THE CHEMISTRY OF MUSCLE. 71 



yielding material has been depleted; the processes of restoration 

 have not_kept pace with those of consumption. But the accumu- 

 lation of the acid fatigue substances is doubtless the immediate 

 cause of the loss of irritability which we usually designate as fatigue. 



Theories of Muscle Contraction. — It is generally admitted that no theory 

 of muscle contraction yet proposed is satisfactory. Such a theory should 

 explain the mechanism by means of which the shortening of the muscle is 

 produced, the nature of the energy which is thus transformed into mechanical 

 work, and the relation of this energy to the chemical reaction that takes place 

 in the stimulated muscle. The measurable manifestations of energy which 

 are observed in the contracting muscle are the change in electric potential, 

 the increased production of heat, and the mechanical work. The electrical 

 change is a fleeting phenomenon which passes rapidly over the muscle, starting 

 from the point stimulated. Whether this electrical change is simultaneous 

 with the chemical reaction or precedes it cannot be stated definitely, although 

 simultaneous records indicate that the electrical change begins at least before 

 either the mechanical or the thermal changes can be recorded. The usual point 

 of view in physiology has been that the chemical change caused by the stimu- 

 lus or products formed from this change give origin to all the forms of energy, 

 electrical, mechanical, and thermal, which are exhibited by the contracting 

 muscle. The older view was to compare the muscle to a heat engine in which 

 the potential chemical energy of the fuel is first converted to heat bj^ combus- 

 tion, and then by appropriate mechanisms a portion of this heat energy is utilized 

 to perform mechanical work. Engelmann * has furnished a specific hypothesis 

 of this character. He assumes that the chemical energy set free in the muscle 

 takes the form of heat, which then acts upon the doubly refractive particles 

 in the dim bands of the muscular fibrillaj and causes them to imbibe water 

 from the adjoining light bands. If the doubly refractive particles are supposed 

 to have a linear shape, then, by imbibition, they would tend to assume a spher- 

 ical form, and thus there would occur a shortening along one diameter and an 

 increase along the diameter at right angles, such as occurs in the contracting 

 muscle. As the muscle cools down the water passes back into the light bands 

 and the phase of relaxation takes place. He has constructed an artificial 

 muscle from a string of catgut which, working on this principle, contracts 

 when heated and relaxes when cooled. When the heating occurs suddenly 

 this model gives curves of contraction identical with those obtained from 

 plain muscle. The apparatus is illustrated and described in Fig. 27, and the 

 curve of contraction obtained from it is shown in Fig. 28. The underlying 

 principle of this hypothesis has met with much criticism. Fick has shown 

 apparently that when applied quantitatively to the work done by muscle 

 it leads to an impossible conclusion. If, in a reversible process, at a temper- 

 ature To, a certain quantity of heat, Qo, is converted to mechanical work, it 

 necessitates, according to the second law of Thermodynamics, the passage of 

 heat, Q, from a higher temperature, T, to a lower temperature, Tj, in accord- 

 ance with the equation, -^ = Q ( tf; fft- )• 



lo \ i2 ii / 



We may assume, in accordance with experimental results, that the efficiency 

 of muscle is equal to 25 per cent, of the total energy, or that Qo = 5Q, and that 

 Tj is the temperature of the body, 37° C, or expressed in the absolute scale, 

 37° + 273° = 310° C. To is the same as Tj. If, now, in the equation we sub- 

 stitute IQ for Qo and 310° C. for T. and solve the equation for T, it gives 

 a value of 387° C., or, expressed in centigrade units, 387° — 273° = 114° C. 

 That is to say, to perform the work indicated the muscle must show a fall 

 in temperature from 114° to 37° C, and it seems clearly impossible to sup- 

 pose that the muscle in contracting attains any such temperature as 114° C. 



This criticism has been accepted by most authors as demonstrating that the 



* Engelmann, "Ueber den Ursprung der Muskelkraft," Leipzig, 1893; also 

 Pfliiger's "Archiv," 1873, p. 155. 



