cause of the motion of the heart.—The motion of the heart, and the constancy and regularity of its movements, are circumstances so remarkable that they could not fail early to excite a deep interest among medical philo sophers when they had once turned their attention to the explanation of vital phenomena. When we contemplate the heart commencing its movements at an early period of foetal existence, and never resting from its apparently unceasing toil until the latest moments of life, and when we remember the uniform and regu lar manner in which all its actions are accom plished—all conspiring for the proper per formance of the deeply important functions assigned to it, we are at first impressed with the idea that it is regulated by laws different from similar textures of the body, and altogether peculiar to itself. It must have been under the influence of similar impressions that the older medical philosophers approached this subject, and it is in this manner only that we can account for many of the strange specula tions on the heart's action which they have left recorded.
We find one sect attempting to explain it by a peculiar innate fire. Sylvius, the head of the chemical sect, had recourse for its explanation to an effervescence excited by the intermixture of the old and alkaline blood with the acid chyle and acid pancreatic lymph.* Descartes supposed that a constant succession ofexplosions occurred in the heart from steam generated there, which propelled the blood through the body. Stahl got at once out of the difficulty by affirming that the heart was more particu larly under the guidance of the anima or soul. But we cannot here dwell longer on these ob solete and to us in the present time almost incredible opinions, and the only use to which they are now applicable is to serve as beacons to keep us, in all our inquiries into the pheno mena of living bodies, within the strict path of facts and observation, and to forcibly impress upon us into what strange and fatal errors even the brightest intellects may fall, when they leave the inductive method of investigation, and wander into the alluring but dangerous regions of hypothesis. And the effects of these errors are only the more to be dreaded as they are often clothed in the most seductive in genuity. It ought also still more forcibly to inculcate upon us the important truth, which, though generally in our mouths, is not unfre quently forgotten in practice,—that as the material world and all which it contains have been placed by the Author of Nature under arbitrary and fixed laws, it is impossible to ex tend our knowledge of these by theorizing in the closet, and that this can only be effected by the patient interrogation of Nature herself.
It was not until the time of Scnac and hailer that accurate notions began to be enter tained on the nature of the heart's action.
The cause of the movements of the heart is distinctly referable to the same laws which regulate muscular contractility in other parts of the body, only modified to adapt it for the per formance of its appropriate functions. Like all the other muscles it is endowed with irrita bility, which enables it to contract upon the application of a stimulus. The ordinary and
natural stimulus of the heart is the blood, which is constantly flowing into its cavities. The greater irritability of the inner surface over the outer is evidently connected with the manner in which the stimulus is habitually applied. When the blood is forced on more rapidly towards the heart, as in exercise, its con tractions become proportionally more frequent ; and when the current moves on more slowly, as in a state of rest, its frequency becomes pro portionally diminished. If the contractions of the heart were not dependent upon the blood, and their number regulated by the quantity flowing into its cavities, very serous and in evitably fatal disturbances in the circulation would soon take place.
As the heart continues to contract often for a very considerable time after the vente cam have been tied, and after the blood has ceased to pass through its cavities, or after it has been removed from the body, this has been supposed by some to indicate that there is something in the heart's structure or in its vital properties which enables its movements to proceed inde pendent of all other circumstances. But in all these cases a stimulus has been applied in some form or other to the heart. If the heart has been allowed to remain in its place, though the circulation of the blood may have come to a stand, part of it may yet remain in the different cavities of the organ ; or if the pericardium has been opened, the impression of the external atmosphere may act as a stimulus. The expe riments of Walther and Haller formerly men tioned upon the comparative irritability of the two sides of the heart, and the different results obtained when the one side of the heart was emptied of blood, and when it was retained in the other, arc sufficient to show the effect which the presence of blood in the cavities of this organ has upon the continuance of its action after the circulation has ceased. If the heart has been removed from the body and emptied of its blood, it must naturally follow that its different cavities will be filled with atmospheric air ; and it has been well ascertained that this acts as a very powerful stimulant upon the inner surface of the heart.* Every circumstance connected with these experiments is in exact conformity with the opinion that the movements of the heart are only called into action by the application of a stimulant. Thus, when the irritability of the heart becomes more languid, and when the blood or the atmospheric air in its cavities becomes insufficient to raise it to contraction, strong and energetic movements may still generally be excited by having recourse to a more powerful stimulant, such as the prick of a scalpel or the application of galvanism. Since the heart is highly en dowed with irritability, various other mild fluids besides the blood are capable of exciting it to contraction. As every organ, however, has its irritability adapted for the function which it is destined to perform, so we find that the heart, the central organ of the circulation, is most fitly called into action by the blood, its appro priate and natural stimulant.