CENTRAL NERVOUS SYSTEM. 645 



send one branch caudad, while the other passes cephalad (Fig. 151). The 

 length of these branches is difficult of determination, but it appears that the 

 one passing cephalad is probably the longer as a rule, and that it may extend 

 over nearly the entire length of the cord. By means of collaterals, these main 

 branches are connected with cells within the cord, probably both efferent and 

 central. Through the central cells arranged in series, pathways are formed 

 by which the incoming impulses may produce an effect at parts of the system 

 remote from the point of entrance, as well as pass almost directly to the effer- 

 ent cells in the neighborhood where they enter. 



Of these afferent roots there are thirty-one on either side, and for each 

 dorsal root there is a corresponding ventral one. Due allowance being made 

 for components which have failed to develop, the cranial nerves can be 

 homologized with them. Considering, then, the longitudinal extension of 

 the cord, it falls into a series of segments marked on each side by a pair of 

 spinal nerves. 



Segmentation. The segmentation thus indicated is most evidently 

 marked by the arrangement of the efferent or ventral spinal nerves. The 

 studies on the relations between the efferent nerve-fibres and the cell-bodies 

 which give origin to them indicate that the latter are located at the same level 

 in the cord as that at which the fibres springing from them emerge. This 

 permits us to infer that the cells of origin for any ventral root tend to concen- 

 trate in the segment from which that root springs. 



The afferent nerve-fibres have in part at least a somewhat extended course 

 through the cord, and are less strictly limited to the segment with which they 

 make their superficial connections. At the same time, a number of central 

 cells belong to each segment, and must be more closely connected with the 

 dorsal and ventral nerves with which they are immediately associated, than 

 with any others. Nevertheless the human spinal cord shows but poorly 

 the segmental disposition of the elements in it when compared with that 

 of lower vertebrates, like the snakes for example, in which the concentra- 

 tion of the nerve-cells about the region of emergence of the roots is more 

 evident. 



Bilateral Symmetry. The body being in the main bilaterally symmet- 

 rical, it is to be expected that the nervous system which controls it will be 

 constructed in the same manner. Such is, indeed, the case. Architecturally 

 this symmetry is not perfect, since each cell on one side is not exactly bal- 

 anced by a corresponding cell on the opposite side, but the number of cells 

 in corresponding regions is approximately the same, and for physiological 

 purposes the bilateral symmetry is quite complete. Yet this arrangement is 

 not without exception. 



Dorsal and Ventral Plates. In the human fetus the shape of the me- 

 dullary tube, the tube from which, later, the brain and spinal cord are 

 developed is shown in cross section in Figure 166. 



Slight indentations on either side of the tube are here evident on the inner 

 wall. They divide each side of the tube into a dorsal and ventral portion, 



