His CHORDATA 



tract. Besides the neurenteric canal there long persists at the anterior 

 rnd an opening to the exterior, the neuropore. 



In all vertebrates, in contradistinction to the lower chordates, the brain 

 is large and sharply marked off from the spinal cord (medulla spinalis). 

 The spinal cord is a cylindrical structure (flattened in Cyclostomes, 

 fig. 511) which is marked in the middle line above and below by two 

 longitudinal grooves, the dorsal and ventral fissures of the cord (sp, sa, 

 fig. 79). The central canal (CV) is greatly narrowed by the growth of the- 

 IUTVOUS tissue, in which 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 contrasted with that of the inverte- 

 brates 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 by infolding. The distinction in color 

 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 

 Amphioocus), which have no medullated fibres, although the same general 

 structure occurs. 



The gray matter surrounds the central canal, but extends on either 

 side dorsally and ventrally into the white matter, so that in section it 

 resembles somewhat the letter H, with its dorsal (fig. 79, HH) and 

 ventral horns (VH}. These horns and the dorsal and ventral nerve 

 roots arising from them, divide the white matter on either side into three 

 tracts, the dorsal (H), ventral (s), and lateral (S) columns of the cord. 



Corresponding to each muscle segment two nerve roots arise from the 

 cord, a dorsal root, with a ganglon (spinal ganglion) at some distance 

 from the cord, and a ventral root, without a ganglion. The dorsal root 

 contains mostly sensor}' fibres i.e., those carrying nervous impulses to the 

 cord and is afferent, while the ventral roots are efferent and contain only 

 motor elements (Bell's Law). These roots unite into a mixed root, which 

 then divides into dorsal and ventral branches. 



The brain of vertebrates in general corresponds in its fundamental 

 plan (figs. 525, 526), best seen in development, with the brain of man. 

 At first, in the embryos of a few lower vertebrates there is a stage with 

 two divisions, an anterior arcliencephalon and a posterior metenceplialon 

 which passes into the spinal cord. This condition is transitory and 

 gives place to a brain with three regions by the division of the archen- 

 cephalon into a. fore brain (prosencephalori) and amid brain (mesencephalon), 

 the metencephalon forming the hind brain. Usually this stage is reached 

 before the closure of the medullary folds. Formerly it was stated that a 



