68 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 the products 

 of metabolism and exhaustion of energy-yielding material, co- 

 operate to produce the conditions actually observed; but the 

 former of these, the formation of metabolic products, seems to be the 

 protective mechanism that is especially adapted to save the muscle 

 from complete exhaustion. In what way these products depress 

 the irritability and contractility 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 O and the CO 2 and lactic acid which we recognize among the 

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

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

 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 

 develop movement. Regarding the means used in the muscle to 

 transform the original chemical or internal energy to mechanical 

 movement we have no or very little positive knowledge. Numer- 

 ous theories of a more or less speculative character have been pro- 

 posed. It has been suggested (Weber) that the muscular force is 

 essentially due to the elasticity of the muscle. It is known that the 

 elasticity of substances may change with conditions, and it is 

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



