72 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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so-called 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 trans- 

 formed into mechanical work, and 

 the relation of this energy to the 

 chemical reaction that takes place 

 in the stimulated muscle. The 

 measurable manifestations of en- 

 ergy which are observed in the 

 contracting muscle are the change 

 in electric potential, the increased 

 production of heat, and the me- 

 chanical 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, al- 

 though simultaneous records indi- 

 cate 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 

 stimulus or the products formed 



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Fig. 27. Engelmann's artificial muscle. 

 The artificial muscle is represented by the 

 catgut string, m. This is surrounded by a 

 coil of platinum wire, w, through which an 

 electrical current may be sent. The catgut 

 is attached to a lever, h, whose fulcrum is at 

 c. The catgut is immersed in a beaker of 

 water at 50 to 55 C., and "stimulated" 

 by the sudden increase in temperature -caused 

 by the passage of a current through the coil. 

 (After Engelmann.) 



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 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 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 spheri- 

 cal 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 supported this hypothesis 

 by microscopical observations upon the relations of the dim and light bands 

 in the contracted and relaxed fibrils (p. 20), and, moreover, 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 



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

 also Pfliiger's "Archiv," 1873, 7, 155. 



