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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



In the case of neither vision nor hearing do we find in man any subcortical 

 cell-groups capable of acting as centres ; that is, after the destruction of the 

 appropriate cortical 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 

 wanting. 



Association Fibres and Association Centres. — Common experience 

 shows us that we can voluntarily 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 



Fig. 111.— Lateral view of a human hemisphere, showing the bundles of association-fibres (Starr): 

 A, A, between adjacent gyri; Ii, between frontal and occipital areas; C, between frontal and temporal 

 areas, cingulum ; D, between frontal and temporal areas, fasciculus uncinatus; E, between occipital and 

 temporal areas, fasciculus longitudinalis inferior ; C, N, caudate nucleus; 0, T, optic thalamus. 



nerve-impulses pass first to the visual area, and then in an indirect manner 

 arouse the cortical cells controlling the muscles of the hand. This connec- 

 tion of the two areas is accomplished through the so-called association-fibres 

 of the cortex. These fibres are formally defined as those which put into 

 connection different parts of one lateral half of any subdivision of the central 

 system (see Fig. 111). 



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 would bear out the 

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

 another. 



