108 PRACTICAL PHYSIOLOGY 



recorded. Again, electrical stimulation of a nerve or a muscle can 

 be a much more powerful stimulus than that resulting from the 

 maximal discharge of a motor nerve-cell ; consequently the fact that 

 peripheral stimulation can make the muscle again lift the weight 

 after voluntary impulses fail, is no proof that the fatigue was central. 

 Further, when a nerve or muscle is stimulated by electrodes placed 

 upon the skin, it is impossible to produce equal stimulation of all 

 fibres. When the muscle appears to be fatigued by peripheral 

 stimulation, then a return to volitional stimulation, by producing 

 equal stimulation of every fibre, leads to an apparent recovery of 

 voluntary power. In this way is to be explained the apparent 

 paradox, that a muscle fatigued by either voluntary or peripheral 

 stimulation shows a recovery of power when stimulated in the 

 opposite way. 



In order to investigate this subject we shall compare the curve of 

 voluntary fatigue taken with a spring ergograph from the human 

 abductor indicis, with the curve obtained from the frog's gastro- 

 cnemius, with its circulation intact and stimulated through the 

 sciatic nerve. 



Porter's Spring Ergograph. A simple form of this instrument is 

 shown in Fig. 44 to consist of a rigid upright iron bar which is 

 clamped to the table. From the upper end of this projects a 

 horizontal straight steel spring, the free end of which carries an 

 ordinary writing point. The spring carries on its under side a short 

 vertical steel arm, the lower end of which fits over the distal end of 

 the second phalanx of the index finger. When the abductor indicis 

 contracts the spring is pushed up ; by sliding the vertical arm along 

 the spring the magnification of the movement and the strength of 

 the spring can be altered. The hand is placed along the vertical 

 side of the wooden support and the three outer fingers tied to it, 

 leaving the thumb and index finger free. The forearm should be 

 fixed to the bench in some form of support, but care must be taken 

 not to tie down the arm sufficiently tightly to interfere with its 

 circulation. 



The subject of the experiment should sit comfortably and with his 

 eyes shut, should not be spoken to nor in any way have his attention 

 diverted, but should confine himself to giving a maximal contraction 

 of his muscle every time he hears the beat of a metronome, which 

 is set to give a beat every second. The observer takes the time of 

 the experiment in minutes and so calculates the number of con- 

 tractions recorded ; further, he has to see that the vertical arm does 

 not slip out of position along the finger. In this way take 300 to 

 600 contractions on a drum revolving at an extremely low rate 

 (Fig. 106). 



At first sight the most striking feature of the curve is the more or 

 less rhythmical waxing and waning in the height of the contractions ; 

 this seems to be purely central in origin and to be due to variations in 

 the strength of the voluntary impulse communicated to the muscle. 

 Practice to a large extent does away with this rhythm. When the 



