DETERMINATION OF THE LATENT PERIOD 109 



As the muscle-record and the time-record are separate, 

 certain error is likely to be introduced in inferring the 

 time- value of any point on the muscle-curve (fig. 64). 

 This error becomes relatively serious when the total time 

 to be measured is very small. There is, again, the difficulty 

 of exactly determining the point of flexure which represents 

 the beginning of mechanical response. More troublesome 

 still is the error due to the inertia of the recording-lever. 

 On account of this and the mechanical inertia of the respond- 

 ing muscle itself, the latent 

 period thus obtained appears 

 somewhat in excess of the 

 true value. 



In the apparatus which I 

 employed, these difficulties 

 have been reduced to a 

 minimum. In the first 

 place, the curve of response 

 or phytogram is at the same 

 time a chronogram. The 

 error which might arise 

 from an inference based on 

 a neighbouring time-record 

 is thus eliminated. I will later explain also the means 

 that make it possible to determine the point of flexure, 

 representing the beginning of the responsive movement, 

 with relative accuracy. And lastly, the error due to the 

 inertia of the recording part of the apparatus is reduced 

 to a minimum by making the writing-lever excessively 

 light. In the muscle recorders the weight of the recording- 

 lever is about 3-5 grams. The lever which I employ weighs 

 only '04 gram. The recording part of my apparatus is thus 

 nearly a hundred times lighter than that used for muscle 

 records. 



The accuracy of the time-record when made by the 

 response recorder itself may be gauged from records giving 

 simultaneous tracings of the exciting standard tuning-fork 



Fig. 64. — Latent period of hyoglos- 

 sus muscle : aa, moment of 

 stimulation ; ab, latent period. 

 Time tracing 200 per second. 

 (Brodie.)j 



