224: CIRCULATION. 



tact of the air, for the heart will pulsate in a vacuum. 1 The 

 heart does not receive its irritability from the nervous sys- 

 tem, for, when removed from the body, it has no connection 

 with the nervous system ; and it is not probable that it re- 

 ceives 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. 



It is unnecessary to refer to the various experiments 

 which have demonstrated the independence of the contrac- 

 tions of the heart. They are of such a simple nature that they 

 may be verified by any one who will take the trouble to ex- 

 cise the heart of a frog or turtle, place it under a small bell- 

 glass so that it will not be subject to possible irritation from 

 currents of air, and watch its pulsations. In such an observa- 

 tion as this, it is evident that for a certain time contractions, 

 more or less regular, will take place ; and the experiments 

 referred to above show that they take place without any ex- 

 ternal influence. In short, it is evident that the muscular 

 fibres of the heart possess an irritability, by virtue of which 

 they will contract intermittently for a time, though no stim- 

 ulus ~be applied; as ordinary striped muscular fibres have an 

 irritability, by virtue of which they will respond, for a time, 

 to the application of a stimulus. 



It is manifestly necessary that the action of the heart 

 should be constant, regular, and powerful ; and when we say 

 that the irritability inherent in its muscular tissue is such 

 that it will contract for a time without any external stimulus, 

 we by no means assume that this is the cause of its physiolog- 

 ical action. It is only an important and incontestable prop- 

 erty of the muscular fibres of the heart, and its regular action 

 is dependent upon other circumstances. 



In the first place, we have to inquire what makes the ac- 

 tion of the heart constant. The answer to this is, that the 



1 JOHN REID, in Cyclopcedia of Anatomy and Physiology, vol. ii., p. 611. 



