206 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



the case of complete lesion of the center on one side, that of the 

 other can be taught to assume its function. Some recorded cases 

 seem to indicate that this substitution is possible in the young, 

 while in the old it is more difficult or impossible. We express our 

 thoughts not only in spoken but also in written symbols. As this 

 latter form of expression involves a different set of muscles, and a 

 different educational experience, it is natural to assume that the 

 complex associations concerned, or, to use a convenient expression, 

 the memory centers, should involve a different part of the cortex. 

 It is, in fact, observed that some aphasics exhibit the symptom 

 of loss of power to write, a condition designated as agraphia. 

 The area in which the motor associations for the act of writing 



are located is placed 

 in the middle or sec- 

 ond frontal convolu- 

 tion contiguous to the 

 cortical motor centers 

 for the muscles of the 

 arm and hand (W, 

 Fig. 92). 



Sensory Aphasia. 

 In sensory aphasia * 

 (amnesia) the individ- 

 ual suffers from an 

 inability to under- 

 stand spoken or writ- 

 ten language, and as 

 the cortical centers for 

 hearing and seeing are 

 situated in distinct 

 parts of the brain, we 

 should expect that the 

 mechanism for the association, in one case of visual memories of 

 verbal symbols with certain concepts and in the other case of 

 auditory memories, should also be located in separate regions. 

 Inability to understand spoken language, or word-deafness, is, in 

 fact, usually attributed to a lesion involving the first temporal 

 convolution contiguous to the cortical sense of hearing (H, Fig. 92), 

 while loss of power to understand written or printed language, word- 

 blindness, is traced to lesions involving the inferior parietal con- 

 volution, the gyrus angularis, contiguous to the occipital visual 

 center (V, Fig. 92). These two conditions may occur together, 

 but cases are recorded in which they existed independently. It 



* Consult Starr, "Aphasia," "Transactions of the Congress of American 

 Physicians and Surgeons," vol. i, p. 329, 1888. 



H 



Fig. 92. Lateral view of a human hemisphere; 

 cortical area V, damage to which produces "mind- 

 blindness" (word-blindness) ; cortical area H, damage 

 to which produces "mind-deafness" (word-deafness); 

 cortical area S, damage to which causes the loss of 

 audible speech ; cortical area W, damage to which abol- 

 ishes the power of writing. (Donaldson.) 



