THE CEREBRUM 



607 



by an inability to express ideas by writing, a condition usually spoken 

 of as agraphia. This area and the general arm center in the pre-central 

 convolution are united by association fibers. 



The Language Zone. These different areas are connected with one 

 another by association fibers, and, taken collectively, constitute the language 

 zone, which in people who are congenitally right-handed is located on the 

 left side of the brain only. Their situation and relations are shown in Fig. 

 250. In this figure the dotted lines coming from the eye (v) and ear (a) 

 represent the visual and auditory tracts 

 through which nerve impulses pass 

 to the visual centers (V) and the au- 

 ditory (A) respectively. Similar lines 

 coming from the muscles involved in 

 speech and writing might also be repre- 

 sented to indicate the paths of the 

 nerve impulses to the muscle sense 

 areas not shown in the diagram. The 

 single continuous lines on the surface 

 of the cortex represent nerve-fibers 

 which associate the visual and audi- 

 tory centers with the speech and writ- 

 ing centers. The double lines asso- 

 ciate the visual and the auditory areas 

 with the frontal association area on the 

 one hand and the association area with 

 the motor speech and the motor writ- 

 ing area respectively. The dotted lines 

 coming from the speech and writing 

 centers represent the tracts coming 

 from the areas in the pre-central con- 

 volutions, and through which nerve im- 

 pulses pass to the muscle of the larynx, 

 tongue, mouth, and lips, and to the 

 muscles of the hand. The anatomic 

 and physiologic association of the vari- 

 ous areas is essential to the registration 

 of the impressions made on the ear 

 and eye and for the expression of the 

 ideas evolved from them by words 

 (speech) and signs (writing). Their 

 collective action is essential to the ac- 

 quisition of language. Destruction of 

 any part of this cerebral mechanism 

 is attended by an impairment of or a total loss in either the power of ob- 

 taining auditory images of words heard and visual images of words seen, or 

 the power of expressing ideas by speech and writing. To this pathologic 

 condition the term aphasia has been given. 



Aphasias are of many degrees and kinds, though they may be included 

 in the two general divisions, motor and sensor. 



Motor aphasia may be either aphemic or agraphic. In aphemic aphasia 



FIG. 250. DIAGRAM SHOWING THE RE- 

 LATION OF THE CENTERS or LANGUAGE 



AND THEIR PRINCIPAL ASSOCIATIONS. A. 



Auditory center. V. Visual center. M. 

 Motor speech center. E. Motor writing 

 center. O O. Intellectual center. (After 

 Grasset) 



