CENTRAL NERVOUS SYSTEM. 247 



groups, because the number of pyramidal fibres is very much less than is the 

 number of cells which they control. This discrepancy is in sonic measure 

 relieved by the formation of "geminal" fibres already described. Moreover, 

 the branching of the pyramidal fibres near their termination is very probable, 

 and the most plausible view at present is that each pyramidal fibre by means 

 of its collaterals controls, perhaps indirectly, a considerable number of cord 

 cells, and probably the cells controlled by any one fibre form a more or less 

 compact group. 



Mapping- of the Cortex. — Having sketched the relations of the pyramidal 

 cells forming the motor region of the cerebral cortex to the parts lying below, 

 we turn to study the arrangement, size, subdivisions, and comparative anatomy 

 of this region, and then to examine the relation of it to the other parts of the 

 cortex. The observations here quoted are those on the monkey only. 



On glancing at Fig. 99 it is evident, first, that the areas for the face and 

 leg are widely separated from each other, that the arm-area lies between 

 them, and that the area for the trunk, though less schematically placed, is 

 located between that for the arm and leg. This arrangement is more typi- 

 cally represented on the mesial (Fig. 100) than on the convex surface of the 

 hemisphere, and in the former locality the serial order of the cortical areas 

 corresponds with the order of the muscle-groups which they control. 



The Size of the Cortical Areas. — Evidently there is no direct relation 

 between the extent of a cortical area and the mass of muscles which it con- 

 trols. Certainly in man the mass of muscles in the leg is three times greater 

 than that in the arm, and this latter many times greater than that of the face 

 and head ; yet it is for the last area that the greatest cortical extent is found. 

 Mass of muscle and extent of cortical area do not therefore iro together. 



When the movements effected by the muscles represented in these several 

 areas are considered, we find that such movements become more complex and 

 more accurate as we approach the head, and it therefore accords with the facts 

 to consider the extent of the motor anas as correlated with the refinement 

 of the movements which they control — a relation which may depend even 

 more on the multiplication of the pathways bringing in impulses than on 

 those which send them out. 



Subdivision of Areas. — The areas which have been described are further 

 subdivided, the subdivisions in the arm-area being the clearest. Here it is 

 found that the stimulation of the upper part of the arm-area gives rise to 

 movements which start at the shoulder, while stimulation at the lower part 

 of this area gives rise to movements first involving the lingers, ami especially 

 the thumb. The centres from which these several reactions may lie obtained 

 occupy, as Fig. 99 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 centres lying in regular order between these 

 two. A similar arrangement appears in the subdivisions of the cortex con- 

 trolling the leg, and in the face-area as well. 



