CHAP, x SPINAL CORD 155 



nerves, to the muscles. It may therefore be inferred 

 that the controlling 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 

 peripheral 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 (Fig. 51). 



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 oft" posteriorly into a 

 fine thread-like portion, the filnm terminale (f. t), 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. /), 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. it 1 ), and the neural 

 canal in which it lies is lined with a stout, tough mem- 

 brane, 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 exten- 

 sion inwards of the pia mater. The ventral fissure is 

 a distinct cleft. Thus the two fissures divide the cord 



