CHAP, i.] THE SPINAL CORD. 871 



ascending degeneration with the exception of certain fibres which 

 do not degenerate at all, and of which we shall speak later on. 



When the spinal cord is cut across, for instance in the dorsal 

 region, all the fibres of the white matter do not degenerate either 

 in the part of the cord above the section or in the part below. 

 Some fibres, and indeed some tracts of fibres degenerate, and some 

 do not. Further, some tracts degenerate in the cord above the 

 section, and thus undergo what has been called an ascending 

 degeneration ; other tracts degenerate in the cord below the 

 section, and thus undergo what has been called a descending 

 degeneration. These terms must however be used with caution. 

 When a nerve trunk is cut across, the degeneration actually 

 descends, in the sense that the progress of the degenerative 

 changes may be traced downwards; they begin at the section 

 and travel downwards at a rate sufficiently slow to permit a 

 difference being observed between the progress of degeneration at 

 a spot near the section and that at one farther off. After section 

 of or injury to the spinal cord, however, it is not possible to 

 trace any such progress either upwards or downwards; in the 

 tracts both above and below the section or injury, degeneration 

 either begins simultaneously along the whole length of the 

 degenerating tract, or progresses along the tract so rapidly 

 that no differences can be observed as far as the stage of de- 

 generation is concerned between parts near to and those far 

 from the section or injury. When, for instance, the cord is 

 divided in the cervical region, subsequent examination of the 

 tracts of so-called descending degeneration shews that the de- 

 generation is as far advanced in the lumbar region far away 

 from the section as in the cervical region just below the section. 

 Applied to the spinal cord, therefore, the term descending de- 

 generation means simply degeneration below the seat of injury 

 or disease, ascending degeneration means simply degeneration 

 above the seat of injury or disease. We may add that the 

 histological features of the degeneration of fibres in the spinal 

 cord are not wholly identical with those of the degeneration 

 of fibres in a nerve trunk. Thus, the neurilemma with its nuclei 

 being absent from the fibres of the cord, no proliferation of nuclei 

 takes place ; the axis-cylinder and medulla simply break up, are 

 absorbed and disappear. 



Similar degenerations, ascending, or descending or both, are 

 seen when the section is not carried right through the whole 

 cord, but particular parts of the cord are cut through or simply 

 injured. And similar degenerations occur as the consequences 

 of disease set up in parts of the cord. 



In this way the results of sections of or of other injuries to or 

 of diseases of the spinal cord have enabled us to mark out certain 

 tracts of the white matter as undergoing degeneration and others 

 as not, and moreover certain tracts as undergoing descending and 



