FUNCTIONS OF THE CEREBRAL HEMISPHERES 457 



The prefrontal region is in all probability the seat of this highest plane of 

 association. Insanity always involves alteration of personality and depends 

 on failure of development or on disintegration processes (subevolution or 

 dissolution of this region) (Fig. 235). In monkeys and cats Franz has found 

 that destruction of the frontal lobes causes a loss of recently formed habits. 

 He concludes from his experiments that the fro/atal lobes are the means by 

 which we are able to learn and to form habits, i e. to regulate our behaviour 

 in accordance with the needs of our position in society. 



THE TIME RELATIONS OF CENTRAL NEURAL REACTIONS 

 In the spinal animal a stimulus of any particular quality and localisation 

 always evokes an appropriate reaction. A certain period of time necessarily 

 elapses between the moment at which the stimulus is applied and the moment 

 at which the resulting reaction takes place. This interval is spoken of as 

 the simple reaction time, and in the spinal animal is entirely independent 

 of consciousness. Many reactions, even in the intact animal, are also, as we 

 may say, involuntary and are not modified perceptibly by our consciousness 

 of their occurrence : such reflexes as the withdrawal of the hand when it 

 comes in contact with a hot surface, the shutting of the eyelid when the con- 

 junctiva is touched, the drawing up of the leg when the sole of the foot is 

 tickled. Not only are these carried out in the absence of voluntary impulses, 

 but in many cases it is almost, if not quite, impossible to check the reaction 

 by any effort of the will. 



When the leg is drawn up in response to a painful or nocuous stimulus 

 applied to the foot, a certain amount of time is involved in each of the 

 following links in the chain of processes which determine the reaction : 



(1) The conversion in the peripheral sense-organ of the mechanical 

 stimulus into a nerve process. 



(2) The passage of a nerve impulse up the nerve from the end organ to the 

 spinal cord. 



(3) The passage of the impulse across two or more synapses in the grey 

 matter of the cord. 



(4) The passage of the impulse down the motor nerve fibres from the 

 spinal cord to the muscles. 



(5) The processes occurring in the end-organs of the muscle. 



(6) The latent period in the muscle fibre itself. 



With a weak stimulus No. 1 is impossible to measure. With a strong 

 stimulus it may be so short as to be practically negligible. (2), (4), (5), 

 and (6) represent quantities for the measurement of which we have all the 

 necessary data. 



In any given reflex therefore we may add these periods together and 

 subtract them from the total reaction time ; we thus get a ' reduced re- 

 action time,' which represents the time involved in the passage of the impulse 

 through the central nervous system, and in the conversion of an afferent 

 impulse into an aggregate of co-ordinated motor impulses. It is found 

 that the reduced reaction time accounts for the greater part of the total 



15* 



