CHAPTER XXIX. 



SPECIAL PHYSIOLOGY OF NERVES. 

 SPINAL NERVES. 



The thirty-one pairs of nerves which leave the vertebral canal 

 by the openings between the vertebrae are called spinal nerves, in 

 contradistinction to the cranial nerves, which come out through 

 the base of the skull. They are attached to the spinal marrow 

 by two bands, the anterior and posterior " roots," which unite 

 together in the intervertebral canal to form the trunk of the 

 nerve. Just before the junction of the two roots the posterior 

 one is enlarged by a ganglionic swelling. 



The spinal nerves are all " mixed nerves," that is to say, they 

 contain both efferent and afferent fibres ; but these two sets of 

 fibres run separately in the anterior and posterior roots of each 

 nerve. The spinal nerves are thus joined to the spinal marrow 

 by two nervous cords, each one of which contains only efferent 

 or afferent channels. About seventy years ago Charles Bell dis- 

 covered that the anterior roots carry the efferent fibres and the 

 posterior the afferent. Hence, the anterior are commonly spoken 

 of as the motor roots, and the posterior as the sensory roots of the 

 spinal nerves. The experiment to show this difference is simple, 

 but requires very delicate manipulation. If the anterior roots of 

 the nerves supplying the hind leg of a recently-killed frog be 

 divided, the muscles of the limb are cut off from the centres in 

 the spinal cord, and therefore the leg hangs limply, and does not 

 move if it is pinched when the frog is suspended ; whereas the 

 limb on the sound side, upon which the anterior roots are intact, 

 will move energetically when the motionless one is irritated. If 

 the distal ends of the divided anterior roots be stimulated, the 

 muscles of the paralyzed limb are thrown into action ; but stimu- 

 lation of the proximal end gives no result. Further, if the two 

 webs of this frog be compared, the blood vessels running across 

 the transparent part of the web on the injured side will be found 



519 



