THE STRUCTURE OF THE SPINAL CORD. 541 



cut across, the degeneration actually descends, in the sense that the progress 

 of the degenerative changes may be traced downward ; they begin at the 

 section and travel downward at a rate sufficiently slow to permit a difference 

 being observed between the progress of degeneration at a spot near the sec- 

 tion and that of one further off. After section of or injury to the spinal cord, 

 however, it is not possible to trace any such progress either upward or down- 

 ward ; in the tracts both above and below the section or injury, degenera- 

 tion 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 degeneration is concerned, between parts near 

 to and those far from the section and injury. When, for instance, the cord 

 is divided in the cervical region, subsequent examination of the tracts of 

 the so-called descending degeneration shows that the degeneration 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 degeneration 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 ab- 

 sorbed, 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 deeply 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 others as undergoing ascending degen- 

 eration. Further, the delimitation of tracts of white matter by the process 

 of degeneration agrees so well with the results of the embryological method 

 as to leave no doubt that the white matter does consist of tracts which differ 

 from each other in nature and in function. 



The several tracts thus indicated vary in different regions of the cord. 

 They may be broadly described as follows : 



I. Descending tracts, that is to say, tracts which undergo a descending 

 degeneration in the sense noted above. 



The most important and conspicuous is a large tract (Fig. 123, cr.P.) 

 occupying the posterior part of the lateral column, coming close upon the 

 outer margin of the posterior horn, and for the most part not reaching the 

 surface of the cord. We shall have to return to this tract more than once, 

 and may here simply say that it is most distinctly marked out by both the 

 embryological and the degeneration methods, that it may be traced along 

 the whole length of the cord from the top of the cervical region to the end 

 of the sacral region, and that it enters the cord from the brain through the 

 structures called the pyramids of the bulb, which we shall study later on. 

 These pyramids cross over or decussate as they are about to pass into the 

 cord, forming what is known as the decussation of the pyramids, and the 

 tract of fibres in question shares in this decussation. Hence this tract is 

 called the crossed pyramidal tract or more simply the pyramidal tract. 



A smaller, less conspicuous descending tract occupies the median portion 

 of the anterior column (Fig. 123, d.P.}. This is not only much smaller but 

 also much more variable than the crossed pyramidal tract, is not present in 



