L'.-.S 



AN AMERICAN TEXT-ROOK OF PHYSIOLOGY. 



from localities outside the third frontal gyrus. Experiments show that in 

 animals stimulation of the cortex in the region corresponding to the third 

 frontal gyrus causes contractions of many of the muscles employed in 

 speech. 1 



fhe interesting observation was also made that in the normal right-handed 



pei-oii the muscles of phonation could not he co-ordinated tor speech from 

 the right hemisphere alone. Thus the symmetrical portion of the right 

 hemisphere has not the same physiological value. 



Besides this lesion, which involves the cortex in front of the motor region 

 proper, numerous other lesions — namely, those which involve the tracts run- 

 ning between the areas of special sensation (vision and hearing,for example), 

 and the motor or expressive region — produce corresponding disturbances (see 

 Fig. 1T2). 



An individual in whom the association tracts between the visual and motor 

 areas have been interrupted can, for instance, see an object presented to him 

 in the sense that he gets a visual impression ; hut because of the interruption 

 of the association fibres the object is not recognized, and the impulses reach- 

 ing this sensory area (licit no re- 

 sponse from those muscles the 

 motor centres for which are 

 located outside of the receiving 

 cortex. 



Upon attempting to picture 

 the anatomical arrangement in 

 anything like the completeness 

 demanded by the physiological 

 reactions, it is necessary to 

 postulate the existence of asso- 

 ciation pathways between each 



area, whether sensory or motor, 



Pig. 112. Lateral view of a human hemisphere; cor- ■ . 



tical ana V, damage to which produces "mind-blind- '-^^i all the others. J Ills arrange- 



ness"; cortical area //. damage to whirl, produces me nt is to be regarded as modi- 

 " mind-deafness " ; cortical area S, damage to which 



the loss of audible speech; cortical area If, dam- fied in several Ways, 



age to which abolishes the power of writing. Jn t]|( . firgt plaC6j t])( , ( . onnec _ 



tion between a given sensory and a given motor area differs widely accord- 

 ing to the area.- concerned. The connection, for example, between the visual 

 area and the motor area for the arm is probably represented by more nerve- 

 element-, and these better organized, than the connection between the gusta- 

 tory area and that for the movements of the leg. 



When, therefore, it i- -aid that such connections exist, it must he added 

 always that the nexus i> different for the several regions concerned, and what 

 i- more, that in man, at least, it i- different for the two hemispheres. 



Relative Importance of the Two Hemispheres. — The cerebral cortex 



Semon ami Horsley : Philosophical Transactions of the Royal Society, 1890, vol. 181. Also 

 -. medicinische \Vo<-!i<-n-<rhri/t. N<>. .".]. lx'.Hi. 



