610 STRUCTURE OF THE SPINAL CORD. [CH. XLII. 



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 left 

 half of fig. 452. But as they are there all put together, it will 

 be a better way of studying the subject to enumerate the 

 ascending and descending tracts with separate 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 no degeneration in the 

 cord ; the fibres of the anterior roots come off from the large 

 cells of the anterior horn, and degeneration is found only on the 

 distal side of the point of section, in the motor nerve-fibres of 

 the nerves. 



Cutting the posterior roots between the spinal ganglia and the 

 cord leaves the peripheral part of the nerve healthy, and degene- 

 ration 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. 452, 3) 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 one or other collec- 

 tion of grey matter either in the cord itself, or in the medulla 

 oblongata. 



Fig. 453 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. 



i, 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. 



According to Schafer's recent work, 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. This is not shown in the diagram. 



A fibre of the posterior root is also shown ; this originates from 

 the cell G of the spinal ganglion ; the process of this cell bifurcates, 

 one branch (B) passing to the periphery where it ends in an 



