CH. XLII.J TRACTS IN THE WHITE MATTER. 609 



addition to the columns of the white matter which are marked 

 out by the points from which the nerve-roots issue, and which 

 are the anterior, the lateral and the posterior, the posterior 

 is further divided by a septum of the pia mater into two almost 

 equal parts, constituting the postero-external column, or column 

 of Burdock (fig. 452, 2), and the postero-mediari, or column of 

 Goll (fig. 452, i). In addition to these columns, however, it 

 has been shown that the white matter can be still further sub- 

 divided. This subdivision has been accomplished by evidence 

 of several kinds, that the parts, or as they are called, tracts in 

 the white matter, perform different functions in the conduction 

 of impulses. 



The methods of observation are the following : 



(a) The embryological method. It has been found by examin- 

 ing the spinal cord at different stages of its development that 

 certain groups of the nerve-fibres put on their myelin sheath at 

 earlier periods than others, and that the different groups of fibres 

 can therefore be traced in various directions. This is also known 

 as the method of Flechsig. 



(b) Wallerian or degeneration method. This method depends 

 upon the fact that if a nerve-fibre is separated from its nerve- 

 cell, it wastes or degenerates. It consists in tracing the course of 

 tracts of degenerated fibres, which result from an injury to any part 

 of the central nervous system. When fibres degenerate below a 

 lesion the tract is said to be of descending degeneration, and when 

 the fibres degenerate in the opposite direction, the tract is one of 

 ascending degeneration. By the modern methods employed in 

 staining the central nervous system, it has proved comparatively 

 easy to distinguish degenerated parts in sections of the cord and 

 of other portions of the central nervous system. Degenerated 

 fibres have a different staining reaction when the sections 

 are stained by what are called Weigert's and Pal's methods, 

 which consist in subjecting them to a special solution of 

 hsematoxylin, and then to special differentiating solutions. The 

 degenerated fibres appear light yellow, whereas the healthy fibres 

 are a deep blue. Marchi's method is even better. After harden- 

 ing in Midler's fluid Marchi's solution (a mixture of Muller's fluid 

 and osmic acid) stains degenerated fibres black, and leaves the 

 rest of the tissue unstained. Accidents to the central nervous 

 system in man have given us much information upon this sub- 

 ject, but this has of late years been supplemented and largely 

 extended by experiments on animals, particularly upon monkeys ; 

 and considerable light has been shed upon the conduction of 

 impulses to and from the nervous system by the study of the 



K.P. B B 



