arterial; and unless we had some common cause in operation (as we have in the general distribution of new blood through the coronaries) it is difficult to account for the simultaneous occurrence of diastole and systole in these two sides. It is an assumption without any proof, that the right side of the heart is more sensitive than the left, and that, by reason of this, it is able to respond to the feebler stimulus of venous blood: indeed this assumption is in opposition to the known law, that the acuteness of sensibility is in direct relation to the arterialized condition of the blood. We may conclude, therefore, that the action of the blood in exciting the heart, is exerted upon the vessels and fibres of the walls of the organ (and perhaps in certain additional reactions in the vessels elsewhere), rather upon the lining membrane of the cavities, although we are not prevented from supposing that there is a certain and definite action of the same kind in the latter quarter.

In the action of the heart, therefore, we detect the harmonious co-operation of extra-organic and intraorganic agents, with which we are already familiar, but the theory is not complete except it will account for the presence of the rhythm in the heart, and the absence in all other muscular structures. Why it is present in the heart we may find on examination. The valvular construction of the organ may be one reason, for this necessitates a continual admission of fresh

supplies of blood. By this arrangement it may be supposed that the fibre of the heart is placed in the conditions most favourable to the exaltation of its mobility,-for in other muscles we see that this faculty is directly proportionate to the free supply of oxygenated blood. A chief cause, however, may be supposed to be the intimate relation which exists between the heart and the blood,-a relation which is rather what it might be imagined to be in a structure intermediate between erectile tissue and muscle, than in ordinary muscle. This is not only seen in the phenomena which have been noticed as occurring in the heart of a frog during action, but also in the peculiar vascularity of any heart, when compared with the other muscles of the same animal. As contributing to the peculiarity of the action of the heart, we must also mention the shortness of the vessels, and the existence of the foramina thesbesii, all of which are favourable to the speedy entrance and escape of blood; and finally, we must notice the great variations in the quantities of blood contained in the walls and cavities of the heart. Now, on comparing voluntary with involuntary muscle, there is good reason to believe that the more abundant vascularity of the former is a chief cause of the greater readiness and rapidity of action; and for the same reason, therefore, we may suppose the heart to possess a greater mobility than either; hence there is no great difficulty in supposing

that the heart is able to respond to the rhythmical variations of force which can be pointed out as acting upon the organ. On the other hand, the very reasons which enable us to suppose that the heart may have a rhythmical action, prevent us from expecting this elsewhere. In other muscles, the more scanty supply of blood-vessels argues a duller mobility, and the absence of such marked variations in the contents of these vessels-the circulation in them being continuous rather than intermittent-argues also a feebler manifestation of the rhythmical variations of force. In other muscles, therefore, we have a feebler rhythm of force, and a duller capacity of responding to this rhythm; and therefore the comparative absence of the conditions which determine the rhythm of the heart may be the reason why such a rhythm is absent elsewhere.

In conclusion: the heart is subject to the laws which govern other mobile fabrics, and its history would seem to offer no exception to what has gone before. The systole refers to the common causes of contraction; the diastole to the common causes of dilatation. The rhythm also may be supposed to be nothing more than the obedience of a natural susceptibility to certain periodical variations in the supply of intra-organic force, the existence of which variations can be satisfactorily demonstrated.

CHAPTER V.

OF THE NERVOUS INFLUENCE AS AN AGENT IN VITAL MOVEMENT.

IN the preceding chapters, it has been found that nervous influence, and the other forces of organization agree in their mode of action, and that all these vital and physico-vital agencies co-operate harmoniously with the several varieties of extra-organic force,— and hence we may infer, with very sufficient reason, that nervous influence is nothing more than a modification of cosmical force.

In another work,* it has been shown that the nervous system is formed upon the same archetypal plan as the other parts of the body, and even of the body itself, and that the same plan is extended to what are called inanimate bodies; and in this unity of the material framework of things there is another

*See "Proteus," p. 65, &c.

reason for supposing the nervous influence to be a variety of cosmical force.

It has been shown, also, in the same work that all forms of physical and vital force are truly correlative: and hence a final argument in favour of the view that the nervous influence is a variety of cosmical force.

We are not taught, however, by these considerations to do away with the idea of nervous influence, though we regard it in this point of view, or to ascribe its workings to electricity, or heat, or light, any more than we should confound the existence of light, or heat, or electricity, and say that either of these agents was identical with the others. Correlation, in fact, involves the idea of unity, but it as distinctly retains that of difference. Nor are we to confound nervous influence with mind, for-if it be as we have said-it is clear that it can have no exclusive claim to be considered as identical with this

essence.

The nervous influence, therefore, is to be regarded as one of several modes of cosmical force, which reacts harmoniously with all companion agencies, in the determination of vital movement.