686 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



within which all the impulses from a given sense-organ reach the cortex. 

 The areas in turn may be marked off into centres, formed by the groups of 

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

 given segment of a limb, or in the visual area are represented by those cells 

 especially 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 muscles for instance, those controlling the 

 arm an( J 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. 



Another method of studying the cortex is to regard the character of the 

 movement obtained by stimulating a single area, as that of the arm. Figure 



FIG. 186. Showing in the arm-area (monkey's brain) the localization of movements having 

 different characters (after Horsley). 



186 shows that stimulation of the upper arm-area gives rise in the first in- 

 stance to movements of extension, whereas the lower arm-area yields those of 

 flexion. This basis of subdivision is, however, not so useful as the analysis 

 into centres. As the smallest subdivisions, the centres are most convenient 

 for further study. 



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 stimu- 

 lation of it, indicating that the special effect can be produced by the passage 

 of impulses from the point of stimulation toward the infracortical structures. 

 If, in addition, a cut be made below the cortex and parallel with its surface, 

 then stimulation of the cortex above this section is ineffective, thus indicating 

 that the impulses pass from the cortex directly into the substance of the hem- 

 isphere along certain nerve-tracts, which by this operation were sectioned. 

 Further, if the bit of cortex thus separated from the underlying white sub- 

 stance be removed and the faradic current be applied to the white substance 

 beneath, a reaction of the same type and involving the same muscles can be 

 obtained, although it differs from that to be gotten from the cortex itself, in 



