128 NERVOUS SYSTEM. 



purpose. The functions of the convolutions have not been assigned, 

 except for a very small portion of the brain-surface and for some 

 of the simpler actions. For the most part, our knowledge of the 

 localization of brain-functions is confined to "motor areas," in which 

 it has been determined that stimulation of a certain group of cells 

 will cause a definite action. Besides this, certain other centres are 

 located, as of sight and speech. 



How are the motor areas determined? 



When the surface of the brain is exposed in animals or in man, 

 the stimulation of certain areas of the cortex by a mild electrical 

 current will give rise to motion in the peripheral muscles ; and it is 

 found that the stimulation of the same region in the same or other 

 animals will cause the same results. These centres of motor im- 

 pulses are situated almost entirely upon the convolutions about the 

 fissure of Kolando (Fig. 36). 



What is known regarding the localization of sensory areas ? 



This has not been, by any means, so definitely fixed as for motor 

 centres ; but the centres for sensation may be said to exist, and 

 probably in the convolutions of the posterior portion of the cere- 

 brum. The centre for vision in the convolutions about the poste- 

 rior branch of the fissure of Sylvius is generally accepted (14, 15, 

 in Fig. 36). The centre for hearing is tolerably defined in the 

 temporo-sphenoidal lobe along the posterior branch of this fissure 

 (16, Fig. 36). The speech-centre is also located with seeming ac- 

 curacy along the anterior branch of the fissure of Sylvius and in the 

 island of Reil. This centre seems to be much more developed upon 

 the left side of the brain. In Fig. 36 this centre may be indicated 

 roughly by reference to the tongue-centres (8 and 9). 



Do the evidences of pathology agree with these experiments ? 



Injuries and diseases involving the motor areas are followed by 

 paralysis so well defined that it is frequently possible to locate 

 the seat of the lesion from its result upon the muscular system. 

 Tumors, abscesses, and depressed bone, for example, are capable 

 of accurate localization in this way. The more indefinite sen- 

 sory paralyses do not so accurately point out their origin. On 

 the whole, the evidence of pathology bears out in full the experi- 

 mental results. The crossed action of all the nervous structures 

 is especially to be noted. In the case of a right paralysis in 



