THE CEREBRO-SPINAL NEKVOU8 SYSTEM. 479 



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, how- 

 ever, a matter of great difficulty to trace them by mere dissection, and 

 so other methods have been resorted to. One of these is based upon the 

 fact that nerve-fibres undergo degeneration when they are cut off from 

 the centre with which they are normally 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 trac- 

 ing such degenerated tracts. The second method consists in observing 

 the development of the fibres of the various tracts; some tracts of fibres 

 receive their medullary substance later than others, and are to be traced 

 by their gray appearance. The chief tracts which have been made out 

 are the following: (1) TJie direct pyramidal tract (Fig. 333, 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, as far as 

 the mid-dorsal region of the spinal cord. It consists of the fibres of the 

 pyramids which do not undergo decussation in the medulla. They are 

 probably fibres chiefly for the arm and constitute about one-fourth or 

 one-fifth of the whole motor tract. There is reason for believing, how- 

 ever, that the fibres of this tract undergo decussation throughout their 

 course, and also that fibres pass over from it through the anterior com- 

 missure to join the lateral pyramidal tract; (2) the Crossed or lateral pyra- 

 midal tract (Fig. 333, L. P. T.) can be traced from the anterior pyra- 

 mids of the medulla, and consists of motor fibres which decussate in the 

 anterior fissure and pass downwards in the lateral columns near the 

 posterior cornu of the gray matter. They may be traced downwards as 

 far as the lower end of the cord. The number of fibres which decussate 

 in the medulla, and consequently the size of this tract, varies. The 

 fibres which most constantly cross over are those for the leg. The pyra- 

 midal tracts end in the gray matter of the anterior cornua; (3) Direct 

 cerebellar tract, r>. c. T., which corresponds to the peripheral portion of 

 the posterior lateral column between the crossed pyramidal tract and 

 the edge of the cord, can be traced upwards directly to the cerebellum 

 and downwards as far as the mid-lumbar region; (4) Posterior median 

 column or Fasciculus of Goll, is found on either side of the posterior 

 commissure, and is traceable upwards and terminates as the fasciculus 

 gracilis of the medulla. It is traceable downwards as far as the mid- 

 dorsal region. The portion of the posterior column between the poste- 

 rior median column and the posterior roots of the spinal nerves, known 

 as (5) the Fasciculus cuneatus, Burdactts, or Poster o-external column, 

 is composed of fibres of the posterior roots on their way to enter the gray 

 substance and the posterior median column at different heights. The 

 antero-lateral column contains fibres from the anterior cornua of the 

 same as well as of the opposite side; (G) Lateral limiting layer (L. L. L.) 



