CHAPTER IX 



DETERMINATION OF THE LATENT PERIOD 



Difficulties of accurate determination of Latent Period — Advantages of 

 Resonant Recorder — Simultaneous tracings of tuning-fork exciter 

 and Resonant Recorder — Automatic stimulation at a definite 

 moment — Identical value of latent period in successive determina- 

 tions — Accurate measurement of time-interval shorter than '005 

 second — Latent period little affected by inertia of recorder — Tabular 

 statement of value of different specimens of Mimosa — Effect of 

 season on latent period. 



When the motile pulvinus of Mimosa is subjected to an 

 exciting shock, a short time elapses between the incidence 

 of this shock and the initiation of the responsive movement. 

 This short interval is known as the Latent Period. In a 

 responding muscle, similarly, contraction does not occur 

 instantaneously on the application of stimulus. The latent 

 period in this case is determined from the record of the 

 muscle-twitch. When after the appHcation of stimulus the 

 muscle has not yet begun to contract, the record appears 

 as a straight line. Then on the commencement of con- 

 traction, the recording-lever is jerked up and the curve 

 likewise bends upwards. The length of the straight portion 

 of the record, between a mark that represents the incidence 

 of the shock and the flexure at the initiation of response, 

 gives us the duration of the latent period. We have, 

 however, to determine the time-value of this length. This 

 is done by means of a sinuous curve drawn below the record 

 by a tuning-fork, vibrating 100 or 200 times in a second. 

 Experimenting in this manner, the latent period for frog's 

 muscle has been determined at about "oi second. 



There are still several difficulties to be encountered 

 in making this determination with any great exactness. 



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