(b.) Of the organic force, not of a nervous cha

racter, as an agent in the action of the heart.

The immediate influence of the blood in this case is not readily illustrated; but it is not difficult to perceive that the present theories upon this subject are very unsatisfactory. That the blood does not stimulate contraction may be argued from the histories of the common blood vessels and visceral cavities, which have been given in a former page, between which histories and that of the heart there is an intimate connexion,for whatever the differences in form and function when fully developed, they all spring from the same origin. To suppose that the blood excites contraction in the heart, moreover, is to create a difficulty which has already been insisted upon; for, if this were the case, how is it that the blood finds entrance into the organ?

There need, however, be no doubt upon this question if we institute once more a comparison between the opposite conditions of plethora and anæmia. In anæmia, for example, (especially in the variety which is associated with the consumptive habit of body,) we find rapidity and smallness of the pulse, together with other signs of emptiness in the vascular system. There is in fact the same evidence of contraction in the ventricular cavities as was found in the state immediately antecedent to syncope, for as we argued in this case, a proportionate flood of blood must be sent into the

In ple

vessels if these cavities were fully dilated. thora, on the contrary, the pulse is generally full and slow; and hence we should argue, in the first place, that the diastole is perfect, and a large quantity of blood taken into the heart at this time; and secondly, that the presence of the blood does not immediately and of necessity give rise to a systole. Comparing, indeed, the state of plethora with that of anæmia, the contrary would seem to be the fact, and the rapid systole in the one case would seem to be ascribable to the small quantity of blood which is taken into the heart, together with its impoverished and watery quality; and the deferred systole, in the other case, to the presence of a larger quantity of richer and more genial blood. The effects, indeed, are analogous to those witnessed in the vessels under similar circumstances, and it must be admitted that the heart has a greater capacity of dilatation in plethora than in anæmia. And so clear in other respects is the significance of the same facts, that it seems as reasonable to suppose that the blood excites the vessels of erectile tissue to contraction, as that it has this influence upon the heart, which after all, is but one of many forms of blood vessel.

It would appear, also, on further examination that the influence of the blood is an agent of extreme importance in determining the rhythmical action of the heart. At the ventricular systole the blood is distributed to the vessels, and among the rest to the

coronary arteries: and this abundant supply of the vivifying stream cannot be without its influence upon the walls of the heart. We cannot suppose it to excite the systole, however, for the rush is absolutely simultaneous with the opposite condition, namely, with the swelling out of the heart into the diastole. Moreover, there is a short pause before the supervention of the systole. The blood, in fact, rushes into the walls of the heart at the instant when the diastole commences, remains during the continuance of this state, and throughout the short pause which supervenes, and hence the flood of new blood can scarcely be supposed to be the cause of the systole. There is no difficulty, however, in believing that it is one of the causes of the diastole. The very rushing into the vessels may contribute to this. The reaction, also, which takes place between the oxygen of the blood and the vascular coats, which we know to be one main cause of the capillary force, will co-operate with the arterial tension in producing the same result, for a definite extrication of force is the direct consequence of this reaction. When, therefore, we consider the rush of blood in the coronary arteries, and the attendant actions in the capillaries, we may easily understand that the pulse of blood in the vessels of the heart may be an important coagent with the pulse of nervous influence which takes place at the same time, in bringing about the diastole. But

this is not all that concerns the action of the blood upon the heart and on further examination we may find, in the same quarter, a reason why the systole should follow the diastole. The jet of blood in the coronary arteries is withheld, as we know, in consequence of the diastole, and hence the arterial tension must be less perfect. The blood, also, at the same time is passing, or has already passed, into the veins, and become deprived of its arterial character. It has in fact given up, in great measure, the principle which is the main stimulus of activity and life,namely, the oxygen; and therefore it is less fitted to sustain the diastole, than it was when freshly injected into the vessels. The arterial tension and capillary action are in fact reduced, if not removed; and hence we may conclude that as this tension and action was a cause of the diastole, so the removal will tend to bring about the systole, by allowing, that is to say, the heart to return to its former condition of contraction.

To this explanation it can scarcely be objected, that there is no intermittent variation in the quantity of blood contained in the walls of the heart. At each systole the blood is expelled from the cavities, and we can hardly suppose that this contraction should take place without expelling some of the blood in the walls as well. Indeed, if we watch the action of the heart in a frog, in which the sternum has been

removed, without doing any injury to the vessels, we may be convinced of this; for at each diastole the organ becomes turgid with blood, not merely in the cavity, but in every vessel of the coats; while at the systole the walls are pale and the cavity emptied.

It is possible, also, that there may be a pulsatile generation of force in the pulmonary and some other tissues, where from the looseness of the structure, and the comparative absence of the pressure or support of surrounding parts, the flow of blood may be intermittent rather than continuous. Any pulses of vascular force originating in this way may react with the pulse of nervous influence (either through the nervous circle, or else by direct conduction through contiguous tissues), and with the pulses of vascular force belonging to the coronary system, in the induction of the diastole; while the common pause in the generation of force in all these quarters which is necessitated by the cessation of the jet of blood into the arterial system, may bring about the systole. It is possible, also, that the blood within the cavities of the heart, in so far as its surface is concerned, may act upon the lining membrane, just as it does upon the coats of the capillaries; but this action must be very insignificant when compared with the immense surface acted upon in the walls of the heart itself. Moreover, on the one side of the heart the cavities contain venous blood, and on the other side,