CHAP. XVIII.] THE NERVOUS SYSTEM. 



213 



efferent fibres, and convey nervous impulses from the spinal 

 cord to the periphery. The fibres growing into the posterior 

 root are afferent fibres, and convey nervous impulses from the 

 periphery to the spinal cord. The afferent fibres usually con- 

 vey sensory impressions, and the efferent motor impulses. 



The thirty-one pairs of spinal nerves, containing within the 

 one sheath both afferent and efferent fibres, issue at regular 

 intervals from the spinal canal, and are distributed to all parts 

 of the body, the 

 efferent or motor 

 fibres being distrib- 

 uted to the muscular 

 tissue, and causing 

 contraction of the 

 muscles ; the afferent 

 or sensory fibres end- 

 ing commonly in the 

 skin, and conveying 

 nervous impulses to 

 the nerve - centres 

 which give rise to 

 sensation. 



The nervous im- 

 pulses transmitted by 



the afferent nerves to 

 the spinal cord are 



the cord to the brain, 

 and the nervous im- 



FIG. 123. SECTIONS OF SPINAL CORD AND 

 NERVE-ROOTS. A, anterior view; B, side view; C, 



USUallv forwarded bv transverse section; J), two isolated roots of a spinal 

 J * nerve. 1, anterior fissure; 2, posterior fissure; 3, 



groove formed by anterior roots; 4, groove formed 

 by posterior roots ; 5, fibres of anterior root ; 6, 

 fibres of posterior root; 6, ganglion on posterior 



pulses sent out along root, 

 the efferent fibres to 



the muscles are also commonly transmitted by the spinal cord 

 from the brain. And if the spinal cord be so injured as to 

 practically sever its connection with the brain, it will be found 

 that paralysis and insensibility of the body below the injury 

 will ensue, showing conclusively that, severed from the brain, 

 the ability of the spinal cord to give rise to sensation, or to 

 control muscular contractions, is seriously impaired. Yet the 

 gray matter in the spinal cord has a certain power of its own, 

 for after the cord has been severed from the brain, spasmodic 



