FACTORS VARYING THE CHARACTER OF THE CONTRACTION 81 



together with monopotassium phosphate and certain toxins, are said 

 to be responsible for the development of fatigue in muscle. They 

 are spoken of collectively as the fatigue substances. 



The phenomena of fatigue are also exhibited by human muscle 

 when subjected to excessive stimulation. We then become cognizant 

 of a peculiar strained f eeUng and eventually also of pain which prevents 

 us from continuing these efforts. It appears that these sensations are 

 the direct result of an irritation of the muscle-spindles and of the cor- 



'ftstC 



Fig. 47. — Fatigue of Muscle. 

 A gastrocnemius muscle of the frog stimulated successively 150 times. 

 1st, 50th, 100th, and 150th contractions are recorded. 



The 



responding receptors in the tendons and joints. Under ordinary con- 

 ditions the tests upon human muscles require the use of an instrument, 

 which is known as the ergograph. The one devised by Mosso^ consists 

 of a support for the arm and a weight which acts in a sliding path or 

 across a pulley and is connected with the tip of one of the fingers, 

 preferably the index finger of the right hand. A spring ergograph, 

 or dynamograph, has been devised by Waller. It consists of a strong 



Fig. 48. — Fatigue Curves of Frog's Muscle. (Waller,) 



oval steel spring which is compressed by the hand, while a pointer is 

 moved across a graduated scale to indicate the degree of compression 

 as well as the power of the group of muscles used in this act. In either 

 method, the displacement of the weight or of the spring may be 

 registered upon a kymograph by means of a writing lever, the resulting 

 record being known as an ergogram. 



The fatigue of human muscle may be illustrated either by recording a 

 series of voluntary twitches or a long-continued tetanus of, say, the muse. 



1 Arch. ital. de biologie, xiii, 1890; also see: Treves, ibid., xxix, xxx, and xxxi, 

 1898-1900, and Schenck, Pfiuger's Archiv, Ixxxii, 1902. 



