THE NERVOUS SYSTEM. 95 



(or down) in the white substance of the posterior columns, and enter the 

 grey matter at various heights at the base of the posterior cornu, perhaps 

 some pass directly upward without entering the grey matter. Those that 

 enter the grey matter pass in various directions, some to join the lateral 

 cells in the anterior cornu, some join the cells in the posterior vesicular 

 column, and some pass across to the other side of the cord in the anterior 

 commissure,, whilst others become again longitudinal in the grey matter. 



It should be here mentioned that the cells in the posterior vesicular 

 column are connected with medullated fibres which pass horizontally to 

 the white matter of the lateral columns and there become longitudinal. 



Course of the fibres in the cord. The nerve fibres which form the 

 white matter of the cord are nearly all longitudinal fibres. It is, howevjer, 

 a matter of great difficulty to trace these fibres by mere dissection, and so 

 some other methods must be adopted. One method is based upon the fact 



P.M.C. 



FIG. 319. Diagram of the spinal cord at the lower cervical region to show the track of fibres; 

 d. p. t., direct pyramidal tract; c. p. t., crossed pyramidal tract; * direct cerebellar tract; p. M. c., 

 posterior medium column. (Gowers.) 



that nerve fibres undergo degeneration when they are cut off from the 

 centre with which they are connected, or when the parts to which they 

 are distributed are removed, as in amputation of a limb; and information 

 as to the course of the fibres has been obtained by tracing the course of 

 these degenerated tracts. The second method consists in observing the 

 development of the various tracts; some have their medullary substance 

 later than others, and are to be distinguished by their more grey appear- 

 ance. The chief tracts which have been made out are the following: 

 (1) The direct pyramidal tract (Fig. 319 d.p.t.), a comparatively small 

 portion of the inner part of the anterior columns, which is traceable from 

 the anterior pyramids of the medulla to the mid-dorsal region of the spinal 

 cord. It consists of the fibres of the pyramids which do not undergo 

 decussation in the medulla. There is reason for believing, however, 

 that these fibres of the direct pyramidal tract undergo decussation through- 

 out their course, and fibres pass over through the anterior commissure 

 to join the lateral pyramidal tract (vide infra); (2) the Crossed pyra- 

 midal tract (Fig. 319, c.p.t.) can be traced from the anterior pyramids 



