FUNCTIONS OF THE CEREBRAL HEMISPHERES 457 



grouping by auditory impressions. If the whole of the auditory associa- 

 tions be destroyed by an extensive lesion in the first and second temporal 

 convolutions, the resulting loss of word appreciation, sensory aphasia, will 

 be attended with great diminution of mental powers. It must be remem- 

 bered that the area of Werhicke is not a sensory centre, but a centre of 

 association between the various sense-impressions, especially those of 

 hearing and sight. It may therefore be spoken of as an intellectual centre. 

 Pure motor aphasia of course exists, but is always anarthria and is due to a 

 lesion in the lenticular zone, i.e. in the lenticular nucleus and its neighbour- 

 hood, in the anterior part and the genu of the internal capsule, and possibly 

 in the external capsule. 



It is important to make a distinction between loss of sanity and loss 

 of intellectual powers. The essential factor of sensory aphasia is the exist- 

 ence of intellectual impairment, though in his behaviour the patient may 

 appear perfectly normal. On the other hand, in insanity there may be 

 perfect retention of the intellectual processes, which depend on the proper 

 working of the lower association centres. The personality of the individual, 

 and therefore finally his behaviour, involves a further association on a higher 

 plane of these intellectual processes and therefore control in accordance with 

 the relation, past, present, or future, of the individual to his environment. 

 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 (sube volution 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 frontal 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 : 



