832 



THE NERVE SYSTEM 



the cord. The metamorphosis of the neuroepithelial columnar cells has been described (p. 810) 

 The lateral walls thicken considerably, the central slit-like canal widens as the walls bulge out- 

 ward in an angular manner, and the central tubular gray becomes a fluted column with dorsa 

 and ventral ridges (or horns) enveloped by a rapidly growing mantle of axone bundles whicJ 

 become myelinic in successive stages. The bulging of the thickening walls in the dorsal anc 

 ventral as well as lateral directions produces the ventral fissure and the postseptum. 



The segmented nature of the spinal cord has been alluded to before with regard to the seg 

 mental derivation of the cerebrospinal ganglia and the disposition of the outgrowing nerv< 



Ventral aspect. 



Dorsal aspect. 



FIG. 605. Distribution of cutaneous nerves. 



bundles. There is a further mode of division into longitudinal systems based upon functional 

 relationships. Two main categories of activity characterize the mechanism of the nerve system 

 and find somatic expression in its architectural plan: First, actions in relation to the external 

 world (somatic involving skin, muscle, skeleton, etc.); second, internal activities concerned 

 with the processes of nutrition and reproduction (visceral involving the alimentary tract, vas- 

 cular [blood and lymph], excretory, and reproductive systems). In each there is a twofold 

 activity on the part of the nerve system: reception of stimuli and motor responses. In the cord 

 (and to some extent in the brain as well) the following functional divisions may be distinguished 

 and located anatomically (Fig. 602): 



Somatic sensor elements. 

 Somatic motor elements. 



Visceral sensor elements. 

 Visceral motor elements. 



