GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 



129 



a myograph, a contraction excited by a single induction shock will show a rise 

 as rapid and as high as normal, but the fall of the curve will be greatly pro- 

 longed (see Fig. 57). 



Often the crest of the curve will exhibit a notch, which shows that relaxa- 

 tion may begin and be checked by a second contraction process which carries 

 the curve up again and holds it there for a considerable time. In the above 

 experiment the contracture effect followed the primary contraction immediately. 



If the muscle be frequently excited, the characteristic prolongation of the 

 contraction disappears, and the curve becomes normal ; but if the muscle be 

 allowed to rest, there is a return of the condition. Both high and low tempera- 

 tures act like exercise to prevent this peculiar effect of veratria from showing 

 itself. 



Barium salts, and to a less degree calcium and strontium, act similarly to 

 veratria to prolong the relaxation of the muscle without lessening the rapidity 

 and height of the contraction. Potassium and ammonium salts act to kill the 

 muscle, and, as the death-process develops, excitation produces prolonged local- 

 ized contractions. This effect seems to be quite different from that of veratria, 

 being accompanied by a rapid lessening of the power of the muscle. Sodium 

 salts in strong solution may increase the irritability and induce fatigue, which 

 is always accompanied by a prolongation of the curve of relaxation. 



The condition of continued contraction caused by veratria is a form of 

 " contracture." The true nature of the condition is still under discussion; 

 the fact that the veratria contracture passes off if the muscle is worked, shows 

 that it is not in the nature of a fatigue effect. Since more heat is produced 

 during contracture than during rest (Fick and Boehme), it is to be regarded as 

 an active contraction process and not an increase of elasticity. The fact 

 that the crest of the veratria curve often exhibits a notch, and that the second 

 rise, leading to the prolonged ridge, may be higher than the primary rise, has 

 been interpreted to mean that the muscle contains two different forms of muscle- 

 tissue which, like the pale (rapid) and red (slower) striated muscles of the rab- 

 bit, have different rates of contraction. The first rise is supposed to be due to 

 the quicker and the second to the slower form of muscle. A similar double 

 crest is seen in the contraction curves of muscles the irritability of which 

 has been heightened by sodium carbonate, and indeed in the curves from 

 muscles of normal frogs after their irritability has been increased by frequent 

 excitations. 



Liberation of Energy by the Contracting- Muscle. The law of con- 

 servation of energy applies no less to the living body than to the inanimate 

 world in which it dwells. Every manifestation of life is the result of the 

 liberation of energy which was stored in the body in the form of chemical 

 compounds. When a muscle is excited to action it undergoes chemical 

 changes, which are accompanied by the conversion of potential to kinetic en- 

 ergy. This active energy leaves the muscle in part as thermal energy, in part 

 as mechanical energy, and, to a slight extent, under certain conditions, as elec- 

 trical energy. In general, the sum of the liberated energy is given off as heat 



9 



