4CS ClIORDArA 



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

 end an opening to the exterior, the neuropore. 



In all \'ertebrates, in contradistinciion to the lower chordates, the brain 

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

 The spinal cord is a cylindrical structure (llattened in Cycloslomes, 

 tig. 511) which is marked in the middle line alio\'e and below b}- two 

 longitudinal grooves, the dorsal and \-cn[i\\\ jissiircs of tiic lord [sp. .ui , 

 tig. 70). The central canal ((."() is greatly narrowed by the growth of tlie 

 nervous tissue, in which two lavers are distinguished, one containing almost 

 solelv nerve tibres, the other both tibres and ncr\'e or ganglion cells. 

 The arrangement of these layers is contrasted with that of the inverte- 

 brates in tliat the ganglion-cell laver — the grav mailer — lies in the centre, 

 the librous layer — ;;7;//(' mailer (11) — on the periphery, a reversed jiosition 

 consequent upon the developiment bv infolding. The distinction in color 

 depends upon the fact that white medullatei.1 liln-es run in the corte.x, 

 while in the grav matter grav non-mcdullatetl tibres are jiresent lielween 

 the nerve cells. The color distirtclions fail in the cwdostomes (and 

 AitipliioMis), which hax'c no meduUated tibres, although the same general 

 structure occurs. 



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

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

 resembles somewhat the letter II, with its dorsal (tig. 70, ]IH) and 

 voilral hor)!s [VH). These horns and the dorsal and ventral ner\e 

 roots arising from them, di\ide the white matter on either side into three 

 tracts, the dorsal (//), ventral (s), and lateral (.S) columns of III c 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 sensory tilires — i.e., those carrying nervous impnilses to the 

 cord — and is afferent, while the ventral roots are elierent 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 correspontls in ils fundamental 

 plan (figs. 525, 526), best seen in de\'elopment, with the brain of man. 

 At first, in the embr\-os of a few lower vertebrates there is a stage with 

 two divisions, an anterior arcliciiccphahvi and a jHisterior iiuiciiccphaloii 

 which passes into the spinal cord, 'litis condition is transitory and 

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

 cephalon into afore brain [proseneeljlialoii) and amid braii: (mesoiceplialoii). 

 the metencepihalon forming the ///;/(/ brain. Usuallv this stage is reached 

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



