532 CHOEDATA. 



in position throughout life. The muscles which are ventral have 

 largely been transferred from the back, and the cause of the migra- 

 tion is to be recognized to a large extent in the progressive devel- 

 opment of the appendages. The dorsal position of the muscles is 

 only a part of a general fact, that the skeletal axis divides the body 

 into a dorsal zone, containing only animal organs, and a ventral 

 zone, chiefly vegetal in character. Besides, the muscles, the cen- 

 tral nervous system, and the most important sense organs eyes, 

 nose, ears belong to the dorsal zone. 



The central nervous system of vertebrates consists of brain and 

 spinal cord. Like that of all chordates it is distinguished from 

 that of other segmented animals annelids, arthropods, in which 

 there is a dorsal brain and a ventral nerve chain in its purely 

 dorsal position. It is further distinguished from that of all non- 

 chordates by its tubular character, that is, by the presence of a 

 central canal in the axis of the elongate central system (fig. 76), 

 lined by a special epithelium, the ependyma, and containing a 

 fluid, the liquor cerebrospinalis. This central canal is the result 

 of the mode of development, the nervous system arising by an in- 

 rolling of the ectoderm and not by a splitting from it as in the 

 invertebrates (fig. 9). Besides the neurenteric canal already 

 referred to (p. 502), there long persists at the anterior end an 

 opening to the exterior, the neuropore. In all vertebrates, in con- 

 tradistinction to the lower chordates, the brain is large and sharply 

 marked off from the spinal cord. 



The spinal cord is a cylindrical structure (flattened in Cyclo- 

 stomes, fig. 555) which, in the middle line above and below, is 

 marked by two longitudinal grooves, the dorsal and ventral fissures 

 of the cord (sp, sa, fig. 76). The central canal (Cc) has its lumen 

 greatly narrowed by the growth of the nervous tissue, in which, 

 .as in the ganglia of the invertebrates, two layers are distinguished, 

 one containing almost solely nerve fibres, the other both fibres and. 

 nerve or ganglion cells. The arrangement of these layers is con- 

 trasted with that of the invertebrates in that the ganglion-cell 

 layer the gray matter lies in the centre, the fibrous layer white 

 matter ( W) on the periphery, a reversed position consequent 

 upon the development TDJ infolding. The distinction in color 

 indicated in the names depends upon the fact that white medullated 

 fibres run in the cortex, while in the gray matter gray non-medullated 

 fibres are present between the nerve cells. The color distinctions 

 fail in the cyclostomes (and AmpJiioxus), which have no medullated 

 fibres, although the same general structure occurs. 



