012 



HEART. 



Upon what does this irritability of the heart 

 depend ? This has been one of the most keenly 

 agitated questions in physiology, as a great part 

 of the experiments, and much of the reasoning 

 upon the nature of muscular irritability, have 

 been furnished by this organ. As, however, 

 the general doctrines entertained on this subject 

 have already been full} discussed under the 

 article CONTRACTILITY, we shall here confine 

 ourselves to a few of the leading facts connected 

 with it which have a special reference to the 

 heart. The two principal questions on this 

 point since the time of Haller have been, whe- 

 ther does it depend upon nervous influence ? 

 or is it a property of the muscular fibre itself 

 independent of the nerves ? 



We have seen that the nerves distributed 

 upon the heart are the par vagum and sympa- 

 thetic. Numerous experimenters have removed 

 portions of the par vagum on both sides of the 

 neck without the slightest diminution of the 

 strength of the contractions of the heart. These 

 experiments we have frequently performed with 

 the same results. There can now be no doubt 

 that the sudden death which occasionally fol- 

 lows this operation is not to be attributed to the 

 cessation of the heart's action, as some of the 

 older experimenters believed, but, as Legallois 

 has shewn, it depends upon an arrestment of 

 the movements of the muscles attached to the 

 arytenoid cartilages. Portions of the sympa- 

 thetic have also been destroyed in the middle 

 of the neck without any effect upon the con- 

 traction of the heart, except what could be 

 sufficiently accounted for by the pain of the 

 incisions and the terror of the animal. A por- 

 tion of both of the sympathetic and pneumo- 

 gastric nerves may be removed in the neck 

 with the same results; in fact we cannot, in 

 the dog and most quadrupeds, cut the par 

 vagum in the middle of the neck without also 

 dividing the sympathetic. Magendie affirms 

 that all the sympathetic ganglia of the neck, 

 along with the first dorsal, may be removed 

 without any sensible derangement of the parts 

 to which their nerves are distributed. Brachet* 

 supposes that the reason why the excision of the 

 sympathetic ganglia in the neck does not always 

 arrest the heart's action, is because there is 

 another source of nervous influence for the 

 cardiac nerves placed below this in the cardiac 

 plexus or ganglion. He accordingly put this 

 opinion to the test of experiment, and he as- 

 sures us that the total destruction of the cardiac 

 plexus was followed by the sudden and perma- 

 nent arrestment of the heart's action. Now 



may be readily increased or renewed under those 

 circumstances, by mild excitants applied to its inner 

 surface, are completely opposed to the supposition 

 that the heart is called into contraction in a manner 

 similar to those sympathetic movements more lately 

 described under the term excito-motary. Though 

 this mode of explanation may be considered quite 

 legitimate when applied to those sympathetic move- 

 ments which do not require the intervention of the 

 brain for their performance, such as deglutition, 

 respiration, &c., it is certainly pushing the doctrine 

 far beyond its proper limits to apply it to the 

 explanation of the movements of the heart. 

 * Du systeme nerveux ganglionairc, p. 120. 



when we consider the nature of such an ex- 

 periment as this, with the chest of the animal 

 laid open, the respiration arrested, and the 

 heart exposed during the time the experimenter 

 is searching and tearing for the plexus placed 

 deep behind the aorta and pulmonary artery, 

 and which would require a considerable time 

 to display even in the dead body when unem- 

 barrassed by the movements of the heart, we 

 must be more astonished that the action of the 

 heart had not completely ceased before the ex- 

 periment was finished, than that it should have 

 continued so long. Besides, even allowing that 

 this experiment could be relied upon, we have 

 sufficient evidence, from the facts stated above, 

 to entitle us to conclude that the heart is not 

 dependent for its movement upon any influence 

 constantly transmitted along its nerves from the 

 central organs of the nervous system, the 

 brain and spinal marrow. Brachet is himself 

 obliged to admit, from other experiments which 

 he performed, that the division of the sympa- 

 thetic at the lower part of the neck is not suffi- 

 cient to arrest the heart's action, so that this 

 experiment is intended to shew that its irrita- 

 bility depends upon the ganglia of the sympa- 

 thetic itself. The independence of the irrita- 

 bility of the heart upon the brain and spinal 

 marrow can be very satisfactorily proved in 

 another manner. The occurrence of acephalous 

 monsters,* and the experiments of Wilson 

 Philip,f Clift,| and Brachet demonstrate that 

 the brain or spinal marrow may be naturally 

 wanting ; that one or both of them may be 

 removed entirely, or destroyed in small portions 

 at a time, without arresting the heart's action. 

 We may here observe that the experiments of 

 Legallois,|| Wilson Philip, Wedemeyer,^[ Bra- 

 chet, and many others, in which the action of 

 the heart was arrested by crushing large portions 

 of the brain or spinal marrow, though they do 

 not prove the dependence of the irritability of 

 the heart upon the brain and spinal cord, at 

 least shew, what the effects of mental emotions 

 upon the movements of the heart had already 

 pointed out, that it can be influenced to a great 

 and most important extent through these organs. 

 The advocates for the dependence of the irrita- 

 bility of the heart upon the nerves appear to 

 have pretty generally abandoned the opinion 

 that this is derived from the central organs of 

 the nervous system, and now maintain the 

 doctrine, which was more prominently deve- 

 loped by Bichat, that this is derived from the 

 sympathetic, the ganglia of which, according 

 to him, are independent sources of nervous 

 influence. From the manner in which the 

 sympathetic is distributed upon the heart, it is 



* The heart is generally though not always ab- 

 sent in acephalous monsters. 



f Experimental Inquiry into the vital functions. 



J Phil. Trans. 1815. 



$ Systeme nerveux ganglionaire. 



f| Legallois performed these experiments on the 

 spinal cord alone, and supposed he had proved 

 that the movements of the heart were dependent 

 upon that portion of the nervous system. 



TJ Physiol. Untersuchungen Uber das Nerven- 

 system, c. p. 235. 



