56 HUMAN PHYSIOLOGY. 



stimuli, and produce single and multiple contractions. Physical 

 agents, as heat and electricity, also act as stimuli. A muscle heated 

 rapidly to 30 C. contracts vigorously, and reaches its maximum at 

 45" C. Of all forms of stimuli, the electric is the most generally 

 used. Two forms are used the induced current and the make-and- 

 break of a constant current. 



Changes in a Muscle During Contraction. When a muscle is 

 stimulated, either indirectly through the nerve or directly by any 

 external agent, it undergoes a series of changes, which relate to its 

 form, volume, optic, physical, chemic, and electric properties. These 

 changes, in their totality, constitute the muscular contraction. 



1. Form. The most obvious change is that of form. The fibers 

 become shorter in their longitudinal and wider in their transverse 

 diameters, and the muscle as a whole becomes shorter and thicker. 

 The degree of shortening may amount to thirty per cent, of the 

 original length. 



2. Volume. The increase in transverse diameter does not fully com- 

 pensate for the diminution 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 contrac- 

 tion 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 as- 

 sumes its original 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 assumes 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 refrangibility of the isotropic and a de- 

 crease 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 

 these is an intermediate point, at which the striae almost entirely 



