from the body. The most remarkable examples of this pro- 

 longed action were in the heart of the sturgeon. In one instance, in an experiment on a large alligator, we found the 

 heart pulsating, in situ, twenty-eight hours after the animal had been killed by the injection of a solution of woorara. 

 The heart was then excised and continued to beat during a long series of experiments, until it was arrested by 

 powerful compression with the hand after it had been filled with water and the vessels tied. 



a It is stated by Friedlander that no portion of the heart, however small, will contract rhythmically unless it con- 



