126 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



contractions of voluntary muscles are tetanic in character, there is a great deal 

 which remains to be explained. 



Effect of Artificial compared with Normal Stimulation. Experiment shows 

 that, with the same strength of irritant, a muscle contracts more vigorously 

 when irritated indirectly, through its nerve, than when it is directly stimulated. 

 Rosenthal describes the following experiment : If the nerve of muscle A be 

 allowed to rest on a curarized muscle B, and an electric shock be applied in 

 such a way as to excite nerve A and muscle B to the same amount, muscle A 

 will be found to contract more than muscle B. 



Further, it has been found that muscles respond more vigorously to volun- 

 tary excitations than to any artificial stimulus which can be applied to either 

 the nerve or muscle. This shows itself, not only in the fact that a muscle can 

 by voluntary stimulation lift much larger weights than by electrical excitation, 

 but that after a human muscle has been fatigued by electrical excitations it 

 can still respond vigorously to the will. An illustration of this is given in 

 Figure 54. 



FIG. 54. Voluntary excitations are more effective than electrical. The flexor muscles of the second 

 finger of the left hand of a man were excited first voluntarily, a, then electrically, a-6, and then volun- 

 tarily, 6. The electrical excitation consisted of series of induction shocks, which were applied once 

 every two seconds, during about half a second, the spring interrupter of the induction coil vibrating 

 23 times per second. Each time the muscle contracted it raised a weight of one kilogram. Each of the 

 contractions recorded, whether the result of electrical or voluntary excitation, was a short tetanus. 



Fatigue of Voluntary Muscular Contractions. Mosso and his pupils have 

 done a large amount of work upon the fatigue of human muscles when excited 

 by voluntary and artificial stimuli under varying conditions. The results at 

 which they arrived all favor the view that human muscles differ but little from 

 those of warm-blooded animals, and that the facts which have been ascertained 

 by experiments upon cold-blooded animals, such as the frog, can be accepted 

 with but slight modifications for the muscles of man. In the experiment 

 recorded in Figure 55 we see the effect of repeated tetanic contractions, excited 

 by electricity, to fatigue a human muscle. Normal voluntary contractions, if 

 frequently repeated, provided the muscle has to raise a considerable weight, 

 likewise cause fatigue. 



It is doubtful whether, in an experiment such as is shown in Figure 55, the 

 loss of the power to raise the weight is due to fatigue of the muscles. It is 

 more likely that the decline in power is really due to fatigue of the central 



