external temperature that these effects are produced; they are equally observed when the animal heat varies in its production. The heating of the organism which accompanies muscular activity, or which results from taking very hot drinks, produces the acceleration in the superficial circulation, which throws out this excess of heat to the surface. Inanition, muscular repose, the drinking of iced waters, &c., slacken the circulation near the surface and check its cooling action.

In the

Such are, as far as we can explain them in a short chapter, the origin and the distribution of heat in the animal organism. The part played by the circulation of the blood in the distribution of heat, perhaps demands more ample details; and, indeed, we have treated it more fully elsewhere.* present chapter we have studied heat only as manifestation of force, and have merely designed to show that, notwithstanding all appearances, heat is of the same nature in the inorganic world and in organised beings.

CHAPTER IV.

ANIMAL MOTION.

Motion is the most apparent characteristic of life; it acts on solids, liquids, and gases-Distinction between the motions of organic and animal life-We shall treat of animal motion only-Structure of the muscles-Undulating appearance of the still living fibre-Muscular wave-Concussion and myography-Multiplicity of acts of contraction-Intensity of contraction in its relations to the frequency of muscular shocks-Characteristics of fibre at different points of the body.

MOTION is the most apparent of the characteristics of life; it manifests itself in all the functions; it is even the essence of several of them. It would occupy much space to explain the

* Physiologie médicale de la Circulation du Sang. Paris, 1863; and Théorie physiologique du Choléra, Gazette Hebdomadaire de Médecine. 1867.

mechanism by which the blood circulates in the vessels, how air penetrates into the lungs, and escapes from them alternately, how the intestines and the glands are perpetually affected by slow and prolonged contractions. All these movements take place within the organs without the exercise of the will; frequently even the individual in whom they occur is unconscious of them; these are the acts of organic life.

Other movements are subjected to our will, which regulates their speed, energy, and duration; these are the muscular actions of locomotion, and the different acts of the life of relation. We shall treat specially of this order of phenomena, which are more easy to observe, and to analyse. Suffice it here to say that the absolute division between the acts of organic life and those of the life of relation ought not to be accepted unreservedly. Bichat, who established it, based it upon anatomical and functional differences which are of less importance now than they were in his time. The muscular element of organic life is unstriped fibre obedient to the nerves of a particular system called the great sympathetic, on which the will has no action; motions produced by this kind of fibre are manifested some time after the excitement of the nerve or of the muscle, and continue for a considerable time. In fact, the object of those acts which are intended to maintain the life of the individual imprints upon them a special character. The muscular element of the life of relation consists of a fibre of striated appearance, whose action, under the control of the will, is dependent upon nerves emanating directly from the brain or from the spinal marrow. These movements become evident rapidly as soon as they are provoked by excitement; they are of brief duration, and are, generally, not indispensable to the maintenance of the life of the animal.

Although this distinction is, in a general way exact, it is plain that it is too arbitrary, and that numerous exceptions to the anatomical and physiological laws which it tends to establish may be quoted. Thus, the heart, an organ directly indispensable to organic life, and not under the governance of the will, is a structure which much resembles the voluntary muscles. Certain fishes of the genus tinca have striated muscles

in the large intestine, as Ed. Weber has pointed out. Very often, on the other hand, the will has no power over certain muscles which, by their structure, and by the nature of the nerves which animate them, belong to the system of the life of relation. Habit, besides, by repeated exercise, appears to extend the action of the will over the muscles, almost indefinitely. The young animal shows, by the awkwardness of his movements, that he is not in full possession of his muscular functions; he seems to have to study the simplest acts, and performs them badly; while the gymnast, or the skilled piano forte-player executes prodigies of agility, strength, or precision, without any apparent effort of the will proportionate to the result obtained. Many physiologists think, and we are of the same opinion, that there exist in the brain, and in the spinal marrow, centres of nervous action which acquire certain powers, by force of habit. They attain to the command and co-ordination of certain groups of movements without the complete participation of that portion of the brain which presides over reasoning and the consciousness of our actions.

Let us lay aside these questions, which are still under investigation, and examine into the production of motion in a voluntary muscle. The organ which generates motion is composed of several elements. Simple as it is supposed to be, it requires the intervention of muscular fibre, of the blood vessels, which unceasingly convey to it the chemical elements at whose expense the motion is to be produced, and finally, of the nerve which excites motion in the fibre.

When the physiologist desires to analyse the actions. which take place in the muscles, he does not deal, in the first place, with voluntary motions, whose complexity is too great.. The operator isolates a muscle, and induces motion in it, by bringing to act upon its nerve artificial excitements which he has under his control.

To give an idea of the part played by each of the elements. of the motive apparatus in the production of movement, it is sufficient to operate upon the leg of a frog. By laying bare and severing the sciatic nerve, the influence of will upon the muscle may be suppressed, so that the latter will only execute

such motions as are produced by excitation, electric or otherwise, applied to the portion of the nerve which remains in communication with it. On the sides of the sciatic nerve are an artery and a vein. Compression of the artery will prevent the blood from reaching the muscle; compression of the vein will produce stagnation of the blood. The influences which different states of circulation produce upon the muscular function may then be observed; and, finally, by making an incision in the skin of the foot, the muscle will be laid bare, and cold, heat, or the various poisonous substances by which its action is modified, may be brought to bear directly upon it.

When the nerve of a frog thus prepared is excited by an electric discharge, a very brief convulsive movement in the muscle is produced; this motion is called Zuckung by the German physiologists, and we propose to call it shock, in order to distinguish it from true contraction. It is so rapid that its phases cannot be distinguished by the eye, so that, to appreciate its characteristics aright, recourse must be had to special instruments. Registering apparatus only can supply this need, for they faithfully render all the phases of motion communicated to them. The general disposition of these forms of apparatus, which for a long time were used almost exclusively in the service of meteorology, is generally known. The indications of the barometer, of the thermometer, of the force or the direction of the wind, of the quantity of rainfall, &c., register themselves under the form of a curve which, according as it is elevated or depressed, expresses the increase or diminution of intensity of the phenomenon to be registered. The time during which these variations are accomplished may be estimated by the length occupied by the curve upon the paper, which travels in front of the marking pen with an ascertained and perfectly regular speed.

The use of instruments of the same kind has been introduced into physiology by Volkmann, Ludwig, and Helmholtz. We have endeavoured to extend the employment of them to a great number of phenomena, and we have constructed many instruments whose description would be out of place here. The apparatus which registers muscular motions bears the name of myograph; it shows the disturbance of the muscle by

means of a curve which readily allows us to study its phases. We have fully explained elsewhere the nature of this instrument, the experiments for which it is suitable, and the results which it gives.* At present we shall limit ourselves to a summary description of the chief results of myography.

In order to explain thoroughly the function of the apparatus, let us reduce it in the first place to its essential elements. Fig. 2 shows a muscle of the calf of a frog's leg, m, suspended by a clip by means of the bone to which the upper part of the muscle is attached. The tendon, t, of the muscle has been cut and then tied by a thread to the lever, L, one end of which can be raised or lowered while the other is fixed; the nerve, n, is susceptible of electric excitement, which produces certain contractions followed by relaxations in the muscle, that is to say shocks. Each of these movements of the muscle is communicated to the lever, which is raised or lowered, ampli

* Du Mouvement dans les Fonctions de la Vie. Paris, 1867: G. Bailliere.