THE PHENOMENON OF CONTRACTION. 47 



muscles of the arms or fingers the series of electrical variations fol- 

 lows at the rate of 47 to 50 per second. Increase in strength of con- 

 traction in these muscles causes no change in rate, although a cor- 

 responding variation in the intensity of the electrical changes is 

 observed. When different muscles are studied by this method, 

 quite a marked difference in rate is obtained. Piper reports such 

 observations as the following: M. deltoideus, 58 to 62; M. gas- 

 trocnemius and M. tibialis anterior, 42 to 44; M. quadriceps 

 femoris, 38 to 41; M. masseter, 88 to 100, and M. temporalis, 80 

 to 86. Assuming that these figures represent the rate of dis- 

 charge of nerve impulses per second by the nerve cells from 

 which arise the motor fibers to the muscles named, it is evident 

 that the various spinal and cranial motor centers may possess 

 quite widely different rhythms, although for each particular 

 center the rate is more or less fixed. Among the motor centers 

 thus far studied it will be noted that the cells of the N. trigeminus 

 possess the highest rate of discharge. There has been much 

 discussion as to whether or not we can obtain simple as well as 

 compound contractions by voluntary stimulation of our muscles. 

 It has been pointed out that in very rapid contractions, such as 

 occur in the trilling movements of the fingers in playing the 

 piano, the duration of the separate contractions is so brief as to 

 suggest that they may be of the order of simple contractions. 

 Direct investigation of such movements by the older method 

 of recording with levers (von Kries) or by the newer method of 

 photographing the electrical oscillations shows, on the contrary, 

 that even the shortest possible voluntary contractions are brief 

 tetani made up of a short lasting series of contractions fused 

 together. In all probability, therefore, our motor centers, when- 

 ever they are stimulated by a so-called act of the will, discharge 

 rhythmically a series of nerve impulses. As we shall see later, 

 it is possible that certain of these centers, when stimulated 

 reflexly, may discharge a single nerve impulse and thus arouse 

 a simple muscular contraction (see Knee- kick). 



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. 23. 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. 



