TEXT-BOOK OF PHYSIOLOGY. 



by the induced currents may be made to succeed one another so rapidly that 

 they become fused together, producing a spasm or tetanus of the muscle. 

 The rapidity with which the induced current appears and disappears, its 

 brief duration, the ease with which its strength can be regulated, combine 

 to render it a most efficient stimulus for either muscle or nerve. 



PHENOMENA FOLLOWING A MUSCLE STIMULATION. 

 PHYSICAL PHENOMENA. 



Physiologic investigation has made it apparent that when a nerve impulse 

 reaches a muscle, it occasions a disruption of certain complex energy- 

 holding compounds and their subsequent oxidation to simpler compounds. 

 Coincidently with the chemic changes there is a transformation of the 

 potential energy of the molecules into kinetic energy which manifests 

 itself under three forms, heat, electricity and mechanic motion, or a change 

 of shape of the muscle. These phenomena vary in extent in accordance with 

 the intensity of the impulse as well as the frequency of its repetition. 

 Though the chemic changes are the first effects of the action of the nerve 



FIG. 21. SHOWING THE CHANGES IN A MUSCLE AND MUSCLE-FIBER DURING 



CONTRACTION. 



impulse and the ones on which other phenomena depend, it will be found 

 convenient to consider the most evident effect, the physical change in the 

 shape of the muscle, first. 



Change of Shape. The most obvious change in a muscle following the 

 arrival of a nerve impulse is that relating to its form. The muscle not only 

 becomes shorter, but at the same time thicker. The extent to which it may 

 shorten when unopposed may amount to 30 per cent, or more of its original 

 length. The increase in thickness practically compensates for the diminu- 

 tion in length, for there is no observable diminution in volume. The change 

 in form of the entire muscle results from a corresponding change of form of 

 its individual fibers as determined by microscopic examination, each of 

 which becomes shorter and thicker. The successive changes in both the 

 muscle and the individual fibers are represented in Fig. 21. 



When the contraction begins both the dim and bright bands diminish in 

 length, but at the same time increase in breadth. This continues until the 

 contraction reaches its maximum. The diminution in the length of the 



