686 



THE TRACTS OF WHITE MATTER. [Book hi. 



takes, produce effects almost immediately above or below, on 

 the same or on the other side, or may have to travel some length 

 upwards, on the same side, before it produces its effects, or may 

 produce no effect on the spinal cord itself, but spend its whole 

 strength on the spinal bulb. 



The method of degeneration, confirmed by the method of 

 development, has shewn that certain fibres, starting as axis- 

 cylinder processes of certain cells in the region of the cerebral 

 cortex which Ave shall speak of as the motor area, finding their 

 way to the spinal bulb through the crus cerebri, form the ante- 

 rior pyramids of the bulb, cross at the decussation of the pyra- 

 mids to the lateral column of the cord and there form a definite 

 strand, the "crossed pyramidal tract" (Figs. 113, 114). This 



asc.a.l. 



Fig. 113. Diagram to illustrate the general arrangement of the sev- 

 eral Tracts of White Matter in the Spinal Cord. (Sherrington.) 



The section is taken at the level of the fifth cervical nerve. The relations of 

 the tracts in different regions of the cord are shewn in Fig. 114. 



The ascending tracts, tracts of ascending degeneration, are shaded with dots, 

 the descending tracts, tracts of descending degeneration, are shaded with lines ; 

 the shading is in each case put on one side of the cord only, the reference letters 

 being placed on the other side. 



cr.P. crossed pyramidal tract, or more shortly pyramidal tract. d.P. direct pyra- 

 midal tract, shaded on the side opposite to that on which cr.P. is shaded, 

 in order to indicate the difference of the two as to crossing. C.b. cerebellar 

 tract, s.lr. and c.r. together indicate the median posterior tract or tract of 

 fibres of the posterior roots, c.r. representing, as is explained more fully in the 

 text, the cervical and s.lr. the sacral, lumbar and dorsal roots, asc.a.l. the 

 antero-lateral ascending tract, desc.l. the antero-lateral descending tract. 

 The area, not shaded, marked x, is the small descending tract or rather 

 patch mentioned in the text as observed, in certain regions of the cord, in 

 the external posterior column rz. The small area at the tip of the posterior 

 horn, marked L, is the posterior marginal zone or Lissauer's zone. 



tract lying in the dorsal region of the lateral column, lateral to 

 the posterior horn, and marked out by a ' descending ' degen- 

 eration, may be traced down the whole length of the cord, 

 diminishing as it goes. It diminishes because fibres succes- 

 sively leave it to make connections, also of contact merely not 



