628 PHYSIOLOGY CHAP. 



It should be noted in conclusion that the post-mortem integrity 

 of Broca's organ in persons who had suffered from motor aphasia 

 may be more apparent than real, unless a careful microscopical 

 examination has been made. Marie's cases, in which there was 

 no motor aphasia despite destruction of Broca's convolution in 

 right-handed patients, are not irreconcilable with the generally 

 accepted theory, as slowly developing changes in the opercular 

 portion of the third left convolution may be associated with a 

 progressive functional development of the corresponding right con- 

 volution, which is the motor centre for articulate speech in left- 

 handed persons. 



The fact that in young persons motor aphasia due to lesions 

 of Broca's centre quickly disappears was used by Mingazzini as 

 an argument in favour of Gowers' theory, which assumes that 

 up to a certain age both hemispheres co-operate in the formation 

 of the motor images of speech, and that the function of the right 

 brain is only later transferred to the left hemisphere in right-handed 

 people, and vice versa in the left-handed. 



If we accept this hypothesis there is no difficulty in assuming 

 that in certain individuals, particularly in the ambidextrous, 

 the function of speech may be distributed throughout life 

 in an approximately equal degree to both hemispheres, so that 

 even a sudden lesion of one does not abolish speech (Mingazzini). 

 This theory, which invalidates Marie's arguments, is supported 

 by all the clinical cases of motor aphasia, due to destruction of 

 the left centre of Broca, in which speech gradually returns after 

 a longer or shorter interval. The following case reported by 

 Oppenheim (1909) is of great importance as a physiological 

 experiment on man. In a patient in whom Broca's centre had 

 been exposed, motor aphasia occurred each time the brain was 

 compressed, and disappeared when the pressure was removed. 



Lesions of Wernicke's centre produce word deafness; those 

 of the cortex of the occipital lobe or angular gyrus word blindness. 

 The centres of auditory and visual word memory are not 

 equally important in the mechanism of speech; obviously the 

 former preponderates. The child learns to speak by exercising its 

 auditory perceptions. As the association paths that connect the 

 auditory word centre with the motor word centre become developed 

 it makes its first attempts to talk, and speech gradually becomes 

 more perfect as the cortical and sub-cortical centres, and paths 

 and peripheral organs of speech, attain full development. 



Lesions of the subcortical paths and peripheral organs produce 

 disturbance of articulation or dysarthria, but the capacity for 

 internal or mental speech then remains intact. Lesions of Broca's 

 and Wernicke's centres may produce alterations on the sensory 

 side of speech, and total or partial incapacity for phonetic 

 expression (aphasia or dysphasia) with more or less disturbance 



