THE BRAIN 



731 



varying according to the extent of the lesion. Sometimes the impair- 

 ment of intelligence is very marked ; in other cases, as in the so-called 

 cases of " motor " aphasia, it is but little affected. If the auditory 

 area be destroyed, there results a loss of spoken word appreciation 

 (sensory aphasia), with great diminution of mental powers. If the 

 occipital lobe be affected, there will result alexia, or word blindness 

 an inability to appreciate the meaning of written words. 



It will be seen that it is improbable that there is any localized 

 speech centre, as suggested by Broca. Disorders of speech are due 

 to local lesions, bringing about a loss of continuity between the cortical 

 centres concerned in the power of speech. " Motor " aphasia, there- 

 fore, is probably not due to a lesion of Broca's area, but principally 

 to a lesion of the lenticular nucleus, causing an inability to articulate, 

 often combined with a lesion of the sensory area of Wernicke. 



* -I 



Fio. 446. DIAGRAM OF SPEECH CENTRES. (Aiter Bramwell, from Purves Stewart's 

 "Diagnosis of Nervous Diseases.") 



Similarly, in the power to read and to clothe thoughts in writing 

 there is involved a number of centres connected by association fibres 

 (see Fig. 446). 



Reaction Time. Various methods have been devised for measuring 

 the '' reflex time," or, as it is usually termed, the " reaction time," 

 of conscious processes, such as sight, hearing, the response being by 

 movement or speech, etc. The subject is usually told to perform 

 some movement, recorded on a drum, such as to open an electric key 

 or to depress a lever, directly he receives the stimulus, which is also 

 recorded e.g., the ringing of a bell, the appearance of a coloured disc. 

 The " reaction time " is made up of the time taken in the conduction of 

 the impulse to and from the brain, and of the processes in the brain. 

 From the knowledge of nerve conduction and of the reflex times of 

 simple arcs we can measure the reaction time of more complicated 

 cerebral processes. The more complicated the process, the longer 

 the time. Thus, if a man has to move his right hand when a red disc 



