INFLUENCE OF THE NERVOUS SYSTEM ON THE HEART. 59 



stances in the human subject. Its physiological movements are capable of being influ- 

 enced in a remarkable degree through the nervous system, notwithstanding this insensi- 

 bility and in spite of the fact that the muscular fibres composing it are capable of con- 

 traction when removed from all connection with the body and that the regular pulsa- 

 tions can be kept up for a long time by the mere passage of blood through its cavities. 

 The influence thus exerted is so great, that some eminent authorities have held the opin- 

 ion that the cause of the irritability of the organ was derived from the nerves. One of the 

 most distinguished advocates of this opinion was Legallois. This observer arrested the 

 action of the heart of the rabbit by suddenly destroying the spinal cord, from which he 

 drew the conclusion that the heart derived its contractile power from the cerebro-spinal 

 system. The experiments which we have already cited, showing the continuance of the 

 heart's action after excision, disprove this so completely, that it was not thought worth 

 while to discuss this view while treating of the cause of its rhythmical contractions. 

 The same may be said with regard to the experiments of Brachet, in which he endeav- 

 ored to prove that the contractility of the heart is derived from the cardiac plexus of the 

 sympathetic system of nerves. The fact that the heart does not depend for its contrac- 

 tility upon external nervous influence may be regarded as long since definitely settled ; 

 but within a few years the discovery in its substance of ganglia belonging to the sympa- 

 thetic system has revived, to some extent, the view that its irritability is derived from 

 nerves. It is not necessary to follow out all the experiments which combine to demonstrate 

 the incorrectness of this view. Bernard, by a series of admirably-conceived experi- 

 ments upon the effects of the woorara poison, has succeeded in demonstrating the dis- 

 tinction between muscular and nervous irritability. In an animal killed with this re- 

 markable poison, the functions of the motor nerves are entirely abolished, so that gal- 

 vanization or other irritation does not produce the slightest effect ; yet the muscles re- 

 tain their irritability, and, if artificial respiration be kept up, the circulation will con- 

 tinue for a long time. The heart, by this means, seems to be isolated from the nervous 

 system as completely as if it were excised ; and galvanization of the pneumogastric 

 nerves in the neck, which, in a living animal, will immediately arrest its action, has no 

 effect. On the other hand, poisoning by the sulphocyanide of potassium destroys the 

 muscular irritability and leaves the nerves intact. By these experiments, which we 

 have frequently repeated, we can completely separate the nervous from the muscular 

 irritability and show their entire independence of each other ; and there is every rea- 

 son to suppose that the heart, like the other muscles, does not derive its contractility 

 from any other system. It is evident, however, that the heart is often powerfully influ- 

 enced through the nerves. Sudden and violent emotions will occasionally arrest its ac- 

 tion and have been known to produce death. Palpitations are to be accounted for in 

 the same way. Some of the modifications which we have already considered, depending 

 on exercise, digestion, etc., are effected through the nerves; and it is through this system 

 that the heart and all the important organs of the body are made to a certain extent 

 mutually dependent. It becomes interesting and highly important, then, to study their 

 influences and follow out, as clearly as possible, the action of the nerves which are dis- 

 tributed to the heart. 



The anatomical connections of the heart with the nervous centres are mainly through 

 the sympathetic and the pneumogastric nerves. We can study the influence of these 

 nerves to most advantage in two-ways; first, by dividing them and watching the effect 

 of depriving the heart of their influence, and second, by exciting them by means of a 

 feeble current of galvanism. It is well known that in an animal just killed the " nervous 

 force " may be closely imitated by galvanism, which is better than any other means of 

 stimulation, as it does not affect the integrity of the nerves and the amount of the irrita- 

 tion may be easily regulated. 1 



1 We shall not discuss the effects upon the heart of sudden destruction of the great nervous centres. It has been 

 shown that the heart becomes arrested when the brain is crushed, as by a blow with a hammer, when the medulla 



