THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 295 



the cerebral cortex. We know upon which areas the auditory, visual, and olfac- 

 tory impulses impinge, and less accurately that in which the thalamic radiation, 

 mediating general bodily sensibility, terminates. Destruction of these areas 

 causes impairment or loss of the corresponding sensations with reference to the 

 opposite side of the body or the opposite half of the field of vision. Total loss 

 of cutaneous sensibility even within circumscribed areas never results from cor- 

 tical lesions; and it seems probable that the thalamic centers are in themselves 

 sufficient for a certain low grade, non-discriminative consciousness or awareness 

 of cutaneous stimulation. This is particularly true of painful sensations, which 

 seem to be for the most part of thalamic origin (Head, 1918). Furthermore, 

 the various parts of the cerebral cortex are so intimately linked together by as- 

 sociation fibers that when afferent impulses reach a given projection center they 

 must not only activate this center, but be propagated to other parts of the cortex 



Motor speech center 



Auditory speech center Visual s P eech center 

 Fie. 225. The cortical areas especially concerned with language. 



as well. In view of these facts it is best to express the known facts of cortical 

 localization in terms of the relation of particular areas to the known projection 

 fiber systems. 



Aphasia. Some idea of the significance of the so-called association centers 

 may be obtained from a study of the group of speech defects included under the 

 term "aphasia." In right-handed individuals these result from lesions in the 

 left hemisphere. Destruction of the triangular and opercular portions of the 

 inferior frontal gyrus usually causes loss of ability to carry out the coordinated 

 movements required in speaking, but does not impair the ability 'to move the 

 tongue or lips (Fig. 225). This defect is known as motor aphasia. Broca's 

 center, as this particular part of the cortex is sometimes called, is located in 

 Campbell's intermediate precentral area; and motor aphasia serves as a good 

 illustration of the importance of the entire intermediate precentral area for the 



