THE PHENOMENON OF CONTRACTION. 45 



individual under different circumstances. Von Kries,* for instance, 

 states that the rate of stimulation in voluntary movements may 

 vary according to the character of the movement. In slow, sus- 

 tained movements the rate is from 8 to 12 per second, while in 

 short, sharp, rhythmical movements of the fingers the rate may be 

 as rapid as 40 per second. The fact that movements of this latter 

 character the trilling movements of the fingers of the pianist, 

 for instance may last for only T ^ of a second or less, is considered 

 by some authors as a proof that they are not tetanic contractions, 

 .and that therefore we can voluntarily make either long-continued 

 tetanic contractions or quick, simple contractions. Von Kries 

 has shown, however, that when these quick, rhythmical movements 

 of the fingers are recorded the curves, even of such brief contractions, 

 show that they are short-lasting tetani. It is the usual belief, 

 therefore, that all voluntary movements are tetanic in character 

 and that it is not possible for us, by a so-called act of the will, to 

 cause a simple contraction, that is, to cause the motor nerve cells 

 to discharge a single motor impulse. This general conclusion is sup- 

 ported by the results of artificial stimulation of the motor regions 

 of the brain. In experiments of this kind made by Horsley and 

 Schafer it was shown that, at whatever rate the stimulus might 

 be applied to the motor cells, they responded by motor discharges 

 of about 10 per second, so far as this could be determined from 

 the contractions of the muscle. The interesting conclusion from 

 the whole discussion, therefore, is that our motor centers, under 

 the stimulus of the will, discharge motor impulses at a certain low 

 rate, which, while somewhat variable, averages in ordinary move- 

 ments about 10 per second. 



The Ergograph. Voluntary contractions in man may be re- 

 corded in a great many ways, but Mosso has devised a special in- 

 strument for this purpose, known as the ergograph. It has been 

 much used in quantitative investigations upon muscular work 

 and the conditions influencing it. The apparatus is shown and 

 described in Fig. 22. The person experimented upon makes a 

 series of short contractions of the flexor muscle of the middle 

 finger, thereby lifting a known weight to a definite height 

 which is recorded upon a drum. In a set of experiments the 

 rate of the series of contractions that is, the interval of rest 

 between the contractions is kept constant, as also is the load lifted. 

 Under these conditions the contractions become less and less ex- 

 tensive as fatigue comes on, and finally, with the strongest voluntary 

 effort, the contraction of the muscles is insufficient to lift the weight. 

 In this way a record is obtained such as is shown in Fig. 23 

 In such a record we can easily calculate the total work done by 

 * Von Kries, " Archiv fur Physiologic/' suppl. volume, 1886, p. 1. 



