CH. 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 

 both. 



Divisions of the Nervous System. — The nervous system 

 is divisible into (i) the ce?itral nervous system, consisting of 

 the brain and spinal cord (Figs. 6 and 7), and (2) \k\& 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, and (2) spinal nerves 

 (Fig. 51) arising from the spinal cord. 



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 posteriorily into a fine thread- 

 like portion, the filum tertninale {f.t), while opposite the 

 fore-limbs, and just anteriorly to the filum terminale, it pre- 

 sents a couple of enlargements, 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.f), extends along 

 its lower surface. 



The cord is covered with a delicate pigmented mem- 

 brane known as the pia mater {p.w) and the neural canal in 

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

 mater {Vig. 52, d.m). Between the two is a cavity 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 sliow 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 



