104 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



particles of the anisotropic substance, these particles having a definite arrange- 

 ment. Striated muscle-fibres present not only cross markings, but under 

 favorable conditions longitudinal striations, these being most evident in the 

 dark bands. These longitudinal striatious are looked upon with great interest 

 as indicating that the particles of anisotropic material are arranged in long 

 chains as incomplete fibrilla?. According to this view the muscle-fibre is com- 

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

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

 and so placed as to give the effect of transverse disks (Z, n, Q, Fig. 36). 



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 (, Fig. 36, C) 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. 36, D), the original dark bands have less and the lighter bands greater 

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

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

 of the sarcoplasm by the anisotropic substance ; the cause of the imbibition is 

 the liberation of heat by chemical changes which occur at the instant the 

 muscle is excited. Engelmann l has shown that dead substance containing 

 anisotropic material, such as a catgut string, can change its form, by imbi- 

 bition 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 as a whole is the result of 

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

 tions which influence the activity of the process of contraction must 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 is 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 be altered by 

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

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

 1 Ueber den Ursprung der Muskelkraft, Leipzig, 1893. 



