In opposition to the view above mentioned Volkmann* maintains that the movements of the heart cannot depend upon the central nervous masses. It continues its pulsations after the brain and spinal cord have been removed. When, however, the rhythmical movements of a part depend upon a nervous centre, they cease immediately after the con nection between these parts and the nervous centre is broken. The rhythmical movements of the muscles of respiration depend upon a nervous centre, the medulla oblongata. So soon as this is destroyed they cease. In like manner the heart, were the medulla oblongata, or any other part of the central masses of the nervous system the centre upon which its movements depend, must also cease pulsating so soon as it is removed from the influence of these. According to the experiments of Bidder, however, already mentioned, frogs may live for six weeks after the spinal cord has been destroyed, the circulation, as seen in the web of the foot, going on as livelily as before, and presenting no difference when compared vvith that m the healthy animal. So also when the entire central masses of the nervous system are removed the heart still continues its pulsations until the second day. The movements exhibited by the heart, after the central masses of the nervous system have been destroyed, cannot, according to Volkinann, be explained as mere movements of irritation, due to the inherent irritability of the muscular fibres, acted on by the stimulus of the blood or of the atmospheric air. Mere irritability, acted on by the stimulus of the blood, or of the air, cannot explain why both auricles or both ventricles should contract at one and the same time; and just as little can we in this way explain the suc cessive contraction of auricles and ventricles. To explain the rhythmical order in which these contractions take place it is necessary to sup pose that they, like movements of a similar kind, such a S those of the respiratory muscles, are regulated by a nervous centre. The fact that the heart's movements continue after it has been removed from the body indicates, moreover, that the centre upon which its movements depend must be contained in the organ itself. It has been already mentioned that in different parts of the heart are found small ganglia. These are believed by Volk mann to be the centres on which its move ments depend. These, according to him, act as organs from which the impulse to contrac tion proceeds : they are also so connected with one another as to act in concert, the impulses proceeding in such directions as to give rise to the remlar succession in which the contractions of'' the different parts take place.
Tile effects produced upon the heart's action by stimuli applied to the central masses of the nervous system, and upon which the view that its movements depend upon these parts is chiefly founded, are explained by Volkmann as taking place by reflex action through the medium of the sympathetic gan glia. The fibres which pass from the spinal cord to the ganglia stand to the proper sym pathetic fibres arising in these in the same relation in which the ordinary sensory fibres stand to the motor fibres of the muscles of animal life.
A conclusive way of determining yvhether the movements of the heart, as well as the order in which these take place, depend, or not, upon the ganglia contained in its substance, would be to ascertain whether they still con tinue after the ganglia have been extirpated. These, however, are so small, and apparently so numerous, as to render such an experiment impossible. That the continuance of these
movements after the brain and spinal cord is destroyed, as well as when the heart is re moved from the body, cannot be attributed to mere irritability of the muscular fibres acted on by the stimulus of the blood or of the atmo spheric air, but must be connected with nervous influence, is rendered probable by several circumstances, but especially by the observation first made by Henry, and after wards by Milner t, that solution of opium or of other narcotic substances, when applied to the outer surface of the heart, does not produce any obvious alteration in its action, whereas when introduced into its cavities so as to be brought into contact with its inner surface, their almost immediate effect is to cause this to cease. Again, when stimulus is applied to one of the ventricles of a heart which has just ceased pulsating, the contraction thereby produced does not commence at the point irritated, as might be expected were the irritability of the muscular fibres alone con cerned, but in the auricles, and is followed by contraction of the ventricles. Sometimes, indeed, stimulus applied to the ventricles is followed by contraction of the auricles alone. Even when the stimulus is applied to the apex of the organ, the contraction still com mences in the auricles, and sometimes limits itself to these. The regular order in which its movements take place, so different from those produced in the ordinary muscles by direct application of external stimuli, would imply that the impulse by which they are produced must be conveyed in a certain de finite direction to the different muscular parts of which the heart is composed ; and this can only be supposed to be effected through the medium of its nerves. The mere arrangement of the muscular fibres of the heart seems in sufficient to account either for the general contraction of auricles and ventricles or for the order in which these succeed one another. If, in the case of the heart, the contraction of a single bundle of the muscular fibres may act as a stimulus to the neighbouring fibres, by which they also are excited to contraction, the same thing ought to take place in the muscles of animal life ; the bundles in these, though presenting a different arrangement from those in the heart, are, notwithstanding, in as close contact with one another as are the latter, and have equal facility for stimulating the neighbouring bundles to contraction. The dependence of the rhythmical movements of the heart upon a certain arrangement of its nerves, and moreover that there are certain portions of the same from which the stimuli to contraction proceed, is further indicated by the effects, as shown by Volkmann, which follow incisions made into the heart's sub stance. When a transverse incision is made through the heart, between its auricles and ventricles, the former have been found to continue their contractions much longer than the latter ; and if a longitudinal incision be made gradually proceeding from apex to base, the rhythm is preserved in both portions until the heart has been divided half way; vvhen the incision is continued further, how ever, the movements of either part become irregular. When the ventricle is divided transversely into two portions, that towards the apex either ceases its contractions immediately or con tinues the same only for a short time, whereas that which is still in connection with the auricles goes on contracting, as before.