652 STRUCTURE OF THE SPINAL CORD [CH. XLY. 



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 subject, 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 results of section of different parts of 

 the central nervous system, and of the spinal nerve-roots. 



By these methods the tracts in the white matter have now been 

 mapped out, and the principal ones are shown in the succeeding 

 diagrams. 



It will be convenient to begin by considering the result of cutting 

 through the roots of the spinal nerves. 



Cutting the anterior roots produces chromatolysis of the cells of 

 the anterior horn from which they originate ; this slow atrophy is the 

 result of disuse of the axons which are cut and still remain attached to 

 the cell-bodies. Wallerian degeneration is limited to the motor nerve- 

 fibres on the distal side of the point of section. The fact that chro- 

 matolysis (see p. 193) occurs when the axon of a nerve-cell is cut 

 through, furnishes us with a valuable method of ascertaining from 

 what nerve-cells various tracts originate. 



Cutting the posterior roots between the spinal ganglia and the 

 cord leaves the peripheral part of the nerve healthy, and degeneration 

 occurs in the portion of the root which runs into the cord, because 

 the fibres are cut off from the cells of the spinal ganglion from which 

 they grew. These degenerated nerve-fibres may be traced up the 

 cord for a considerable distance. Each posterior root-fibre when it 

 enters the cord bifurcates, the main branch passing upwards, and the 

 shorter branch downwards, so that the degeneration is seen in a 

 small tract called the comma tract (fig. 403) immediately below the 

 point of entrance of the cut posterior root. The upgoing fibre is 

 contained in the posterior column of white matter, and it terminates 

 in the grey matter either in the cord itself at a higher level, or in 

 the medulla oblongata. 



Fig. 401 represents in a schematic way the manner in which the 

 fibres of the two roots of a spinal nerve are connected to the grey 

 matter in the cord. 



1, 2, 3, 4 represent four cells of the anterior horn. Each gives 

 rise to an axis-cylinder process A, one of which is shown terminating 

 in its final ramification in the end-plate of a muscular fibre M. Each 

 of these four cells is further surrounded by an arborisation (synapse) 

 derived from the fibres of the pyramidal tract P, which comes down 

 from the brain. The pyramidal fibres really terminate around the 

 cells at the base of the posterior horn ; these cells therefore act as 

 intermediate cell-stations on the way to those in the anterior horn. 



