178 



COMPARATIVE PHYSIOLOGY. 



and is provided with a recording apparatus that does not cause 

 enough friction to interfere with its vibrations. 



Fig. 162 illustrates one arrangement that answers these con- 

 ditions fairly well. 



The marker, or chronograph, in the more limited sense, is 

 kept in automatic action by the fork interrupting the current 

 from a battery at a certain definite, rate answering to its own 

 proper note. 



Marey's chronograph, which is represented at h above, and 

 in more detail below, in Fig. 163, consists of two electro-mag- 

 nets armed with keepers, between which is the writer, which 





JV "7*- 



9'... .,■■-..■ . ^ 



- I .' , . . . ■ J 



Fig. 163.— Side view of Marey's chronograph (after McKendrick). a, a, coile of Wire; 

 b, b, keepers of electro-magnets; c, vibrating style fixed to the steel plate e; d, 

 binding screws for attachment of wires; + from interrupting tnning-fork; - to 

 the battery. 



has a little mass of steel attached to it, the whole working in 

 unison with the tuning-fork, so that an interruption of the cur- 

 rent implies a like change of position of the writing-style, which 

 is always kept in contact with the recording surface. 



Fig. 173 shows the arrangements for recording a single 



muscle contraction, and 

 Fig. 174 the character of 

 the tracing obtained. 



A muscle-nerve prepa- 

 ration, which usually con- 

 sists of the gastrocnemius 

 of the frog with the sciatic 

 nerve attached, clamped by 

 a portion of the femur cut 

 off with the muscle, is 

 made, on stimulation, to 



I^G. 164. —Muscle-nerve preparation, showing 

 gastrocnemina muscle, sciatic nerve, and 

 poiticw of femur of frog, for attachment 

 to a vse (after Rosenthal). 



