As an objection to the notion that the voluntary muscles may be made to contract by the direct operation of external cold, we must mention, however, a statement often made by physiologists, that heat is given out during the state of contraction. The fact upon which this opinion is founded is the small increase of temperature which takes place during violent exercise, as of the forearm in the act of sawing,-and this fact is undeniable. The conclusion, however, is altogether gratuitous. In an act of this kind, indeed, it is not with mere muscular contraction that we have to do, but with an alternate series of contractions and dilatations; and it is quite as probable that the elevation of temperature is, connected with the latter phenomena as with the former. Nay, it is more so:for when the limb is wearied and the action at an end, and when also the temperature is highest, the condition of the parts is much more nearly allied to relaxation than to any other state, while the fact that the increased heat continues for some time after the cessation of the contractions, shows that it is not to these that we must look for its immediate cause.

It is difficult to procure any direct evidence of the action of light upon the voluntary muscles, for this is so mixed up with other actions: but, at the same time, we may notice two circumstances which seem to bear upon the question. The first of these is

the more frequent occurrence of spasmodic attacks at night than during the day time. The second is the pale and flaccid appearance of the muscles, which is a common condition in convulsive diseases—a fact which may possibly show that the absence of light (for light is infinitely concerned in healthy nutrition,) has had something to do in inducing that state of muscular fibre in which there is a proneness to contraction.

The influence of electricity in muscular action is very marked, but the manner of operation is difficult and hard to be understood. If, however, we investigate the late discoveries in this department of science, and especially those of Matteucci, we find reason to believe that the action of electricity, when more fully known, will present no essential difference from that of the correlative agents with which we are already familiar.

It is an initial fact in these inquiries, and one that may very easily be tested, that there is in the muscles of an animal during life, and so long after death as the irritability continues, an electric current from the interior to the exterior of the muscles, and in the limbs, from the extremities towards the head. The latter current, which is particularly conspicuous in the hinder limbs of the frog, is of infinite importance in the interpretation of the influence of electricity in muscular action, and our first step must be to acquire

an adequate idea of its existence and reality. In this there is no difficulty. We must take a number of the hind legs of the frog, amputated at the pelvis, and stripped of their skin, and arrange them in a series of wine-glasses half filled with water, each leg being bent over the edge of the two contiguous glasses in such a manner that the foot is immersed in one glass and the thigh in the other. A foot and thigh of different limbs are thus in every glass, except the terminal ones, in the one of which there is (as must needs be) only a foot, and in the other a thigh. On plunging the poles of a galvanometer into the water contained in the glasses which hold the extremities of this muscular pile, there is a contraction of all the limbs, and a divergence of the needle of the instrument in such a direction as to show that a current of electricity has passed from the feet towards the thigh. By adding or diminishing the number of limbs, the divergence of the needle may be increased or lessened, and thus it is found that the intensity of the current is proportionate to the size of the pile. It is found. also that the muscular contractions cease after a time, and with them the divergence of the needle; and in this way we may satisfy ourselves that the development of the electricity is dependent upon the conditions which preserve the muscular irritability, and not upon the mere contact of the limbs as masses of inorganic matter. The nerves would seem to have little to do

in this process, nor are they necessary to connect one limb with another, for it is found that a metal conductor, or pieces of moist paper, will answer this purpose quite as well, if not better; and in this way we may be more fully satisfied that the influence which is generated is electrical in its character.

In this experiment contraction is induced in the limbs which form the pile, and in them we might investigate the operation of the electric agent. It is better, however, to prepare an additional limb in the same way as the others, and to induce action by bringing it within the circuit. In this case also it is of no moment whether the nerve be left to serve as a conductor, or whether we use metal or moist paper for the purpose.

On considering the contractions in this limb when it is connected with the pile, they are found to take place at different times, according to the direction of the current. At first there is contraction both on making and breaking the circuit. If the current pass from the thigh towards the foot, there is a powerful contraction on completing the circuit, and a far slighter one on breaking it; but if the current pass in a contrary direction,-namely, from the foot to the thigh, there is a slight contraction on completing the circuit, and a powerful one on breaking it. There is a change, however, if we continue to transmit the electric influence, and eventually the slighter contrac

tions disappear, there being only contraction on closure of the circuit when the current passes towards the foot, and on disruption when it passes towards the thigh.

Now these results are not altogether unintelligible, if we attentively consider all the premises. It may be granted, in the first place, that there is a current of electricity in the limb which is used to test the activity of the pile, in the same direction as in the limbs which compose the pile,-namely, from the foot towards the thigh. It may be granted, also, that by adding this limb to the pile in the proper manner, the activity of the electric current would be intensified; and if it were inverted so that the thigh were added to the thigh—instead of to the foot—extremity of the pile, it may be granted also that this mal-arrangement will neutralize to a certain extent the energy of the current, just as would be the case if we were to add a pair of plates to a galvanic battery, in such a manner that platinum faced platinum, or zinc zinc, instead of observing the natural order and causing the zinc and platinum to oppose each other. And further, it may be granted that the limb which is made the subject of the experiment is virtually a part of the pile, though it be only connected by conductors, and that it must neutralize or intensify the energy of the electric current, just as much as if it were more obviously a part of the pile.