82 Journal of Comparative Neurology. 



By reason of the association of the nerve units arise those 

 intricate tracts, the unravelHng of which has so occupied anato- 

 mists in recent years. 



We cannot possibly expect to call attention here to even the 

 more important results which have been established with greater 

 or less certainty. The few instances which follow may serve 

 rather as illustrative of that which has preceded. They will 

 show to a certain e.Ktent the practical applications of the views 

 previously discussed. I select as the simplest illustration the 

 psychomotor tract (pyramidal tract) ; i. e. that group of nervous 

 elements which must be called into play when a voluntary mus- 

 clar fibre contracts in response to an act of consciousness. We 

 need for this purpose only two neurones. The first has its 

 nerve cell in the cortex and from it the nerve fibre passes 

 through the inner capsule, the pes pedunculi, the pyramids, and 

 the pyramidal decussation in the cord. Here, in the appropri- 

 ate place in the gray substance of the anterior horn, it forms a 

 terminal brush. 'I'his is applied to the nerve cell of the second 

 neurone, tlie cell of the anterior horn, from which the fibres of 

 the anterior root aris.e, pass to the muscular fibre, and here sub- 

 divide in the motor end plate. 



Kolliker, moreover, indicates that the few lateral twigs 

 which branch off from the axis-cylinder process of the cells of 

 the anterior horn also become fibres of the anterior roots, so 

 that in this way several fibres take their origin from a single 

 cell. 



We must concern ourselves somewhat more minutely with 

 the fibres of the posterior roots, since Golgi, Ramon y Cajal, 

 Kolliker, and others have discovered new facts about their rela- 

 tions. 



The fibres of the posterior roots arise, for tlie most part at 

 least, from the ganglion cells of the spinal ganglia, penetrate 

 into the cord and, after continuing some distance at right angles 

 to the surface of the cord, divide into ascending and descending 

 branches. These longitudinal branches constitute the principal 

 part of the white posterior bundles, giving off, however, at right 

 angles a number of collaterals which now for the first time pen- 

 etrate the gray substance and there in various places break up 

 into terminal brushes. Some of these terminal brushes surround 



