Constancy ni the heart's action.—The con stancy of the heart's action is more apparent than real. After each contraction a state of relaxation follows. The relative duration of the contraction of the auricles and ventricles, according to Laennec, appears to be as fol lows :—a third at most, or a fourth or a little less by the systole of the auricles ; a fourth or a little less by the state of quiescence ; and the half or nearly so by the systole of the ventricles. From this he calculates that the ventricles, when the heart is acting with its usual frequency, rest twelve hours out of the twenty-four, and that in those individuals in whom the pulse is naturally below 50, it must be in a state of relaxation sixteen hours out of the twenty-four.* Now this is a degree of contraction of which many muscles of the body are probably suscep tible, such as the muscles which support the trunk when we sit or walk, and which some, as the diaphragm and intercostals, generally perform.
Regularity of the heart's movements.—The regularity of the heart's movements, so essential to the welfare of the animal, has appeared, even to many modern physiologists, to be inti mately connected with some peculiarity in its structure. We are inclined, however, to agree with Haller, that this is perfectly explicable on the known laws of muscular contractility in other parts of the body. The regularity of the heart's action was another fertile subject of hypothesis to the older physiologists ; and even in the present day we hnd the term "organic instinct" employed to designate it.
The contractions of the heart take place in the order in which the blood flows into its different cavities; and if the blood be the habi tual stimulant upon which its movements depend, this is exactly what we would expect-S. The blood forced in greater quantity into the auricles by the contraction of the termination of the cavm and pulmonary veins, stimulates the auricles to contract and propel an additional quantity into the ventricles ; and this, acting as a stimulant upon the ventricles, excites them to contract and drive the blood into the arteries, when the same series of phenomena is renewed and repeated in the same succession.
The continuance of the heart's action after the circulation has ceased, we have already attempted to explain ; and if these contractions depend upon the presence of a stimulus, they must evidently be in the same order as hi the natural state of the organ, as these have not been interrupted. The continuance of the re gular order of the contractions of the heart after its removal from the body can in general, we think, be satisfactorily accounted for by the substitution of a new stimulant for that of the blood ; the cavities are then occu pied with air instead of blood, and each contraction of the auricle must force an ad ditional quantity into the ventricle, and this, though small in quantity, may be quite suffi cient to excite the ventricles to contraction, when the irritability is not too much impaired.*
It is only in this manner, taken along with the greater irritability of the internal surface over the external, that we can explain the observa tion made by Dr. Knox in the course of his experiments upon the irritability of the heart in fishes, where, when the irritability was nearly exhausted, contractions excited in the auricle were sometimes followed by contractions of the ventricle, when irritation of the outer surface of the ventricle itself produced no effect.-1- Cer tainly, under ordinary circumstances, this regu larity of the heart, so necessary for the proper performance of its functions, is a marked fea ture in its action ; but that it is not either ne cessarily connected with its structure or vital properties, but depends solely on the manner in which its stimulant, the blood, is applied, is proved by various facts. 1st. The movements of the auricles and ventricles generally cease at different times after death ; and though the auricles much more frequently continue to con tract after the ventricles, yet several accurate experimenters have observed the left auricle become quiescent before its corresponding ventriclel 2dly. When the movements of the ventricle have ceased, while the auricles continue to contract, the ventricle may generally be excited to vigorous contractions by the ap plication of a powerful stimulus. 3dly. When the irritability of the heart becomes somewhat languid, two, three, or sometimes six or seven contractions of the auricle may take place be fore the ventricles are roused to contraction ; the evident deduction from which is, that the contractions of the ventricles do not neces sarily follow those of the auricles, unless the contractions of the auricles occasion the application of a stimulant to the inner sur face of the ventricles sufficient to excite them to contraction. 4thly. The movements of the ventricles and auricles will go on in the same manner, though detached from each other by the knife. 5thly. I f we were allowed to argue from final causes in negative cases, we could easily skew that a peculiar endow ment, such as we are contending against, would not be of the slightest advantage in se curing the regularity and constancy of the heart's movements. It appears, then, quite un philosophical to call in the agency of some un known and indefinite principle for the produc tion of these periodic movements, as they have been called, of the different chambers of the heart, when they can be satisfactorily referred to the laws which regulate muscular contracti lity in other parts of the body. We have here a beautiful example of the manner in which nature produces adaptation of means to an end, not by the creation of new properties, which we, in our ignorance, sometimes erroneously attribute to her, but by the employment of those already im1 use in the performance of other functions, only modified to accommodate them to the circumstances under which they are placed.