DEGENERATIONS IN THE SPINAL CORD. 27 



DEGENERATIONS IN THE SPINAL CORD AS THE RESULT OF LESIONS. 



It necessarily results from the "Walleriau law of degeneration of nerve-fibres 

 (Vol. I., p. 356) that every lesion of the nervous system in which fibres are cut or 

 crushed must be followed by degeneration of nerve-fibres either above or below the 

 lesion according to the position of the cells from which the fibres have originally 

 grown, and which serve throughout life to maintain the nutrition of the fibres with 

 which they are connected. Any such degeneration which occurs above the lesion is 

 usually termed " ascending," and below the lesion " descending." 



These terms must not be taken to imply, as is erroneously done by some authors, either that 

 the degeneration starts from the lesion and extends upwards or downwards along the fibres, 

 or that the nervous impulses which the fibres conduct necessarily have an ascending or a 

 descending direction. For it is known (1) that the degeneration in the peripheral part of a 

 cut nerve-fibre occurs simultaneously along its whole course ; (2) that a nerve-fibre may 

 undergo " descending " degeneration, although it normally conveys ascending impulses, e.g., 

 the peripheral afferent nerves. 



Since the terms ascending and descending as applied to nerve-degeneration lead to much 

 confusion of ideas it would be better, if possible, to discard them and to adopt words which 

 merely imply that the degenerative process occurs above or below a lesion. But unfortunately 

 those terms have taken such deep root in the literature of the subject that this course 

 would be very difficult, and might lead to even worse confusion than their retention. 



Similarly, if the nerve-cells from which the fibres are thus derived are removed 

 or destroyed, the fibres, with any branches which they may give off, will degenerate 

 along their whole course. Hence any tracts of fibres in the spinal cord which are 

 derived from cells in parts of the brain, degenerate on destruction of those parts. 

 .Further, those fibres which have grown into the spinal cord or brain from nerve-cells 

 situated external to those organs (e.g., the fibres of posterior roots of spinal nerves and 

 corresponding fibres of cranial nerves) will, if cut between the spinal cord or brain 

 and the ganglion-cells from which they are derived, undergo degeneration from the 

 point of section, not only as far as the surface of the central organ, but also along 

 their whole course within that organ, and the degenerative process will include all 

 their branches. Thus much having been said to explain the meaning of the results 

 which are obtained by the study of nerve-degenerations, we may proceed to consider 

 the degenerations which occur within the spinal cord : (1) from section of the pos- 

 terior roots of the nerves ; (2) from lesion or removal of parts of the brain ; (3) from 

 section or other lesion of the spinal cord itself. 



1. Degenerations resulting from section of the posterior roots. These 

 have been investigated in the dog by Singer and Kahler, in the monkey by Mott 

 and Tooth. It is convenient in describing the effect of sections of the posterior 

 nerve-roots to consider them in the sacro-lumbar and in the dorso-cervical regions 

 respectively, although the course of the degenerative process is essentially similar in 

 both. 



(a.} Section of the posterior roots of the sacro-lumbar nerves. This has been 

 most satisfactorily investigated by Singer in the dog and by Mott in the monkey. 

 Section of these nerves, e.g., of the whole cauda equina, is followed by " ascending " 

 degeneration in the posterior column of the same side along the whole length of the 

 spinal cord and as far as the nucleus gracilis of the medulla oblongata. But the 

 degeneration does not occupy the same position and extent in every part of the 

 cord. At the lowest part it involves the whole of the posterior column (fig. 21, a), 

 but as soon as a point is reached at which uncut nerve-fibres enter the cord, these 

 now occupy the part of the column nearest the horn of grey matter, while degenerated 

 fibres are seen in that part only which is nearest the posterior median fissure (i). 



