xvn THE NERVOUS SYS I I M 181 



periosteum of the bones, and receives the special name of dura 

 mater. The brain and spinal cord themselves are closely 

 covered by a delicate, very vascular membrane, called the pia 

 mater, from which they receive blood-vessels. Between the 

 pia mater and the dura mater is a space containing a little 

 lymph-like fluid, and some loose connective tissue attached 

 partly to one and partly to the other membrane. This loose 

 connective tissue forms a third membrane. It is called the 

 arachnoid membrane. 



The Spinal Cord. The spinal cord is a column of soft 

 substance extending from the brain downwards along the spinal 

 i .inal to, in man, about the level of the second lumbar ver- 

 tebra, where it tapers off into a filament. It is about 18 

 inches long in a man of average height, and about half an inch 

 across. Running along the front of the cord is a deep groove 

 called the anterior fissure, and along the back of the cord 

 another deep cleft called the posterior fissure. The two 

 fissures extend into the cord so far that they nearly meet, 

 leaving only a narrow bridge of tissue connecting the two 

 halves. In the centre of this bridge of tissue is a small 

 canal, called the central canal, which runs along the middle 

 of the cord. Connective tissue from the pia mater passes into 

 the fissures, carrying in with it blood-vessels which, like 

 other vessels from the pia mater at the surface of the cord, 

 pass into the substance of the cord, and so supply it with blood. 



If the spinal cord be cut across, the cord will be seen to 

 be composed partly of white-looking substance lying on the 

 outside, and partly of grey- looking substance lying on the 

 inside. In each half of the cord, on each side of the fissures, 

 the grey matter ami the white matter are correspondingly 

 placed so that tin- apprarance of one half is exactly like that 

 of the other. The grey matter in each half of the cord is 

 somewhat in the form of a crescent with rounded horns, one 

 horn being directed towards the front, and the other towards 

 the back of the cord. These arc the anterior horn and the 

 posterior horn of grey matter. The white matter lirs all 

 round the crescent of gn-y matter except where the crescent in 

 one half is conm-< u-d, as it is, by grey matter to the crescent in 

 the other half. 



The Spinal Nerves. From the spinal cord, at intervals 



