876 THE NERVE ROOTS. [BOOK in. 



channels but simply as communicating channels. Upon this it 

 may be remarked that impulses do not necessarily travel in the 

 same direction as the degeneration; when a spinal nerve trunk is 

 divided the afferent fibres as well as the efferent fibres both 

 degenerate in a descending direction towards the periphery, though 

 the former carry impulses in the other direction. Hence the 

 direction of degeneration is no proof of the direction in which 

 impulses travel ; moreover, as we have seen, degeneration does not 

 actually travel along the fibres of the spinal cord in the same way 

 that it does along the fibres of a nerve trunk. It may be that the 

 descending tracts do carry impulses in a descending direction, that 

 is, efferent impulses, and that the ascending tracts serve to carry 

 afferent impulses ; but the proof that they do thus respectively 

 act must be supplied from other facts than those of degeneration. 

 Moreover, we shall have to return to these ascending and descending- 

 tracts and to study their behaviour along the length of the cord 

 before we can use the facts concerning them as a basis for any 

 discussion as to their functions. 



569. The connections of the nerve roots. If we regard the 

 spinal cord, and apparently we have right to do so, as resulting 

 from the fusion of a series of segments or metameres, each 

 segment, represented by a pair of spinal nerves, being a ganglionic 

 mass, that is to say a mass containing nerve-cells with which nerve 

 fibres are connected, we should expect to find that the fibres of a 

 spinal nerve soon after entering in, or before issuing from the spinal 

 cord are connected with nerve-cells lying in the neighbourhood 

 of the attachment of the nerve to the cord. We should, we say, 

 expect to find this ; but owing to the difficulty of tracing individual 

 nerve fibres through the tangled mass of the substance of the cord, 

 our actual knowledge of the termination of the fibres of the 

 posterior root, and origin of the fibres of the anterior root is at 

 present far from complete. 



With regard to the anterior root, there can be no doubt that 

 a veiy large proportion of the fibres in the root are continuations 

 of the axis-cylinders of cells in the anterior horn. The fibres 

 which can thus be traced are of large diameter and appear to be 

 chiefly if not exclusively motor fibres for the skeletal muscles. In 

 the frog a laborious enumeration on the one hand of the number 

 of fibres in the anterior roots, and on the other hand of the 

 number of cells of the anterior horn in the areas corresponding 

 to the nerve roots has, it is true, shewn a very remarkable 

 agreement in number between the two. We might be inclined 

 from this to conclude that all the fibres of an anterior root start 

 directly from cells in the anterior horn, and that all the cells in 

 the anterior horn end in fibres of the nearest anterior root. 

 But several considerations prevent us from trusting too much to 

 this observation, especially in the case of the higher animals. 

 The anterior root contains other fibres than motor fibres for the 



