dilatation, and hence it seems necessary to suppose that there must be something more than mere relaxation in the diastole.

Another argument in favour of the same conclusion may be found in a case of cardiac-hernia, recorded by Dr. Cruvelhier, in which it is stated that the hand which grasped the protruded heart was opened during the ventricular diastole, notwithstanding a considerable effort to the contrary on the part of the experimenter, -a fact which is altogether at variance with the idea that this state is one of mere relaxation.

These considerations are sufficient to prepare us to expect that the movements of the heart form no exception to other vital movements, and to make us doubt that the ventricular diastole is a condition of relaxation, marking fatigue or weariness after an act of mysterious and exhausting contraction. What, however, is the true nature of this state, and of the systole, will appear if we proceed to examine the heart in relation to its causes of action.

1. Of extra-organic force as the agent in the movements of the heart.

The effects of the agents which belong to this class are somewhat obscure, but they are most apparent in the rudimentary phases of life. Low in the scale of creation, in "exsanguine animals, such as the snail," we learn, on the authority of the great Harvey, that

the rhythmical movements of the heart are suspended in winter, or else repeated so slowly as to escape the attention of an ordinary observer, and the organ itself so shrunk and contracted as to be little more than an ordinary vessel,-a change which, when compared with the state in summer, would seem to argue that the heart has suffered contraction in consequence of the coldness and inclemency of the season.

The influence of extra-organic agents may also be seen in the heart of a chick, if a visit be paid to the hydro-incubating establishment of M. Cantello. The other day, for example, I had ocular demonstration of this, along with my friend Dr. William Addison. On opening an egg of the third day the heart was seen to pulsate regularly, and to continue to do so for some after the embryo was separated from the vascular area, and placed upon a slip of glass. When the heart became still, which it did in a few minutes, we found it possible to renew the action by placing the lower surface of the glass in light contact with the warm water which is used to hatch the eggs; and when it was still a second time the rhythm was renewable in the same manner. Indeed it was found possible by this application of warmth to revivify the heart when it ceased to act, until the organ and the rest of the gelatinous embryo were half dried up by exposure to the atmosphere. It was the same also with an embryo-chick of the sixth day, which was

subjected to the same treatment. On examining the heart in these cases, after it had ceased to act, it was shrunken and collapsed, and the immediate operation of the heat seemed to induce the diastole; but owing to the minuteness and indistinctness of the parts it is not easy to speak positively upon this point. Be this as it may, however, the influence of extra-organic agents is clearly exhibited in these experiments.

Now,

The same influence is also seen in the heart of a cold-blooded animal-of a frog, for example; and here we may obtain a distant glimpse of the causes which induce the rhythm. It is well known that a heart of this kind will pulsate for some time after removal from the body, taking air into its cavities and expelling it again, as it might be blood,-the reactions between the air and the heart being the chief source of the force which is concerned in this action. at first sight, we have no more reason to suppose that the air excites the systole than the diastole; but, if we reflect awhile, we find it easier to entertain the latter supposition than the former. There is no elastic tissue to restore the diastole by its resilience when the systole ceases, and on this account it is difficult to suppose that the air can find entrance, unless it possess some power by which it can resolve the contracted state. Moreover it may be supposed, that a certain generation of force must result from the combination of the atmospheric oxygen with the

tissues with which it comes in contact. And, further, if the air finds entrance into the heart by this means, it follows that it will be less able to retain its position after a portion of its vivifying ingredients have been expended, as they must have been if the diastole is induced in this manner, and we may infer, therefore, that the systole will occur and expel the air, as the natural consequence of the altered and deteriorated quality of this air. But let the explanation be as it may, we may return to the original assertion, and say, that there is no more proof that the air excites the systole than the diastole. The question, however, is extremely complicated, and we must be content to deduce an indirect answer from the more evident operation of intra-organic agents.

2. Of intra-organic force as the agent in the heart's action.

(a.) Of the nervous influence as an agent.

The effects of fear upon the heart are very well marked, and these may serve as the first subject for inquiry. In this state the extreme pallor of the skin and the tendency to evacuation in the several visceral cavities, argue, as we have seen, an actual contraction in the rudimentary muscular structures, and it is the same also in the walls of the heart. When, for example, fear verges upon syncope, and the contrac

tions of the heart are very hurried, and when, at the same time, the paleness of the skin shows how small a quantity of blood is forced into the vessels, the only manner in which it is possible to account for the inconsistency of these phenomena, is to suppose that the walls of the ventricles have become contracted,— for if these cavities preserved their natural dimensions, the increased number of systolic contractions must produce an equivalent distention of the vessels, by which the skin would become hot and flushed, instead of being, as it is, cold and pallid. And hence it may be presumed, that a state of contraction has been induced under the influence of fear, in the walls of the heart, as in other muscular structures.

In the natural action of the heart, also, there is strong presumptive evidence that the systole is cotemporaneous with an interruption in the supply of nervous influence, and not with any increased supply. We know, indeed, that the development of this agent is proportionate to the flow of blood to the sources of innervation; and hence we may conclude that this development must follow an intermittent law in animals provided with hearts. At the systole, indeed, a gush of blood will pass to the nervous organs and produce signs of activity in them; and as the distribution of force will be consentaneous with its production-for in subtlety this agent is analogous to electricity-it follows that the heart is most freely supplied with