204 CENTRAL NERVOUS SYSTEM 



Time Involved in Nervous Processes. Whenever a nerve 

 impulse passes from one neuron to another, there is a delay in 

 its transmission. In the frog, for example, it takes twice as 

 long for an impulse to pass from the middle of the cerebral 

 hemispheres to the optic lobes as from the bulb to the lumbar 

 enlargement, although the distance is much less. If one eyelid 

 be stimulated electrically, both lids wink. The total time 

 required for this reflex is from 0.066 to 0.057 second. Deduct- 

 ing the time required for the impulse to pass to and from the 

 brain along the fifth and seventh nerves, and also the latent 

 period of the orbicularis muscle, there remains 0.05 to 0.04 

 second for the time required in the brain. This time value 

 may be regarded as an average of simple reflex actions. The 

 individual values vary greatly. In general the time is shorter 

 the stronger the stimulus. It is shorter when the reflex is con- 

 fined to one side of the body than in cases where the impulse 

 crosses to the other side, even if allowance is made for the 

 difference in the length of the nervous path. The time depends, 

 of course, on the condition of the cord, becoming greater during 

 exhaustion and disease. The time becomes longer the greater 

 the number of neurons involved. When the afferent impulse 

 arouses sensation and consciousness, and the ensuing response 

 is the result of a volitional effort, then the neurons involved 

 are indefinite in number. The action is now a reaction, and the 

 time is known as the reaction time. This time is shorter when 

 the right hand makes a response to a stimulus to the left than 

 when the response is to either auditory or visual sensations. 

 But the shortest reaction time follows a visual excitation pro- 

 duced by direct electrical stimulation of the retina. Roughly, 

 reaction times for cutaneous sensations are one-seventh of a 

 second; for auditory sensations, one-sixth of a second; for visual 

 sensations, one-fifth of a second. 



A reaction period consists of three stages, corresponding 

 to the times required by the impulse on its afferent path, its 

 efferent path, and in the central nervous system. All three 

 stages may vary independently. The time involved in the 

 central stage is known as the reduced reaction time. It varies in 

 different persons, and is known as the personal equation. When 

 the subject is required to react to one of two or more possible 

 signals, the reaction time may be prolonged from 0.016 to 0.062 



