by the amount of carbon rather than the amount of nitrogen which is excreted. The energy evolved daily by the human body would raise it to a height of about six miles.8

But beside heat, work may be the outcome of the organism; and this through the agency of the muscles. Their absolute obedience to mechanical law in their mode of action has been admirably established by Haughton.9 The work a muscle does, it does in contracting. It is to the mechanism of muscle-contraction that we are indebted for another illustration of our subject.

When work is done by a muscle in contracting, three changes are observed to take place normally in its tissue. First, there is a loss of its electric tension; second, there is an evolution of heat in it; and third, carbon dioxide appears there, and its reaction, before neutral, becomes acid.10

Matteucci was the first to observe and to call attention to the remarkable similarity in structure and in the mechanism of operation, between striated muscular fibre and the electric organ of certain fishes.11 Recently, Marey has repeated and extended his observations.12 In structure, the electric organ is made up, like the muscle, of columnar masses each separable transversely into vesicular sections. In a torpedo weighing seventy-three pounds, there were 1182 of these columns in each of the two organs, with 150 sections, on an average, in each column. 13 In the muscles which bend the fore-arm, there are 798,000 fibrillæ.14 As to the mechanism, alike in muscle and in electric organ, an electric current stimulates action on opening and on closing the circuit, but not when it is flowing; the same phenomena take place in both with the direct and with the inverse current; both are reflex; stimulation of the electric nerve produces discharge, as that of the motor nerve causes muscular shock; an entire paralysis follows nerve-section; curare paralyzes both; and tetanus results in both from rapid currents or from strychnine.15

Still more striking analogies are furnished by the investigation of the susurrus or muscular sound, first noticed in 1809 by Wollaston.16 This sound is produced by all muscles when in the state of contraction, the pitch of the note being not far from thirty vibrations per second. It is evidently only the intermittent contraction of the muscular fibre. A single excitation produces a muscular shock. As this production requires from eight to ten hundredths of a second, it is evident that if another stimulus be

applied before the first has disappeared the two will coalesce; and when twenty per second reach the muscle it becomes permanently contracted or tetanized.17 By means of a very sensitive myograph, Marey has found that in voluntary contraction the motor nerves are the seats of successive acts, each of which produces an excitation of the muscle. In 1877, Marey examined similarly the discharge of the torpedo and found a most complete correspondence between it and muscular contraction.18 Since electric tension disappears from a muscle during contraction, does not the analogy suggest strongly that, like the discharge of the electric organ of the torpedo, muscular contraction is an electrical phenomenon?19 Granting electric discharge to be a necessary concomitant of muscle-contraction, what is the origin of this electricity? That it is not carried to the muscle by the nerves follows from the fact that a muscle will still contract when deprived of all its nervefibres, or when these are paralyzed by curare.20 It must therefore be generated within the muscle itself. To reach a solution of the problem we must obviously follow the analogies of its production elsewhere.

Perhaps no single question in physics has been more keenly discussed than this one of the origin of electric charge. The memorable conflict between Galvani and Volta, between animal electricity and the electricity of metallic contact, succeeded by the even more triumphant overthrow of the latter and the establishment ultimately by Faraday, of the electro-chemical theory; these are facts fresh in all our memories. The justice of time however in this case, if it has been tardy, has been none the less sure. The experiments of Thomson have vindicated Volta and established the contact theory as a vera causa. And more curiously still, it now appears to be proved that both contact and chemical action underlie the production of that very animal electricity so stoutly battled for by Galvani and his associates.

Volta's experiments to prove that a difference of potential is developed by the contact of two heterogeneous metals were not crucial. But Thomson, repeating them with the aid of more delicate apparatus, has shown that whenever copper and zinc are brought in contact, the copper becomes negative to the zinc. In proof that the chemical action of atmospheric moisture was not the cause of the phenomenon, he showed that when a drop of water served to connect the copper and the zinc, no charge at all

was produced.22 The fact may therefore be regarded as established, as the result of numerous and varied experiments, that a difference of electrical potential is always developed at the surfaces of contact of heterogeneous media. Not only is this true of solids in contact with solids, but also of solids in contact with liquids, and of liquids in contact with each other.23 Of course the production of electricity by contact must result from a loss of energy elsewhere. In the opinion of Cumming, it is the loss of energy which is owing to the unsymmetrical swinging of the molecules on the two sides of the surfaces of contact, which reappears as difference of potential between the solids or as the energy of electrical separation.24

But we may carry the sequence yet another step backward. The energy which is thus lost at the surfaces of separation must be heat, and this junction must be cooled thereby. Thus the production of thermo-electricity is seen only to be a special case of a general law, a view to which the well-known Peltier effect gives support. In this phenomenon, when two metals are joined together in the form of a ring and one junction is heated, a current is produced which cools the other junction.25 From a study of these conditions, Thomson has concluded that the absorption of heat in a thermo-electric circuit varies for different metals with the direction of the current. Thus in iron, the current from hot to cold absorbs heat, while in copper the current which absorbs heat is from cold to hot.26 In entire accordance with these results, are the conclusions recently reached by Hoorweg. Whenever two conductors come into contact, motion of heat results in the development of electricity, the current produced existing at the cost of heat at one part of the point of contact, and evolving heat at the other for a result. Hence all voltaic currents are thermo-currents.27

To return to the muscle, it must now be apparent that the electrical charge which appears in its fibre may have its origin in so purely a physical cause as the contact of the heterogeneous substances of which the tissue is built up; the maintenance of this charge being effected by chemical changes going on constantly in the substance of the muscle, by which the carbon dioxide is produced, which is shown to be a measure of the work done.28

Conceding then, that a muscular contraction is conditioned upon an electric discharge, by what mechanism is the contraction effected? Prevost and Dumas supposed each particle of a

muscular fibril to be magnetic. Such a row of particles would indeed attract each other when magnetized and shorten the length of the whole fibril. But the force of contraction would increase as the length diminished; whereas the fact in the case of the muscle is precisely the reverse.29 Hence Matteucci supposed that electrification of the muscular fibre produces a repulsion between its elements, the subsequent return of this fibre, in virtue of its proper elasticity,30 constituting the muscular contraction. Radcliffe's theory is of more recent date and is somewhat analogous. He maintains that each fibre of the muscle, together with its sheath, constitutes a condenser. When charged, the attraction of the two electricities compresses the fibre laterally and thus elongates it. When discharge takes place the normal elasticity of the fibre produces the contraction.31 Assuming, however, that the electrical phenomena take place during the latent period only, other theories of muscular contraction regard these phenomena as simply preparatory, the contraction itself being mechanical. Thus Marey 32 likens the muscular fibre to a string of India rubber which, when stretched, contracts upon warming it; thus transforming heat directly into work. Rouget contends that the muscular fibre is a true spiral spring, which, actively distended during the repose of the muscle, returns upon itself at the moment of contraction; muscular contractility being thus a purely physical property of elasticity. Engelmann, observing that during contraction changes take place in the doubly-refracting power of the alternate disks of the fibril, supposes muscular contraction to result from a passage of water from the isotropic layers into the anisotropic or doubly-refracting ones; thus osmotically increasing the volume of the latter. By this means the ellipsoidal doublyrefracting particles are converted into spherical ones; and since the longer axes of the ellipsoids are parallel to the length of the fibre, the muscle is thereby shortened.33

From this brief review, does it not seem probable that the phenomenon of muscular contraction may be satisfactorily accounted for without the assumption of "vital irritability," so long invoked ?34 May it not be conceded that the theory that muscular force has a purely physical origin is at least as probable as the vital theory?

Time would fail me to discuss the many other phenomena of the living body which have been found on investigation to be non

vital. Digestion, which Prout said it was impossible to believe was chemical,35 is now known to take place without the body as well as within it, and to result from non-vital ferments.36 Absorption is osmotic and its selective power resides in the structure of the membrane and the diffusibility of the solution.37 Respiration is a purely chemical function. Oxyhæmoglobin is formed wherever hæmoglobin and oxygen come in contact and the carbon dioxide of the serum exchanges with the oxygen of the air according to the law of gaseous diffusion.38 Circulation is the result of muscular effort both in the heart and the capillaries, and the flow which takes place is a simple hydraulic operation.39 Even coagulation, so tenaciously regarded as a vital process, has been shown to be purely chemical. Upon the hypothesis of Schmidt it results from the union of two proteids, fibrinogen and fibrinoplastic substance.40 According to the later theory of Hammarsten, fibrin is produced from fibrinogen by the action of a special ferment.41

There is another function which should by no means be omitted from our consideration. This function is that of the nervous system. In structure, this system is well known to us all. In composition, it is made up essentially of a single substance, discovered by Liebreich 42 and called protagon, the specific characters of which have lately been confirmed by Gamgee.43 In function, the nervecell and the nerve fibre are occupied solely in the production, the reception and the transmission of energy, which is believed to be electrical. There is evidently a close analogy between the nerve and the muscle, the nerve-fibre like the fibrilla being composed of cells, and having a positive electric charge upon the exterior surface the tension of which is one-tenth of a volt.44 Indeed a piece of nerve removed from the body exhibits nearly the same electric phenomena as a piece of muscle. Haughton attributes tinnitus aurium to the vibration of nerve cells.45

The main objection raised to the electrical character of nerveenergy is based upon its slow propagation.46 Though thirty-six years ago Johannes Müller predicted 47 that the velocity of nerve transmission never could be measured, yet Helmholtz accomplished the feat very soon afterward.48 His results, like those of subsequent experimenters, show that the velocity of propagation of the nervous influence along a nerve, like that of electric transmission, is only about 26 to 29 metres in a second.49 But it should be borne in mind, as Lovering has pointed out, that electricity has no