676 SPINAL NERVES. [BOOK in. 



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 

 4 spinal 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 statement. 



449. 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 passirig 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 cen- 

 tral 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 whicl) we call a 'sensa- 

 tion ' ; 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 con- 

 sciousness 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 separated from its muscle ( 67) the only 

 change which we can appreciate in it is an electrical change. 

 Similarly in the case of an afferent nerve, the central system is 

 our chief teacher ; in a bundle of afferent fibres isolated from 

 the central nervous system, in a posterior root of a spinal nerve 

 for instance, the only change which we can appreciate is an 

 electrical change. To learn the characters of afferent impulses 



