696 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



This relation is to be carefully noted, for with it is correlated the degree 

 of the disturbances in the reactions of the entire nervous system following 

 removal of parts of the encephalou, the effect being slight when the encephalon 

 is connected with the cord by a small number of fibres, and serious when the 

 connection is by many fibres, as in the case of man and the highest mammals. 



G. PATHWAYS WITHIN THE HEMISPHERES. 



If the guiding idea of the pathway of the nervous impulse through the 

 central system had been rigidly followed, the association tracts in the cerebral 

 hemispheres would have come up for discussion immediately after the descrip- 

 tion of the afferent pathways. The knowledge of the arrangement in the 

 cerebral cortex which has been obtained from the stimulation of it is, how- 

 ever, so much less complicated than that obtained by other methods of inves- 

 tigation that the observations on this head were made introductory to the 

 whole matter of localization, although in so doing the strict sequence of the 

 presentation was interrupted and the emphasis put on the cell-groups which 

 discharge from the cortex to the lower centres. 



Determination of Sensory Areas. The determination of the sensory 

 areas in man has been through the study of brains modified by destructive 

 lesions or congenital defects. 



The cortical centre for smell, inferred from comparative anatomy and 

 physiology to be at the tip of the temporal lobe and closely connected with 



the hippocampal gyrus and the 

 uncus, has been similarly located 

 in man on the basis of pathologi- 

 cal observations ; but the evidence 

 is indirect and incomplete (see 

 Fig. 195). Concerning the loca- 

 tion of taste sensations even less 

 is known. Both of these senses, 

 it must be remembered, are insig- 

 nificant in man, and hence their 

 central locations have not been 

 studied with great care. 



On the other hand, the cortical 

 areas for hearing and sight have 

 been located with much more precision and certainty. 



Damage to the first and second temporal gyri in man causes deafness in 

 the opposite ear, and concordantly conditions of the ear which early in life 

 lead to deafness and deaf-mutism are accompanied by a lack of development 

 in these gyri. 1 Destruction of these temporal gyri on one side always causes 

 deafness in the opposite ear, but there has not yet been reported a case of com- 

 plete deafness due to a double cortical lesion alone. 



1 Donaldson : American Journal of Psychology, 1891. 



FIG. 195. Lateral view of a human hemisphere. The 

 cortical area for smell is shaded (S) ; the cortical area 

 for hearing is shaded ( H). 



