228 



THE NERVOUS SYSTEM 



to protect it from external injury. For, while a very slight 

 disturbance of its structure suffices to disarm it of its power, 

 yet so staunch is its bony enclos- 

 ure, that only by very severe inju- 

 ries is it put in peril. The three 

 membranes that cover the brain are 

 continued downward, so as to en- 

 velope and still further shield this 

 delicate organism. 



17. The Spinal Nerves. The 

 spinal nerves, thirty-one pairs in 

 number, spring from each side of 

 the cord by two roots, an anterior 

 and a posterior root, which have the 

 same functions as the columns bear- 

 ing similar names. The posterior 

 root is distinguished by possessing 

 a ganglion of gray matter, and by a 

 somewhat larger size. The succes- 

 sive points of departure, or the off- 

 shooting of these nerves, occur at 

 short and nearly regular intervals 

 along the course of the spinal cord. 

 Soon after leaving these points, the 

 anterior and posterior roots unite to 

 form the trunk of a nerve, which is 

 distributed, by means of branches, 

 to the various organs of that part 

 of the body which this nerve is 

 designed to serve. The spinal 

 nerves supply chiefly the muscles 

 of the trunk and limbs and the 

 external surface of the body. 



18. The tissue composing the 

 nerves is entirely of the white 



FIG. 58 



A, CEREBRUM, B, CEREBELLUM, 

 D, D, SPINAL CORD 



17. The spinal nerves ? The posterior root ? The nerves, how arranged ? Their office? 



18. The nerve tissue ~t Its oliui-acter ? Course of each nerve-fibre ? 



