CORTICAL LOCALIZATION OF FUNCTION 



963 



of the muscles used in speech (larynx, tongue, jaw muscles). Destruction of this region at least 

 causes a loss or disturbance of articulation of words. 



(b) The auditory receptive centre, clinically known as the centre of "word deafness," is 

 localized in the marginal gyre and adjacent parts of the super- and meditemporal gyres, espe- 

 cially the latter. A patient suffering with a lesion of this area may clearly hear but not under- 

 stand the spoken word. This division of the centre might also be called the lalognostic (word- 

 understanding) centre. 



(c) The visual receptive centre, clinically known as the centre of "word blindness," is 

 localized in the angular gyre. Lesion of this area renders the patient incapable of understand- 

 ing the significance of the words and objects which he sees. 



(d) An emissive "writing" centre, not positively proved to exist, has been localized in the 

 medifrontal gyre, frontad of the motor area for the upper limb. 



(e) Of not a little importance with reference to the intellectual control of the faculty of language 

 is the island of Reil, purely an association centre, serving to connect the various receptive sense 

 areas relating to the understanding of the written and spoken word with the somesthetic emissary 

 centre related to articulate speech and writing. 



FIG. 714. Diagram showing the language zone. The opercula are divaricated to expose the island of Reil. 



The union of the various centres enumerated above forms the cortical zone of language, 

 and is most intensely, if not exclusively, localized, or at least, active, in the left cerebral hemi- 

 sphere in right-handed persons, and vice versa in left-handed persons. 



4. The Association Areas. The remaining area of the cerebral cortex is presumably the 

 organic substratum for the higher psychic activities. At the present time not much is known 

 about them, but broadly stated the frontal association area is concerned rather with the powers 

 of thought in the abstract creative, constructive, and philosophic. The parieto-occipito- 

 temporal association area, on the other hand, seems to be more concerned with the powers of 

 conception of the concrete, for the comprehension of analogies, comparing, generalizing, and 

 systematizing things heard, observed, and felt. 



The great extent of the association areas in the human brain is a somatic expression of man's 

 possession of an associative memory or ability to register and compare sensations far greater than 

 that of the highest ape. The pattern of the fissures and gyres in the brains of the higher anthro- 

 poids and man presents the same general features in all these types. In the course of evolution, 

 however, the regions known as association areas assumed a greater energy of growth and ex- 

 panded in proportion to the rise in functional dignity of these areas. They are regions of " un- 

 stable equilibrium" which afford greater and more complex associations as mental development 

 goes on in the species, and concomitant with this great cortical expansion the associating or 

 coordinating fibre systems became more elaborate, complex, and far-reaching. 



With the aid of the microscope the maturing of the brain elements can be followed from the 

 earliest stages of embryonic life to the adult period. The Flechsig method has shown how the 

 function of nerve fibres within the brain is only established when the myelin sheath has developed. 



