454 ELEMENTARY PHYSIOLOGY less, xi 



space, though small and insignificant in the region of 

 the brain, becomes large in the region of the spinal cord, 

 and here contains a considerable quantity of fluid, called 

 cerebro-spinal fluid. This fluid has the aj)pearance 

 of ordinary lym])h, but there the resemblance ends ; for 

 cerebro-sprnal fluid contains only a minute amount of 

 proteids (globulins), does not clot as true lymph does, and 

 contains a peculiar reducing substance, which is, however, 

 not a sugar. 



3. The Arrangement and General Structure of the 

 Spinal Cord and the Roots of the Spinal Nerves. —The 

 spinal cord (Fig. 142) is a column of greyish-white soft 

 substance, extending from the top of the spinal canal, 

 where it is continuous by means of the spinal bulb with 

 the brain, to about the second lumbar vertebra, where it 

 tapers off" into a filament. Starting at the level of the 

 junction of the atlas vertebra with the skull, the spinal 

 cord gives off laterally thirty-one pairs of spinal nerves 

 whose trunks pass out of the spinal canal by a])ertures 

 between the vertebra;, called the intervertebral 

 foramina, and then divide and subdivide, their ultimate 

 branches going for the most part to the muscles and to 

 the skin. Each nerve originates from the cord by two 

 roots, consequently there are twice as many roots as there 

 are spinal nerves (Fig. 144). After their exit from the 

 s[)inal canal the spinal nerves become connected with a 

 cliain of ganglia which lies parallel to the spinal cord and 

 constitutes the sympathetic nervous system (Fig. 142), 

 which will be described later on. 



Transverse sections of the cord show thac a deep, 

 somewhat broad, fissure, the anterior fissure (Fig. 143, 

 1), divides it in the middle line in front, nearly down to 

 its centre : and a similar deeper but narrower cleft, the 

 posterior fissure (Fig. 143, 2), also extends nearly to 

 its centre in the middle line behind. The pia mater 

 extends more or less into each of these fissures, and 

 supports the vessels which supply the cord with blood. 



