CENTRAL NERVOUS SYSTEM. 697 



In the case of the visual areas in man there is the same sort of evidence, 

 but somewhat more exact. The destruction of the area represented by the 

 cuneus and the surrounding cortex (see Figures 182 and 183) always injures 

 vision, and the failure of the eyes to grow arrests the development of this 

 portion of the hemisphere. 1 



Hemianopsia. It is found, moreover, that injury to the visual area in one 

 hemisphere produces usually a hemianopsia or partial defect of vision in both 

 retinas. The homonymous halves are affected on the same side as the lesion, 

 and the dividing line is usually vertical. The clinical picture corresponds to 

 a semi-decussation of the optic tract and the representation of the homon- 

 ymous halves of each retina in both hemispheres. At the same time the rela- 

 tion is much more complicated than at first sight appears, for the point of 

 most acute vision is often unaffected in such cases ; and for this peculiarity we 

 have no anatomical explanation. 2 



In neither vision nor hearing do we find in man any subcortical cell-groups 

 capable of acting as centres ; that is, after the removal of the appropriate cor- 

 tical region the corresponding sensations and reactions to the stimuli which 

 arouse these sensations are completely and permanently lost. 



From these facts, therefore, it appears that the impulses which give rise to 

 visual and auditory sensations are delivered in certain parts of the cerebral 

 cortex, and unless they arrive there the appropriate sensations are absent. 



Association Fibres. Common experience shows us that we can volun- 

 tarily contract any group of muscles in response to any form of stimulus 

 dermal, gustatory, olfactory, auditory, or visual. When, therefore, the hand 

 is extended in response to a visual stimulus, the nerve-impulses pass first to 

 the visual region, and then are transferred to the cortical cells controlling the 

 muscles of the hand. This connection is accomplished through the so-called 

 association fibres of the cortex. These fibres are formally described as those 

 which put into connection different parts of one lateral half of any subdivis- 

 ion of the central system (see Fig. 196). 



The bundles which are thus shown in the cerebral hemisphere must be 

 looked upon as typical of the arrangement throughout the entire cortex, and, 

 further, the arrangement in the cortex is typical of that in other parts of 

 the central system. Anatomy would suggest, and pathology bears out the 

 suggestion, that it is by these tracts that the impulses travel from one area 

 to another. 



Aphasia. The development of the ideas bearing on this subject has been 

 slow. After the publication of the great work of Gall and Spurzheim (1810- 

 19) on the brain, some pathologists (Bouillaud, 1825; Dax, 1836), especially 

 in France, were in search of evidence touching the doctrine of the localization 

 of function. At the same time the subject of phrenology, as put forward by 

 Gall and Spurzheim, was not in good repute, and anything which looked that 

 way, even in a slight degree, was generally scouted. Broca, however, pub- 



1 Donaldson: American Journal of Psychology, 1892, vol. iv. 



2 Noyes: New York Medical Record, 1891. 



