CENTRAL NERVOUS SYSTEM. 



243 



cortical cells which, on direct stimulation, cause contractions of the skeletal 

 muscles. 



The results here presented were obtained from the electrical stimulation 

 of the monkey's brain by Beevor and Horsley 1 (see Figs. 99, 100). 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-gronp 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 

 subdivisions. These are subdivided into areas for the muscle-groups belong- 

 ing to different members of the body — arms, head, trunk, etc. — as well as 

 those areas within which all the impulses from a given sense-organ reach the 



PoF. 



Fig. 100.— Mesial surface of the brain (monkey). The localization of motor functions is indicated 

 along the shaded portion of the marginal gyrus. The location of the visual area is indicated at the tip of 

 the occipital lobe, and the location of the olfactory area at the tip of the temporal (Ilorsley). 



cortex. The areas in turn are subdivided into centres, comprising the groups 

 of cells, which, for example, control the smaller masses of muscle 1 belonging 

 to a given segment of a limb, or in the visual area constitute those cells espe- 

 cially connected with one part of the retina. There is thus a motor region, 

 the stimulation of which gives rise to the more evident bodily movements. 

 Within this are several subdivisions, the stimulation of one of which is fol- 

 lowed by movements of groups of muscle- — for instance, those controlling the 

 arm — and within such an area in turn come the smaller centres, or those the 

 stimulation of which is first followed by movements at one joint only. 



The physiological characters of these cortical motor centres have been 

 determined by the following observations: 



If a vertical incision be carried around such a centre so as to isolate it 

 from the other parts of the cortex, the characteristic reactions still follow the 

 stimulation of it, indicating that the special etl'eet can lie produced by the 



1 Beevor and Horsley: Philosophical Transactione of the Royal Society, 1SS8-90. 



