In examining the nature of the irritability of the heart, and contrasting it with that of the voluntary muscles, we must not compare its contractions with those excited by volition in the muscles of voluntary motion, for these last are evidently modified by the nervous influence for an obvious purpose; but let us observe both when placed under similar circumstances, and irritated by the application of the same stimu lant applied to the muscles themselves, and we will find that they only differ in this,—that in the voluntary muscles each successive appli cation of the stimulant is generally followed by a single contraction, while in the heart it is followed, except when the contractility is much impaired, by several consecutive con tractions alternated with relaxations. This ten dency to successive contractions is also observed, though not to the same extent, in the muscular coat of the intestines.
We must admit, however, that the contrac tions of the heart proceed under circumstances where it is difficult to point out the presence of any sufficient stimulus, and where, to account for their continuance, we are almost obliged to have recourse to the supposition, that there is some innate moving power in the heart itself. It has been stated, for example, that the move meats of the heart will proceed under the ex hausted receiver of an air-pump. I have repeatedly placed under the bell-glass of an air-pump the heart of a frog when removed from the body and emptied of its blood, and I could never satisfy myself that the frequency or strength of its contractions was at all affected by the withdrawal or renewal of the air ; and though it might be urged that the air is only rarefied, not entirely removed, in the best ex hausted receiver of an air-pump, and that con sequently in such experiments a stimulant still existed in the presence of the rarefied air, yet I would not consider this explanation of the continuance of its contractions by any means satisfactory. In these experiments there is ano ther source of stimulation present which ought to be taken into account, for, as I shall after wards show, the slightest movement of the heart, such as that caused by its contraction, upon the surEice upon which it is placed when removed from the body, is sufficient, from the great irritability of the organ, to act as a stimu lant upon it. If these external stimuli appear to be insufficient to account for the persistence of the contractions of the heart under the cir cumstances we have mentioned, we may have recourse to another explanation drawn from the mechanical structure of the organ; for it is possible, as has been suggested by Dr. Alison, that from the peculiarly convoluted arrangement of the fibres, the outer may, during the con traction of the organ, pinch or stimulate the inner, and so cause this tendency to repeated contractions from one application of a stimu lant. We do not, however, consider that we have succeeded perfectly in accounting for the continuance of the contractions of the heart under all circumstances, but we are unwilling to admit the existence of any peculiar innate and unknown agency in the production of any phenomenon, until it is satisfactorily established that it cannot be accounted for on the known laws which regulate similar phenomena in the same texture in other parts of the body. And it must also be remembered that these move ments of the heart have only been observed when its contractility was still comparatively vigorous, and where sources of stimulation were still present. We ought, besides, to be the more cautious in admitting the existence of this innate moving power, since it is in opposition to a well-known law in the animal eeonomy,—that though the various tissues of an organized body are endowed with certain vital properties, yet the application of certain external and internal stimuli is necessary to produce their manifestations of activity. In fact it is from the action and reaction of these tissues and excitants upon each other, that the phenomena of life result.* 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 fully 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 llaller 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 shown, 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 ns that the total destruction of the cardiac plexus was followed by the sudden and perma nent arrestment of the heart's action. Now 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. Bracket 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 skew 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 aeephalous monsters,* and the experiments of Wilson Philip,f Clift,t 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,H Wilson Philip, WedenieyerN 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 show, 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 perfectly impossible to insulate that organ from the nerve and experiment upon it ; but we think we are justified in concluding from ob servations and experiments derived from other sources, that in all probability the contractility of the heart depends upon a property possessed by the muscular fibre itself without any neces sary intervention of its nerves. The possibility of exciting or increasing the action of the heart by stimuli applied to its nerves has been mixed up with this question. Though it must be admitted that mechanical and chemical stimu lants applied to a considerable surface of the central organs of the nervous system quicken the heart's action, yet experimenters have gene rally acknowledged that these stimulants applied to the nerves of the heart produce no effect upon its movements. Burdach,* however, maintains that he has quickened the heart of a rabbit deprived of sensation by applying caustic potass to the trunk of the sympathetic, or its in ferior cervical ganglion. That the heart can be excited to contraction by the application of galvanism has had many supporters, and many celebrated names are arranged both on the atlirmative and negative sides of the question. That the movements of the heart may be in creased or renewed by the application of gal vanism as the experiment is usually performed, there can be no reasonable doubt ; for if one wire is placed upon the nerve and the other upon the heart, the moist nerve will act as a conductor to the electricity, and the effect pro duced will be the same as if the stimulant had been applied to the substance of the heart itself. Nysten admits that movements of the heart were excited by the galvanism when one of the wires was applied to one of the large arteries from which all the visible filaments of the nerves had been dissected off. Dr. C. Holland,f in a number of experiments, satisfied himself that the tissues of the body conduct galvanism with so much facility, that the heart's action could readily be excited, when one wire was placed upon the heart and the other in the nose, mouth, and even among the moist food in the stomach. I have performed similar experiments with the same results. Humboldt and Brachet assert that they have quickened the movements of the heart by applying both wires to one of the cardiac nerves. If these and the experi ments of Burdach could be relied upon, they would be sufficient to prove that the heart could be occasionally stimulated through the cardiac nerves, but the negative experiments on the other side are so numerous, and the sources of fallacy in judging in this manner of the relative quickness of the heart's action between one time and another so obvious, that we must be allowed to distrust them unless they should be confirmed by other accurate observers.