THE CHEMISTRY OF MUSCLE. 71 



in walking or climbing, may diminish the amount of work obtainable 

 from other unused muscles, such as those of the arms. So also the 

 effect of muscular exercise upon the rate of the respiratory move- 

 ments and upon the heart-rate is explained, as we shall see, in a 

 similar way. It should be added that Lee.* confirming an older 

 observation by Ranke, has published experiments which indicate 

 that the first effect of the so-called fatigue substances is to increase 

 the irritability of the muscle, while the later effect is to diminish the 

 irritability or to suppress it altogether. In this initial favoring in- 

 fluence Lee finds an explanation of the phenomenon of Treppe (see 

 p. 35). After the appearance of complete fatigue a muscle shows 

 usually some return of irritability if given a short rest. But even in 

 the case of a muscle in the body, with its circulation intact, an 

 interval of some hours is required before it regains entirely its power 

 to perform a normal amount of work. It seems probable that the 

 loss of power to do work is referable in part to a using up of the 

 supply of energy-yielding material, but the accumulation 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 engery 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 by combus- 

 tion, and then by appropriat3 mechanisms a portion of this heat energy is utilized 

 to perform mechanical work. Engelmannf 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 fibrillae 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 



* For discussion and experiments, see Lee, Harvey Lectures, 1905-06, 

 Philadelphia, 1906; also "Journal of the American Medical Association," May 

 19, 1906, "American Journal of Physiology," 18, 267, 1907. 



t Engelmann, "Ueber den Ursprung der Muskelkraft," Leipzig, 1893: 

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



