68 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



a very short rest suffices to make the muscle again capable of 

 lifting its load, but a very long interval of rest, two hours, may 

 be required before the muscle is restored entirely to its normal 

 condition. Such a long interval is evidently not necessary for 

 the removal of the acid products, and we must recognize that a 

 part of the fatigue is due to a using up of the material from which 

 the energy is obtained. That is, during contraction the processes 

 of disassimilation or catabolism are in excess of those of assimilation 

 or anabolism, so that at the end of prolonged muscular activity 

 the muscle contains a diminished supply of oxidizable or energy- 

 yielding material. To supply this deficiency new food material, 

 including under this term also the necessary oxygen* must be 

 assimilated by the muscle. We must suppose, therefore, that 

 two factors, accumulation of waste products and exhaustion of 

 energy-yielding material, co-operate to produce the conditions 

 actually observed; but the former of these, the formation of acid 

 waste products, seems to be the protective mechanism that is 

 especially adapted to save the muscle from complete exhaustion. 

 In what way the acid products depress the irritability and con- 

 tractility of the muscles is not known; their presence may, as 

 Ranke supposed, prevent the underlying chemical changes, the so- 

 called physiological oxidations, or their action may be exerted on 

 the contractile machinery alone, that is, the mechanism by means 

 of which the shortening is effected. 



Theories of Muscle Contraction. It is universally admitted 

 that the ultimate cause of the muscle contraction is the chemical 

 change caused by the stimulus. While the nature of this chemical 

 reaction is not known, it is admitted also that it consists in a process 

 of splitting and oxidation whereby large and relatively unstable 

 molecules are reduced to smaller and more stable ones, such as 

 H 2 and the* C0 2 and lactic acid which we recognize among the 

 products. This reaction is exothermic, that is, some oY the chemical 

 or internal energy of the complex compound is liberated as heat; 

 some also as electrical energy, as is explained in a later chapter. 

 Both of these results are so frequently observed in other chemical 

 reactions that they call for no special comment in this case. The 

 particular problem regarding the muscle is how this chemical 

 reaction leads to the shortening of the muscle and thereby 

 makes it do mechanical work. We must assume that there is some 

 mechanism in the muscle by means of which the energy liberated 

 during the chemical change is utilized in causing movement, some- 

 what in the same way as the heat energy developed in a gas-engine 

 is converted by a mechanism into mechanical movement, or the 

 electrical energy in the coils of a motor is utilized by a device to 

 * Verworn, "Archiv f. Physiologie," 1900, suppl. volume, p. 152. 



