94 PHYSICAL EXPRESSION. 



sources of error. Exner found that when he used 

 a visual stimulus, viz. a flash of light, the time was 

 not only exceedingly prolonged, "2168 sec., but very 

 variable. 



"The time required for any reflex act varies, 

 according to Rosenthal, very considerably with the 

 strength of the stimulus employed, being less for 

 the strong stimuli, is greater in transverse than in 

 longitudinal conduction, and is much increased by 

 exhaustion of the cord. It has been stated that the 

 central processes of a reflex action are propagated 

 in the frog at the rate of about eight metres a 

 second; but this value cannot be depended on. The 

 time thus occupied by purely reflex actions must not 

 be confounded with the interval required for mental 

 operations; of the latter we shall speak presently. 



"Inhibition of Reflex Action* When the brain 

 of a frog is removed, reflex actions are developed to 

 a much greater degree than in the entire animal. 

 We ourselves are conscious of being able by an 

 effort of the will to stop reflex actions, such, for 

 instance, as are induced by tickling. There must, 

 therefore, be in the brain some mechanism or other 

 for preventing the normal development of the 

 spinal reflex actions. And we learn by experiment 

 that stimulation of certain parts of the brain has 

 a remarkable effect on reflex action. In a frog, from 

 which the cerebral hemispheres only have been 

 removed, the optic thalami, optic lobes, medulla 

 oblongata, and spinal cord being left intact, a 

 certain average time will be found to elapse 

 * Foster, op. cit., p. 474. 



