540 THE SPINAL CORD. 



column, commonly called the postero-median column or column of Goll (Fig. 

 119. m.p.\ from an outer lateral portion, the external posterior column, com- 

 monly called the postero-external column or column of Burdach (Fig. 119, 

 e. p."), the lateral part of which, nearer the gray matter, has, for reasons 

 which we shall see later on, been called the posterior root-zone. But beyond 

 this neither the irregular septa nor other features will enable us to distin- 

 guish one part of the white matter as different in nature from another. 

 Nor have we better success when with the scalpel we attempt to dissect 

 out the white matter into separate strands. Nevertheless we have convin- 

 cing evidence that the white matter is arranged in strands, or tracts, or 

 columns, which have different connections at their respective ends, which 

 behave differently under different circumstances, which we have every 

 reason to believe carry out different functions, but which cannot be sepa- 

 rated by the scalpel, because each of them is more or less mixed with fibres 

 of a different nature and origin. The evidence for the existence of these 

 tracts is twofold. 



One kind of evidence is embryological in nature. When a nerve-fibre 

 is being formed in the embryo, either in the spinal cord or elsewhere, the 

 essential axis-cylinder is formed first and the less essential medulla is 

 formed later. Now when the developmental history of the spinal cord is 

 studied it is found that, in the several regions of the cord, all the fibres of 

 the white matter do not put on the medulla at the same time. On the con- 

 trary, in certain tracts, the medulla of the fibres makes its appearance early, 

 in others later. By this method it becomes possible to distinguish certain 

 tracts from others. 



Another kind of evidence is supplied by facts relating to the degenera- 

 tion of the fibres of the white matter. We have seen ( 474) that the 

 degeneration of a nerve-fibre is the result of the separation of the fibre 

 from its trophic centre, and that while the trophic centre of the afferent 

 fibres is in the ganglion on the posterior root, that of the efferent fibres is 

 in some part of the spinal cord. In the case of the efferent fibres the 

 degeneration might be spoken of as descending from the spinal cord to the 

 muscles or other peripheral organs. In the case of the afferent fibres of 

 the trunk of the nerve, the degeneration is also one descending from the 

 ganglion down to the skin or other peripheral organ. When, however, the 

 section is carried through the posterior root of a spinal nerve, the degener- 

 ation takes place in the part of the nerve between the section and the spinal 

 cord ; it runs up from the section to and into the spinal cord, and may, 

 therefore, be called an ascending degeneration. Thus we may say that 

 when a nerve-trunk or when a nerve-root is cut completely across, all the 

 fibres, which are thereby separated from their trophic centres, degenerate. 

 When the nerve-trunk is divided, all the fibres below the section undergo 

 descending degeneration. If the anterior root be cut across, all the fibres 

 of the root below the section undergo descending degeneration. If the 

 posterior root be cut across, all the fibres of the root above the section un- 

 dergo 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 degen- 

 erate 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 witk caution. When a nerve-trunk is 



