28 NATURAL HISTORY OF VERTEBRATES. 



to distinguish between the cerebro-spinal and the sympathetic nerve-centres, but we 

 shall see that the latter system is very closely connected with the former in develop- 

 ment, so that we have first to consider the spinal cord and brain, which together 

 constitute the cerebro-spinal nerve-centres, and then briefly the nerves in connection 

 with these organs. 



In the preliminary account of the development of vertebrates, the central nervous 

 system was first seen in the form of a plate of modified epiblast — the medullary plate 

 — which then became grooved and eventually transformed into a tube, closed at both 

 ends. Certain dilatations and thickenings of the anterior end of the tube result in 

 the brain, while the spinal cord is formed by a more uniform differentiation in the rest 

 of its length. 



A comparison of Figs. 3 and 32 will show what the nature of this differentiation 

 is. The cavity of the tube is represented by the now small central canal of the cord 

 lined by the remains of the epiblast, while the rest of the epithelium has been converted 



into the white and gray nervous matter sur- 

 rounding the canal. The distinction of color 

 is diie to the presence of nerve cells m the 

 gray matter, which is chiefly aggregated 

 about the central canal, while the white 

 matter foiins the more superficial substance 



Fig. 32.— Diagrammatic section of spinal cord of of the COrd. TwO fisSUres running along 

 mammal; a, anterior, p, posterior root of spin.al . 



nerve ; c, canal ; g, ganglion ; gr, gray matter ; w, the dorsal and ventral middle lines give 



a marked bilateral symmetry to the cord, 

 which exhibits great uniformity in its thickness throughout, except where it tapers 

 at the tail, and where, in the higher forms, two swellings give rise to the nerves 

 for the anterior and posterior limbs respectively. The gray matter forms a cres- 

 centic outline on a transverse section of the cord, and nerve roots may be seen 

 to extend inwards towards the horns of the crescent. A segmented appearance 

 is given to the cord by the origin of the pairs of spinal nerves, and this segmen- 

 tation extends to the gray matter, which is more abundant in the planes of the origin 

 of the nerves than in the intermediate planes. The spinal nerves arise in pairs, the nei-ve 

 of each side originating by two roots, a dorsal and a ventral. According to what is 

 known as ' Bell's law,' the latter are purely motor, while the foi-mer are purely sensory 

 nerves, and some structural difference can also be recognized, for the dorsal roots con- 

 tain fine fibres and have a ganglion developed on them, while the ventral roots have 

 coarser fibres and no ganglion. The roots join shortly after their origin, and give 

 rise to mixed motor and sensory nerves, generally disposed in three branches, a dor- 

 sal branch to the pai'ts lying dorsad, a ventral branch to the parts lying ventrad, and 

 an intestinal branch to the contents of the body cavity. The last-named branch is 

 ganglionated, and as the various ganglia of the intestinal branches become united with 

 each other, a double ganglionated chain results on the ventral aspect of the vertebral 

 column — the sympathetic nervous system. 



Much more complex are the changes which affect the anterior end of the neural 

 tube, and which result in the formation of the brain or encephalon. At a very early 

 period three dilatations appear, which are known as the three primary cerebral vesicles, 

 but the first and third of these soon become subdivided in such a way as to render 

 five regions distinguishable, which are known as the prosencephalon, thalamencepha- 

 lon, mesencephalon, epencephalon, metencephalon. The cavities of these regions, 



