852 SPINAL NERVES. [BOOK in. 



senses; but, on the other hand, a knowledge of the central nervous 

 system is necessary to a proper understanding of the special 

 senses; and on the whole it will be more convenient to study 

 the former before the latter. 



We may, however, digress here to remark that the question 

 whether an afferent impulse differs in itself from an efferent 

 impulse is one of great difficulty. It is true that the electrical 

 changes, which alone as we have said we can appreciate in an 

 isolated piece of nerve, appear to be the same in both kinds of 

 fibres; in each the electrical change is propagated in both directions 

 and possesses the same features. But it would be hazardous to 

 insist too much on this. Moreover, we must remember that what 

 we call a nervous impulse, especially one provoked by artificial 

 stimulation, constitutes a gross change in the nerve fibre, and 

 that changes of a finer, more delicate nature, such as cannot be 

 shewn by the coarse methods used to detect a ' nervous impulse,' 

 may take place in, and be propagated along, a nerve fibre. We 

 shall have occasion immediately to point out that the condition of 

 an afferent nerve fibre along its whole length is dependent on a 

 nerve cell in the ganglion of the posterior root ; the fibre when 

 cut off from the nerve cell degenerates and dies. This means 

 that in the intact fibre certain influences are propagated along 

 the fibre from the cell in the ganglion to the peripheral endings 

 of the fibre, that is to say in a direction the opposite of that taken 

 by the ordinary afferent nervous impulses ; and it may be that in 

 like manner in efferent fibres some influences are propagated 

 centripetally from the peripheral endings to the central nervous 

 system. Our knowledge of these influences is extremely limited : 

 but it is important to bear in mind the possibility of their 

 occurrence. And we had this in view, when above, in speaking of 

 efferent and afferent fibres, we used the phrase "usually carry 

 impulses." 



560. The proof that the afferent and efferent fibres which 

 are both present in the trunk of a spinal nerve are parted at the 

 roots, the efferent fibres running exclusively in the ventral or 

 anterior root and the afferent fibres exclusively in the dorsal or 

 posterior root, is as follows. 



When the anterior root is divided, the muscles supplied by the 

 nerve cease to be thrown into contractions either by the will, or by 

 reflex action, while the structures to which the nerve is distributed 

 retain their sensibility. During the section of the root, or when 

 the proximal stump, that connected with the spinal cord, is stimu- 

 lated, no sensory effects are produced. When the distal stump is 

 stimulated, the muscles supplied by the nerve are thrown into 

 contractions. When the posterior root is divided, the muscles 

 supplied by the nerve continue to be thrown into action by an 

 exercise of the will or as part of a reflex action, but the structures 



