292 INTRODUCTION TO NEUROLOGY 



speech defects, but Hughlings Jackson extended the connotation 

 of the word to include "a loss or defect in symbolizing relations 

 of things in any way." The lesion which produces the defect 

 affects the association centers rather than the projection centers, 

 for there is no primary sensory defect- no blindness or deafness 

 or loss of general sensation nor is there any motor paralysis. 

 The problems connected with aphasia are very difficult and 

 confused, and there is by no means general agreement on either 

 the fundamental physiological mechanisms involved in speech or 

 on the nature of the lesions which produce the various types of 

 observed speech defects. The enormous literature relating to 

 this subject cannot be summarized here; see the text-books of 

 physiology, physiological psychology, and clinical neurology. 



Lesions of the primary sensory or motor projection centers will not pro- 

 duce aphasia, for in these cases all sensations or all movements related to 

 the injured parts are lost, whereas in aphasia only the correlations involved 

 in speech or other associational processes are impaired and all other sensori- 

 motor correlations may be intact. Of course, the number of associational 

 pathways involved in the communicating of ideas by hearing, reading, 

 speaking, and writing words is very large; and the character of the speech 

 defect will depend in part upon the particular associational tracts affected 

 by the lesion and in part upon the effect of the lesion upon the general in- 

 telligence of the patient (diaschisis effect, see p. 293). The second factor 

 seems to be exceedingly variable and has given rise to much controversy. 



Distinctive names have been given to the more important types of speech 

 defect as clinically observed ; such as agraphia or inability to write correctly, 

 aphemia or inability to utter words, word-blindness (alexia) or inability to 

 comprehend written words, word-deafness or inability to comprehend 

 spoken words, and many others. Evidently an aphasia may result from 

 injury to (1) a sensory association area contiguous to the primary visual or 

 auditory projection centers (sensory types of aphasia), or (2) to a motor 

 association center contiguous to the motor projection centers for the speech 

 muscles (motor types), or (3) to any of the associational tracts connecting 

 these association centers. 



The second, or motor, type of aphasia usually, though not invariably, 

 results from injury to the posterior part of the inferior frontal gyrus (see 

 Fig. 54, p. 121) of the left hemisphere in right-handed persons and of the 

 right hemisphere in left-handed persons. This relation was first discovered 

 by Broca, and the area of motor speech correlations (marked "motor speech " 

 in Fig. 133, p. 283) has since been termed Broca's convolution. 



It should be reiterated that Broca's convolution does not lie in the excit- 

 able motor zone of the cortex. Though the destruction of this area may be 

 followed by defects of speech, the muscles of the larynx, tongue, lips, etc., 

 involved in vocalization are not paralyzed. This case is typical of many 

 other motor association centers of the cortex whose integrity is essential for 

 specific motor combinations, though separate motor centers are present for 

 all of the muscles involved in these movements. 



