850 SPINAL NERVES. [BOOK HI. 



of the spinal cord itself, it will be desirable to say a few words 

 concerning the spinal nerves, that is to say the nerves which issue 

 from the spinal cord. 



We have already seen ( 96) that each of the spinal nerves 

 arises by two roots, an anterior root attached to the ventral or 

 anterior surface, and a posterior root attached to the dorsal or 

 posterior surface of the cord. We have further seen that the 

 latter bears a ganglion, a 'ganglion of the posterior root' or 'spinal 

 ganglion/ and we have ( 97) studied the structure of this 

 ganglion. 



We stated at the same time that while the trunk of a spinal 

 nerve contained both efferent and afferent fibres, the efferent 

 fibres were gathered up into the anterior root and the afferent 

 fibres into the posterior root ; but we gave no proof of this state- 

 ment. 



559. Before we proceed to do so, it will be as well to say a few 

 words on the terms 'efferent' and 'afferent.' By efferent nerve 

 fibres we mean nerve fibres which in the body usually carry 

 impulses from the central nervous system to peripheral organs. 

 Most efferent nerve fibres carry impulses to muscles, striated or 

 plain, and the impulses passing along them give rise to movements ; 

 hence they are frequently spoken of as 'motor' fibres. But all 

 efferent fibres do not end in or carry impulses to muscular fibres ; 

 we have seen for instance that some efferent fibres are secretory. 

 Moreover all the nerve fibres going to muscular fibres do not serve 

 to produce movement ; some of them, as in the case of certain vagus 

 fibres going to the heart, are inhibitory and may serve to stop 

 movement. 



By 'afferent' nerve fibres we mean nerve fibres which in the 

 body usually carry impulses from peripheral organs to the central 

 nervous system. A very common effect of the arrival at the central 

 nervous system of impulses passing along afferent fibres is that 

 change in consciousness which we call a 'sensation'; hence afferent 

 fibres or impulses are often called ' sensory ' fibres or impulses. 

 But as we have already in part seen, and as we shall shortly see 

 in greater detail, the central nervous system may be affected by 

 afferent impulses, and that in several ways, quite apart from the 

 development of any such change of consciousness as may be fairly 

 called a sensation. We shall see reason for thinking that afferent 

 impulses reaching the spinal cord, and indeed other parts of the 

 central nervous system, may modify reflex or automatic or other 

 activity without necessarily giving rise to a " sensation." Hence it 

 is advisable to reserve the terms 'efferent' and 'afferent' as more 

 general modes of expression than ' motor ' or ' sensory.' 



We have seen in treating of muscle and nerve, that the changes 

 produced in the muscle serve as our best guide for determining the 

 changes taking place in a motor nerve ; when a motor nerve is 



