CHAP, x SPINAL CORD 155 



trolling apparatus, by which the functions of the body are 

 regulated, is lodged in either the brain or the spinal cord, or 

 in both. 



Divisions of the Nervous System. The nervous system 

 is divisible into (i) the central nervous system, consisting of 

 the brain and spinal cord (Figs. 6 and 7), and (2) the peri- 

 pheral, nervous system, Consisting of the nerves which pass 

 from the central nervous system to the various parts of the 

 body. The nerves are divisible into (i) cerebral nerves 

 (Fig. 53), arising from the brain, (2) spinal nerves (Fig. 51), 

 arising from the spinal cord, and (3) sympathetic nerves. 



The Spinal Cord. In form the spinal cord (Figs. 6 and 

 7, sp. cd} is irregularly cylindrical. It is continuous in front 

 with the brain, and tapers off posteriorly into a fine thread- 

 like portion, the filum terminale (//), while opposite the 

 fore-limbs, and again just anteriorly to the filum terminale, 

 it presents an enlargement : these are known respectively 

 as the brachial and sciatic swellings. Along its dorsal surface 

 runs a delicate longitudinal line, the dorsal fissure (Fig. 48, 

 d.f), and a distinct groove, the ventral fissure (v.j\ extends 

 along its lower surface. 



The cord is covered with a delicate pigmented mem- 

 brane known as the//# mater (p.w) and the neural canal in 

 which it lies is lined with a stout, tough membrane, the dura 

 mater (Fig. 52, d.m). Between the two is a space filled with 

 a lymphatic arachnoid fluid, which, like the pericardial fluid, 

 preserves the contained organ from shocks. 



Examination of a transverse section of the cord under a 

 low power will show that the dorsal fissure is an extremely 

 narrow vertical wall formed by an extension inwards of the 

 pia mater. The ventral fissure is a distinct cleft. Thus the 

 two fissures divide the cord into paired half-cylinders, right 

 and left, joined in the middle by a narrow bridge. This 



