684 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



To be sure, Ferrier 1 very early pointed out that stimulation of the other sen- 

 sory areas causes movements. It was by means of the movements thus ob- 

 tained that he sought to localize the sensory centres, assuming that the move- 

 ments were in response to sensations caused by the irritation of the cortex. 

 As the result of stimulation of a sensory area the muscles of the sense 



organ itself or those immediately 

 associated with it respond (see Figs. 

 182, 183). 



Shafer 2 has shown in the mon- 

 key that the dorsal portion of the 

 visual area is associated with the 

 upper portion of the retina, the 

 eye being turned downward as the 

 result of stimulating this portion. 



FIG. 183. Mesial view of a human hemisphere. Thecorti- This is interpreted as a movement 



cal visual area is shaded, V; cortical area for smell, S. r , i , -i i , i 



of the eye intended to bring a 



stimulus falling on the upper part of the retina into the centre of the field of 

 vision. When the stimulus is applied to the ventral portion of the area a 

 corresponding upward movement of the eye occurs, and the corresponding 

 relation holds for the stimulation of the lateral and mesial portions of the 

 area. These movements occur in both eyes, although the stimulus is applied 

 to one lobe only, and hence the two retinal fields appear to be superposed in 

 the visual cortex of each hemisphere. 



The experiments on the stimulation of the other sensory areas show, in 

 the first place, that these areas contain cells the stimulation of which causes 

 the contraction of certain muscles immediately associated with the organ of 

 sense, and, in the second place, that while each of the areas is pre-eminently 

 concerned with the reception of impulses from a particular sense-organ, yet 

 no one of them is exclusively sensory. 



Deferring for a moment the other evidence by which the sensory charac- 

 ters have been established, and also the arrangements within the cortex by 

 which any group of muscles can be made to respond to stimuli arriving at any 

 sensory area, we shall follow out the distribution of those cortical cells the 

 stimulation of which causes contractions of the skeletal muscles. 



The results here presented were obtained from the electrical stimulation of 

 the monkey's brain by Beevor and Horsley 3 (see Figs. 184, 185). These 

 experimenters explored the exposed surface of the hemisphere with the elec- 

 trodes, moving them two millimeters at a time, and at each point noting the 

 muscle-group first thrown into contraction. 



As the result of many observations on the monkey, it is possible to map out 

 the cerebral cortex in the following way : The surface of the hemispheres is 

 divided into regions (motor and sensory regions), which are the largest sub- 



1 The Functions of the Brain, 1876. 



2 Proceedings of the Royal Society, London, 1888, vol. xliii. 



3 Philosophical Transactions of the Royal Society, 1888-90. 



