544 THE SPINAL CORD. 



larly leave for the present the small zone of white matter composed of very 

 fine fibres known as the posterior marginal zone or Lissauer's zone (Fig. 123, 

 L.), lying dorsal to the tip of the posterior horn and in the lower regions 

 reaching to the outside of the cord ; for this, too, belongs to the fibres of the 

 posterior root. Leaving these parts out of consideration we may say as 

 regards the rest of the white matter that the present state of our knowledge 

 will not allow us to divide it into special tracts. All this area is largely 

 composed of fibres which do not undergo either ascending or descending 

 degeneration as the result of section, injury, or disease. It has been sug- 

 gested that these fibres either have no trophic centre at all or have double 

 ones, one above and one below, on either of which they can in case of need 

 lean ; so that when the fibre is divided at any level, the upper portion is still 

 nourished from some centre above, and the lower from some centre below. 

 At all events, whether this be the true explanation or not, the fibres in this 

 part of the white matter cannot be differentiated into tracts by a study of 

 their degeneration. Fibres of this kind, which we can speak of neither as 

 ascending nor as descending, also occur in the external posterior column 

 mingled with the fibres of the posterior root. And we may repeat the cau- 

 tion, that even in the several ascending and descending tracts just described, 

 especially in those which we spoke of as less distinct or as more diffuse, many 

 fibres are present which undergo neither ascending nor descending degen- 

 eration. 



481. It may be as well perhaps to insist here once more, that when 

 these several tracts or the fibres running in the tracts are spoken of as ascend- 

 ing or descending, what is meant is that the degeneration takes place above 

 the section or seat of injury or disease in the one case, and takes place below 

 in the other. It has been supposed by many that the nervous impulses which 

 these fibres severally carry, travel in the same direction as that taken by the 

 degeneration, that the ascending tracts carry impulses from below upward, 

 that is to say, carry impulses which arising from peripheral organs pass to 

 various part of the spinal cord or of the brain, that they are, in other words, 

 channels of afferent impulses, and that conversely the descending tracts carry 

 efferent impulses. To this view is often added as a corollary, that the tracts 

 which do not degenerate at all carry impulses both ways, and hence cannot 

 be considered as either afferent or efferent channels, but simply as communi- 

 cating channels. Upon this it may be remarked that impulses do not neces- 

 sarily 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 toward the periphery, though the 

 former carry impulses in the other direction. Hence the direction of degen- 

 eration 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 di- 

 rection, 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 

 behavior along the length of the cord before we can use the facts concerning 

 them as a basis for any discussion as to their functions. 



482. The connections of the nerve-roots. If we regard the spinal cord, 

 and apparently we have a 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 



