CENTRAL NERVOUS SYSTEM. 689 



then the leg-area (see Fig. 153) be stimulated, an electrometer applied to the 

 cut end of the cord will show the passage of nerve impulses, because the 

 electrometer is applied to a tract of fibres on their way to the lumbar enlarge- 

 ment, and the fibres originate in cortical cells within the region stimulated. 

 When, however, the cortical stimulus is made in the arm-area, the electrometer 

 being applied as before, no electric change occurs, for the neurons of the cells 

 in the arm terminate in the part of the cord containing the cell-groups which 

 control the muscles of the arm, and these all lie cephalad to the point of 

 section of the cord. It is evident, therefore, that the arrangement is a com- 

 paratively simple one namely, an extension of the neurons of the several 

 groups of cortical cells from the different areas for the leg, arm, face, etc., to 

 the axial cell-groups which control the muscles of these parts, and which 

 are situated in the cord. Sherrington reports l a degeneration of some fibres 

 as far as the lumbar enlargement even when the lesion is confined to the 

 cortical area for the arm. Assuming the correctness of this observation, it is 

 to be harmonized with the preceding statements to the effect that stimulation 

 of the arm-area does not produce an electrical variation in an electrometer 

 applied to the crossed pyramidal tracts in the mid-thoracic region by the fact 

 that the number of these long fibres is small. 



The cortical cells in the motor region belong to the group of central cells 

 i. e. their neurons never leave the central system and hence they are 

 engaged in distributing impulses within it. To the axial cell-groups in the 

 cord they bring impulses, and therefore from the standpoint of these latter 

 may be considered as afferent, whereas, owing to the fact that they carry 

 impulses away from the cortex, they are sometimes called efferent. Confusion 

 can be avoided, however, by refraining from either term. Just how these two 

 sets, the cortical and the cord elements, are related still requires to be worked 

 out. The number of fibres in the pyramidal tracts indicates that there cer- 

 tainly is not one fibre for each cell in the axial cell-groups, because the num- 

 ber of pyramidal fibres is very much less than is the number of cells which 

 they control. This discrepancy is in some measure relieved by the formation 

 of "geminal" fibres already described. Moreover, the branching of the pyr- 

 amidal fibres near their termination is very probable, and the most plausible 

 view at present is that each pyramidal fibre by means of its collaterals comes 

 into physiological connection with a considerable number of efferent cells, and 

 probably the cells controlled by any one fibre at its terminus form more or less 

 compact groups. 



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, 

 it will be important to study the arrangement, size, subdivisions, and com- 

 parative 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 Figure 184 it is evident, first, that the areas for the head 



1 Journal of Physiology, 1869, vol. x. 

 44 



