HUMAN EVOLUTION 



position in the throat differs in man from that in other primates. 

 The voice box of non-human primates is in slight, to close, 

 contact with the soft palate and the base of the tongue. This 

 difference in position, together with differences in the inner- 

 structure of the voice box itself, is the reason why chimpanzees, 

 for example, cannot make long, resonant sounds. As one conse- 

 quence of man's upright posture and the bending of the cranio- 

 facial axis, the voice box in man is moved down the throat 

 away from contact with the soft palate, thus forming an oral 

 chamber which makes possible the human sort of vocal per- 

 formance (Kelemen, 1949). 



The neural mechanism of speech in the higher brain centers 

 is not fully understood. The wide variation in localization of 

 a primary lesion leading to aphasia— the inability to use spoken 

 or written language with normal symbolic control— indicates the 

 brain mechanism of speech is highly complex. We may classify 

 aphasia into two types, each with three divisions, according to 

 the location of a lesion in the (1) motor, or expressive, and 

 (2) sensory, or perceptive, brain systems, and its location in the 

 (a) cortical, (b) subcortical, or (c) intercortical areas (Rehberger, 

 1946): 



(1) Motor aphasia— failure of symbolic expression. The sym- 

 bolic output in articulation, writing, or gesture is de- 

 ranged. 



(a) Cortical— loss of articulation or (often) writing with 

 loss of word memory but no loss of understanding of 

 spoken or written language. 



(b) Subcortical— loss of articulate speech but retention of 

 word memory. 



(c) Intercortical— inability to repeat words heard or seen 

 but ability to understand their meaning. 



(2) Sensory aphasia— failure to comprehend vocal or visual 

 symbols. 



(a) Cortical— loss of memory of the meaning of words 

 heard and/or seen. 



(b) Subcortical— word memory is intact but is cut off 

 from the usual mental connections which may, how- 

 ever, be recalled by a roundabout path. 



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