THE NERVOUS SYSTEM. 565 



marked in the lumbar region, as well as in the thoracic region (No 

 5, fig. 532). 



(c) Besides these groups, which have their names largely on ac- 

 count of their location, there are distributed throughout the gray 

 matter a very large number of other cells, which are known as intrinsic 

 cells. These send out neuraxons which pass into the white matter 

 of the same or the opposite side, pass up and down the cord, enter the 

 gray matter again, and connect there by their end-brushes with cells at a 

 different level of the cord. The intrinsic cells are, therefore, in the 

 main, commissnral in their function, that is to say, they unite the two 

 sides or different levels of the cord. They are also, themselves, in re- 

 lation with the fibres and cells of the anterior and posterior cornua. 



Columns and tracts in the white matter of the spinal cord. In 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 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 Burdach (fig. 353, 2), and the poster o-median, or 

 column of Goll (fig. 353, 1). In addition to these columns, however, it 

 has been shown that the white matter can be still further subdivided. 

 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 that if the develop- 

 ment of the spinal cord be carefully observed at different stages that cer- 

 tain groups of the nerve-fibres put on their myelin sheath at earlier peri- 

 ods than others, and that the different groups of fibres can therefore be 

 traced in various directions. This is 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 oppo- 

 site direction the tract is one of ascending degeneration. By modern 

 methods of staining of the central nervous system it has proved com- 

 paratively 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 Marchi's methods. Accidents to the 

 central nervous system in man have given us much information upon 

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

 extended by the experiments on animals, particularly upon monkeys; 



