562 



PHYSIOLOGY 



CHAP. 



portion of the centrum ovale in the same hemisphere, or better 

 in symmetrical regions of the two hemispheres. Fig. 284 is a 

 tracing showing the intervals between the moment at which the 

 current passes and the moment at which the reaction commences 

 on exciting the cortex and the centrum ovale. As- will be noted, 

 the difference is not insignificant; in this instance it amounts to 

 y-^ sec., but in other cases it may attain y-g-^ sec. Bubnoff and 

 Heidenhain, who operated on dogs under morphia, even obtained a 

 difference of Vrnrtr 8ec - 



Tins marked delay in response when the cortex is excited 

 shows that the cortical grey matter is not merely a passive inert 

 conductor to the subjacent white matter. It receives the stimulus, 



FIG. 285. Tracing <>t'a voluntary contraction of the opponens pollicis taken at a known vdncily 

 of the recording cylinder. (Schafer.) Shows the elementary vibrations that make up the 

 contraction. 



elaborates it, and enters into the active physiological state known 

 as excitation, which Pfliiger proved to occur on the direct or reflex 

 excitation of the grey matter of the spinal cord. 



To obtain a clear idea of the active state or physiological ex- 

 citation of the cortex, it is useful to compare the character of the 

 muscular contractions evoked by the voluntary impulse with those 

 produced by electrical excitation of the cortex. 



On recording the voluntary contraction of any muscle (e.g. the 

 opponens muscle of the thumb), by some suitable myographic 

 method, the resulting curve shows undulations which are fairly 

 regular as to rhythm, though irregular in amplitude, with a 

 frequency of 10-12 per second (Fig. 285). Horsley and Schafer 

 showed this variation to be fairly constant in the same individual, 

 but variable in different subjects (from 8 to 13 contractions per 

 second), provided the resistance the muscle encounters in contract- 



