50 HITMAN PHYSIOLOGY. 



the muscle as a whole becomes shorter and thicker. The degree of short- 

 ening may amount to thirty per cent, of the original length. 



2. Volume. The increase in transverse diameter does not fully compensate 

 for the dimunition in length, for there is at the moment of contraction a 

 slight shrinkage in volume, which has been attributed to a compression of 

 air in its interstices. 



3. Optic Changes. If a muscle-fiber be examined microscopically during its 

 contraction, it will be observed that when the contraction wave begins, 

 both bright and dim bands diminish in height and become broader, though 

 this change is more noticeable in the region of the bright band. This 

 Englemann attributes to a passage of fluid material from the bright into 

 the dim band. At the time of relaxation there is a return of this material, 

 and the fiber assumes its orignal shape and volume. As the contraction 

 wave reaches its maximum, the optic properties of both the isotropic and 

 anisotropic bands change. The former, which was originally clear, now 

 becomes darker and less transparent, until at the crest of the wave it as- 

 sumes the appearance of a distinct dark band. The latter, the anisotropic, 

 which was originally dim, now becomes, in comparison, clear and light. 

 This change in optic appearance is due to an increase in ref rangibility of the 

 isotropic and a decrease in the anisotropic bands coincident with the passage 

 of fluid from the former into the latter. There is at the height of the 

 contraction a complete reversal in the positions of the striations. At a 

 certain stage between the beginning and the crest of the wave there is an 

 intermediate point, at which the striae almost entirely disappear, giving to 

 the fiber an appearance of homogeneity. There is, however, no change in 

 refractive power, as shown by the polarizing apparatus. After the con- 

 traction wave has reached the stage of greatest intensity, there is a reversal 

 of the foregoing phenomena, and the fiber returns to its original condition, 

 which is one of relaxation. 



Physical Changes. The extensibility of muscle is increased during the con- 

 tracton, the same weight elongating the fibers to a greater extent than during 

 rest. The elasticity, or its power of returning to its original form is corres- 

 pondingly diminished. 



Chemic Changes. The metabolism of muscle during the contraction is 

 very active. There is an increase in the production of carbon dioxid and in 

 the absorption of oxygen. The muscle changes from an alkaline or neutral 

 to an acid reaction, from the development of sarcolactic acid. The muscle 

 also becomes warmer. The electric changes will be treated of in connection 

 with nerves. 



