Chap. VIL] 



THE NERVOUS SYSTEM. 



81 



completely divided into lateral 

 halves by an anterior and pos- 

 terior fissure, the anterior fis- 

 sure dividing it in the middle 

 line in front, and the posterior 

 fissure, in the middle line be- 

 hind. In consequence of the 

 presence of these fissures, only 

 a narrow bridge of the sub- 

 stance of the cord connects its 

 two halves, and this bridge is 

 traversed throughout its en- 

 tire length by a minute central 

 canal, — the canalis centralis. 

 On making a transverse section 

 of the spinal cord, the gray 

 matter is seen to be arranged 

 in each half in the form of a 

 half-moon or crescent, with one 

 end bigger than the other, and 

 with the concave side turned 

 outwards. The convex sides of 

 the gray matter in each half 

 approach one another, and are 

 joined by the isthmus or bridge 

 which contains the central 

 canal. The tips of each cres- 

 cent are called its horns or 

 cornua, the front or ventral 

 horns being thicker and larger 

 than the dorsal. The white 

 matter of the cord is arranged 

 around and between the gray 

 matter, the proportion of gray 

 and white matter varying in 

 different regions of the cord. 

 The white matter, as in the 

 brain, is composed of medul- 

 lated nerves, and the gray 

 matter of cell-bodies and fine 



\Cocc. 



Fig. 69. — Base of Brain, Spinal 

 Cord, and Spinal Nerves. — V, 5th 

 nerve ; VI, 6th nerve ; VII, a, facial nerve, 

 b, auditory nerve; VlII, piieunio-gastric 

 nerve ; VIII, a, glosso-pharyngeal, b, 

 spinal accessory ; IX, hypoglossal ; c^^c', 

 cervical nerve roots ; D^D^'^, dorsal nerve 

 roots ; U—Ip, lumbar nerve roots; S^, S^, 

 4th and 5th sacral nerves ; C'occ, coccyg- 

 eal nerves; i?. P., brachial plexus; L.P., 

 lumbar plexus ; S. P., sacral plexus; Sa, 

 b, c, cervical sympathetic ganglia. 



