188 



COMPARATIVE PHYSIOLOGY. 



seconds, but that presently the arm begins to tremble or vi- 

 brate, and soon the weight must be dropped. The arm was 

 maintained in its position by the joint contraction of several mus- 

 cles, the action of which might be described (traced) by a writer 

 attached to the hand and recording on a moving surface. Such 

 a record would indicate roughly what had happened ; but the 

 exact nature of a muscular contraction in such a case can best be 

 learned by laying bare a single muscle, say in the thigh of a 

 frog, and arranging the experiment so that a graphic record 

 shall be made. 



Using the apparatus previously described (Fig. 173), a series 

 of induction shocks may be sent into the muscle with the result 

 indicated in Figs. 176 and 177, according to the rate of interrup- 

 tion of the current. 



^^^^^^^^^/^^i/uvm/vvw-wwwv/-. 



Fig. 176. — Curve of imperfect tetanic contraction (Foster). Uppermost tracing indi- 

 cates contractions of muscle; intermediate, when tlie shocks were given;lower, 

 time-marliings of intervals of one second. Curve to be read, like others, from left 

 to right, and illustrates at the end a ''contraction remainder." 



If the stimuli follow each other with a certain rapidity, such 

 a tracing as that represented in Fig. 176 is obtained; and if the 

 rapidity of the stimulation exceeds a fixed rate, the result is that 

 seen in Fig. 177. 



P'iG. 177.— Curve of complete tetanic contraction (Foster). 



