of contraction, and proceeded to notice the effects of an additional weight. He proceeded as before, and found a similar train of results. There were, indeed, no signs of action so long as the pin remained; but on removing this, the beam once more ascended, and the muscle began to contract, after a certain degree of extension. It was the same, also, in relation to the degree of contraction, for the beam was not depressed as much as when a lesser weight was in the scale.

From these experiments, M. Schwann concludes that the power of contraction is greater when the muscle is least contracted, and therefore (he argues) that the force of vital contraction cannot augment in an increasing ratio as the particles approximate, as would be the case if the law of attraction were of the same nature as that of gravity, or any other physical force.

It is difficult, however, to understand by what process of reasoning M. Schwann is led to this conclusion. In respect to the law of universal attraction, there is no doubt that bodies will move towards each other with an energy increasing in the ratio of the square of the distance, if these bodies are free to move; and such a law seems to be involved, of necessity, in the very idea of force. When, however, the bodies acted upon are not free to move, and a force of resistance is opposed to that of attraction, the case is entirely altered. Do away with resistance, and bodies will

move towards each other with increasing velocity, according to a certain and definite law, until they merge into one; but concede the antagonistic force, and the results of the pure law of attraction are at once masked. That law, therefore, which M. Schwann requires in muscular contraction is impossible, except he could do away with the matter of the muscle, and all opposing force in other quarters.

Instead of being a valid objection, therefore, the experiment under consideration merely shows that the normal manifestations of the law of attraction in muscles, as in other bodies, is masked by a force of resistance; and hence the only value of the experiment is as a measure of the degree of this resistance. Alter, indeed, the manner in which the facts are stated, and the result is simply this-that a muscle contracts to a certain degree when it has a given weight to raise, to a greater degree when the weight is lessened, and to the greatest degree when the scale is emptied, and when the antagonizing influence is only that which is furnished by the substance of the muscle itself.

M. Schwann would have had reason for his conclusions, if he had found the muscle to contract to the same extent in each of these differing circumstances; but this was not the case, and the amount of shortening was always inversely proportionate to the resistThere is less reason, also, for the arrival at

ance.

this conclusion, when it is remembered that elastic bodies follow precisely the same law; and lastly, as another argument against his conclusions, it must be remembered that these movements are not nervous and vital, but the results of the workings of the galvanic force.

All, therefore, that we learn from this experiment is, that attraction and the antagonizing forces occupy an inverse relation to each other-and this is a truth of universal extent. The deductions of M. Schwann are altogether gratuitous, and so far from the experiment being an objection to the idea that vital and physical contraction are obedient to the same law, it furnishes evidence in favour of this view, by showing, that the force of attraction in each case is affected in the same manner by resistance.

In conclusion: we have reviewed the phenomena of voluntary muscular action in relation to the several correlated aspects of physical force, and from this examination it appears that the muscles are subject to each and all of these agents, as inorganic bodies are subject. Heat, light, electricity, and chemical agency are all correlative of motion, and this latter aspect of force, so far as we may judge, is not different from the motion which is correlative of these forces in inanimate nature.

CHAPTER IV.

C. OF VITAL MOVEMENTS IN THE HEART.

Preliminary Considerations.

If we examine the heart, we find many reasons to believe that it is subject to the law which rules the rest of the muscular system.

The existence of the sigmoid valves at the outlets of the ventricular cavities, is a fact which testifies as to the existence of a power in the heart whose operation is to produce regurgitation of blood: and in those cases, where these valves have been altered by disease, so that the arterial openings are imperfectly closed, the stethoscope reveals to us the reality of such a regurgitation. Now it is certain, that the force by which the blood is made to rush back in this manner is not owing to the resilience of an elastic tissue seated in the walls of the ventricles,

which tissue had been compressed during the systole, for no such structure is to be found: and therefore we are led to suppose that the ventricular diastole must be more nearly allied to dilatation than relaxation.

Again: the peculiar construction of the heart in insects, and other articulate animals, is in favour of the existence of a state of active dilatation in this organ. In these creatures, the heart lies in the general cavity of the body, and floats in the bloodlike liquid with which this cavity is filled. There are no venous trunks, but the blood at once finds entrance into the ventricle through simple valved slits. The heart, moreover, is not a dense muscular viscus, as in the higher animals, but it is a lax membranous tube, little, if at all, superior to the condition of an ordinary small vessel. Notwithstanding this peculiarity, however, the beats follow the characteristic rhythm. The action, in fact, is perfect with the conditions simplified. Here, for example, the diastole cannot be a state of mere relaxation originating in the operation of a vis-a-tergo, for in a heart without veins, and lodged, as it were, in an immense auricle where the blood moves in no definite current, there can be no such force. Here, moreover, the diastole cannot be the consequence of the resilience of elastic tissue, for the walls of the organ are lax and membranous. In such a heart, indeed, there can be no passive cause of