CH. XLV.] TRACTS IN THE SPINAL CORD 651 



(2) Posterior vesicular column of Lockhart Clarke ; generally known 

 as Clarke's column. This is a group of large nerve-cells with their long 

 axis vertical. It lies at the base of the posterior horn, and is best 

 marked in the thoracic region. The axons of these cells pass into 

 the cerebellar tracts. 



(3) Intermedio-lateral group. This is seen in the outer part of 

 the grey matter of the lateral horn, and is most distinct in the upper 

 thoracic and lower cervical regions. 



(4) The middle cell column lies in the middle of the crescent. 



(5) The cells of the posterior horn are usually small ; they are 

 numerous, but are not disposed in special groups. 



Columns and tracts in the white matter of the spinal cord. The 

 columns of the white matter which are marked out by the points 

 from which the nerve-roots issue, are called the anterior, the lateral, 

 and the posterior columns; 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 Burdach, and the postero- 

 median, or column of Goll (fig. 402). In addition to these columns, 

 however, it has been shown that the white matter can be still further 

 subdivided. These tracts in the white matter perform different 

 functions in the conduction of impulses. 



These tracts have been made out by the following methods : 



(a) The emlryological method. It has been found by examining 

 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 so the different groups of fibres can 

 be easily distinguished. 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; this consists in subjecting them to a special 

 solution of hsematoxylin, and then to certain differentiating solutions. 

 The degenerated fibres appear light yellow, whereas the healthy fibres 

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

 in Miiller's fluid, Marchi's solution (a mixture of Miiller's fluid and 



