1907.] on Dexterity and the Bend Sinister. 645 



centres in the brain for vision, hearing, taste, and touch, and that 

 there is a great motor area in the middle of the brain in which are 

 represented all the voluntary movements of the body. And we know 

 further — and that is the significant point for us — that in that motor 

 area of the cerebral cortex there is a region distinctly differentiated 

 into centres, each of which stands in connection with a particular 

 group of muscles, or with the movements produced by these muscles 

 in the upper hmb (Fig. 28). We have here the centres for move- 

 ments of the shoulder, elbow, hand, thumb and fingers, centres stimu- 

 lation of which by electricity in the exposed brain of the monkey or 

 by disease in man, produces movements in the muscles subtending 

 them, and destruction of which, experimentally in animals or by 

 disease in man is followed by paralysis of these muscles. 



Xow, study and experimental interrogation of these hand and arm 

 centres in the brain does not afford any clue to the comprehension of 

 right and left-handedness. Each side reacts apparently in the same 

 manner to the stimulation of its centre in the opposite hemisphere of 

 the brain, and we should not expect to distinguish fine differences. 

 But there is another centre in the brain, the study of which has 

 thrown a flood of hght upon the subject in which we are interested, 

 and that is the speech centre. 



I cannot enter on the psychology and physiology of speech, a vast 

 and controversial topic ; but there is one point on which we are all 

 agreed, and that is, that there is an emissive centre for speech in the 

 third frontal or Broca's convolution, as it is called. We need not 

 localise speech in any such small part of the brain, but of this we are 

 certain : that damage in this part is destructive of speech, but in a 

 different way in the two hemisphere. Damage to Broca's convolution, 

 as by a clot of blood or an embolon, in the left hemisphere deprives 

 the right-handed man of speech but leaves the left-handed man with 

 speech unimpaired ; while damage to Broca's convolution in the right 

 hemisphere deprives the left-handed man of speech and leaves the 

 right-handed man with speech intact. 



The evidence that has been accumulated makes it certain that we 

 have aphasia and paralysis of the right hand when Broca's convolu- 

 tion is damaged in the left hemisphere of a right-handed man, in 

 whom damage of the same convolution in the right hemisphere 

 involves paralysis of the left hand but not loss of speech, and that in 

 the left-handed man the exact opposite of all this holds good. 



Here then we have conclusive proof that the old opinions, that 

 the action of both hemispheres is required in all mental operations, 

 and that either half of the brain indifferently can act alone, must be 

 discarded. The two halves are not double in function in the sense that 

 both are required for speech, since a right-handed man can speak 

 perfectly well when the right half of his brain is damaged ; nor 

 are they independent in function in the sense that the tAvo halves 



