CENTRAL NERVOUS SYSTEM. 



691 



while stimulation at the lower part of this area gives rise to movements first 

 involving the fingers, and especially the thumb. The centres from which 

 these several reactions may be obtained occupy, as Figure 184 shows, narrow 

 fields across the cortex in a fronto-occipital direction. Moreover, the centre 

 for the most proximal joint of the arm is farthest removed from that for the 

 most distal, while the intermediate joints are represented by their several cen- 

 tres lying in regular order between these two. A similar arrangement appears 

 in the subdivisions of the leg, and in the face-area as well. 



Interpreting these facts in the terms of nerve-cells and their arrangement, 

 it appears that in the shoulder centre the neurons of the cortical cells that dis- 

 charge downward pass predominantly to the efferent cell-groups which in the 

 spinal cord directly control the muscles of the shoulder, and that a similar 

 arrangement obtains for the other centres in this region with the correspond- 

 ing cell-groups in the cord. The stimulation of the different portions of the 

 internal capsule where it is composed of bundles of fibres coming from the 

 motor region shows (observations on orang-utang) that the fibres running to 

 the several lower centres are here aggregated, and are ranged in the same 

 order as the cortical centres themselves (see Fig. 188). 



Separateness of Areas and Centres. As we ascend in the mammalian 

 series there is an increase in the perfection with which cells forming the sev- 

 eral centres are segregated, though the areas in the different forms tend to 

 hold the same relative positions. 1 



Figures 189, 190 give the localizations recently obtained in the rabbit's 

 brain by stimulation (Mann). The various areas occupy a large proportion 



of the cortex, and in some cases come 

 very close together, so that they are 

 not easily separated by experiment. 



FIG. 189. Rabbit's brain, dorsal view. The 

 areas indicated are those the stimulation of which 

 causes a movement of the parts named (Mann). 



FIG. 190. Rabbit's brain, lateral view. The 

 areas indicated are those the stimulation of which 

 causes a movement of the parts named (Mann). 



In the lower monkeys (Macacus sinicus) these cell-groups are segregated, 

 so that those associated with the cervical portion of the cord and forming the 

 arm-area are much more together, and quite separate from those associated 

 with the lumbar region, leg-area. In the orang-utang, 2 and to a greater 

 extent in man, a further separation occurs, so that they come to be surrounded 



1 Mann: Journal of Anatomy and Physiology, 1895, vol. xxx. 



2 Beevor and Horsley: Proceedings of the Royal Society of London, 1890-91, vol. xlviii. 



