GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 105 



posed of semifluid isotropic substance, in which are the particles of anisotropic 

 material, arranged to form vast numbers of parallel fibrillar of like structure, 

 and so placed as to give the effect of transverse disks (Z, n, (J, Fig. 39). 



When a striated muscle contracts, each of its fibres becomes shorter and 

 thicker, and the same is true of the dark and light disks of which the fibres 

 are composed. If we examine a muscle-fibre which has been fixed by osmic 

 acid at a time when part of it was contracting, we see that in the contracted 

 part the light and dark bands have both become shorter and wider, but that 

 the volume of the dark bands (Q, Fig. 39, ( ') has increased at the expense of 

 the light bands. 



Further, the dark bands are seen to be lighter and the light bands darker 

 in the contracted part, while examination with polarized light shows that 

 though the anisotropic substance does not seem to have changed its position, 

 (Fig. 39, D), the original dark bands have less and the lighter bands greater 

 refractive power. These appearances would seem to be explained by Engel- 

 mann's view that contraction is the result of imbibition of the more fluid 

 part of the sareoplasm by the anisotropic substance. He has advanced the 

 theory that the cause of the imbibition is the liberation of heat by chemical 

 changes which occur at the instant the muscle is excited. In support of 

 this theory Engelmann ' showed that dead substance containing anisotropic 

 material, such as a catgut string, can change its form, by imbibition of 

 fluid under the influence of heat, and give a contraction curve in many 

 respects similar to that to be obtained from muscle. This theory of 

 the method of action of the muscle-substance, though attractive, can be 

 accepted only as a working hypothesis, and is not to be regarded as proved. 

 Various other theories have been advanced to explain the connection between 

 the chemical changes which undoubtedly occur during contraction and the 

 alteration of form, but none have been generally accepted. Enough has been 

 said to show that the contraction of the muscle a- a whole is the result of 

 a change in the minute elements of the fibrillffi, and that the various condi- 

 tions which influence the activity of the process of contraction musl act chiefly 

 through alterations produced in these little mechanisms. 



3. Elasticity of Muscle. — The elasticity and extensibility of muscle are 

 of great importance, for by every form of muscular work the muscle is sub- 

 jected to a stretching force. Elasticity of muscle i- the property by virtue of 

 which it tends to preserve its normal form, and to resist any external force 

 which would act to alter that form. The shape of muscles may he altered by 

 pressure, but the change is one of form and not of bulk ; since muscles arc 



largely made up of fluid, their compressibility is i asiderable. The elasticity 



of muscles is slight but quite perfect, by which is meant that a muscle yields 

 readily to a Btretching force, hut on the removal of the force quickly recovers 



its normal form. Most of the experiments Upon muscle elasticity have been 

 made after the muscle had been removed from the body, hence under abnormal 



1 fiber den Ursprung </<•/■ Muskdkrafi, Leipzig, 1893. 



