60 CIRCULATION OF THE BLOOD. 



Experiments on the influence of the sympathetic nerves upon the heart are not quite 

 so satisfactory as we might desire. It has been asserted that the action of the heart is 

 immediately arrested by destroying the cardiac plexus. With regard to this, we must 

 take into account the difficulty of making the operation and the disturbance of the heart 

 consequent upon the necessary manipulations. It has been shown pretty conclusively, 

 however, that stimulation of the sympathetic in the neck has the effect of accelerating the 

 pulsations of the heart. The extreme difficulty of dividing all the branches of the sympa- 

 thetic going to the organ leaves a doubt as to whether such an operation would definitely 

 abridge its action. 



We have next to consider the influence of the pneumogastrics upon the heart. Ex- 

 periments on these nerves are made with greater facility than on the nerves of the sym- 

 pathetic system, and the results are much more satisfactory. Like all the cerebro-spinal 

 nerves, the influence generated in the nervous centre from which they take their origin is 

 conducted along the nerve and manifested at its distribution. When they are divided, 

 we may be sure that, as far as they are concerned, all the organs which they supply are 

 cut off from nervous influence ; and, when galvanized in their course, we imitate or ex- 

 aggerate the influence sent from the nervous centre. 



The invariable effect on the heart of division of the pneumogastric nerves in the neck 

 is an increase in the frequency and a diminution in the force of its pulsations. One or two 

 writers have denied this fact, but it is confirmed by the testimony of nearly all experi- 

 menters. To anticipate a little in the history of the pneumogastric nerves, it may be 

 stated that, while they are exclusively sensitive at their origin, they receive, after having 

 emerged from the cranial cavity, a number of filaments from various motor nerves. 

 That they influence certain muscles, is shown by the paralysis of these muscles after divi- 

 sion of the nerves in the neck, as, for example, the arrest of the movements of the glottis. 

 Having this double property of motion and sensation, and being distributed in part to 

 an organ composed almost exclusively of muscular fibres, which, as we have seen, is not 

 endowed with general sensibility, we should expect that their section would arrest, or at 

 least diminish, the frequency of the heart's action. What explanation, then, can we offer 

 for the fact that this seems actually to excite the movements of the heart? We shall be 

 better prepared to answer this question after we have studied the effects of galvanization 

 of the nerves in a living animal or in one in which the action of the heart is kept up by 

 artificial respiration. 



Numerous experiments have been made with reference to the effects on the heart of 

 galvanic currents, both feeble and powerful, passed through the pneumogastrics before 

 division, of currents passed through the upper and lower extremities after division, etc., 

 a full detail of which belongs properly to the physiological history of the nervous system. 

 In this connection, a few of these facts only need be stated. 



It has been shown by repeated experiments, which we have frequently confirmed, that 

 a moderately-powerful interrupted galvanic current passed through both pneumogastrics 

 will arrest the action of the heart, and that the organ remains quiescent so long as the 

 current is continued. This experiment has been performed upon living animals, both 

 with and without exposure of the heart. The arrest is not due to violent and continued 

 contraction of the muscular fibres ; on the contrary, the heart is relaxed, its ventricles 

 are flaccid, and its fibres are for the time paralyzed. The question then arises whether 

 this action be exerted directly on the heart through the nerves, or whether an influence 

 be conveyed to the nervous centre and transmitted to the heart in another way. This 

 is settled by the experiment of dividing the nerves and galvanizing alternately the ex- 

 tremities connected with the heart and those connected with the nervous centres. It has 



oblongata or the spinal cord is suddenly destroyed, and even the crushing of a foot, in the frog, has been known to 

 product this effect. In fine, this may be done by any extensive injury to the nervous system ; but this fact does net 

 teach us much with regard to the physiological influences of the nerves. For example, while crushing of the brain 

 arrests the heart, the brain may be removed from a living animal and the heart will beat for days. Experiments 

 upon the influence of the medulla oblongata and spinal cord are by no means satisfactory. 



