542 



THE SPINAL CORD. 



the lower animals, being found in man and the monkey only and being better 

 developed in man than in the monkey, and reaches a certain way only down 

 the spinal cord, generally coming to an end in the thoracic region, "it, too, 

 comes down from the pyramid, and is a continuation of that part of the pyra- 

 mid which, unlike the rest, does not decussate in the bulb; thus the tract 

 which coming down from the left side of the brain runs in the left pyramid 



FIG. 123. 



cr.P. 



asc. a. I 



d.P 



Diagram to illustrate the General Arrangement of the Several 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 shown in Fig. 127. The ascending tracts, 

 tracts of ascending degeneration, are shaded with dots, the descending tracts, tracts of descending 

 degeneration, are shaded with lines; the shading 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 pyramidal 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, cr. 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 de- 

 scending 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 

 r.z. The small area at the tip of the posterior horn, marked L, is the posterior marginal zone of 

 Lissauer's zone. 



in the bulb, passes down into the left anterior column of the cord. Hence 

 this smaller tract is called the direct pyramidal tract 



These two are the most conspicuous and important descending tracts, but 

 names have been given to two other descending tracts. One, known as the 

 antero-lateral descending tract, is a large tract placed in the antero-lateral 

 column, and seen in section (Fig. 123, desc. I.) as an elongated area stretch- 

 ing from the pyramidal tract toward the anterior column and reaching at 

 times as far as the anterior fissure. The area is large, however, because the 

 tract is very diffuse, that is to say, the fibres with descending degeneration, 

 or fibres which degenerate below the section or injury, are very largely 

 mixed up with fibres which do not degenerate ; in this respect this tract 

 contrasts with the pyramidal tract, which is to a much greater extent com- 

 posed of fibres with descending degeneration, though even in it there are 

 a considerable number of fibres which do not degenerate. Indeed, this 

 antero-lateral descending tract is so diffuse that it hardly deserves to be 

 called a tract. 



The other is a small, narrow, comma-shaped tract (Fig. 123, ), situated 

 in the middle of the external posterior column which has been observed in 

 the cervical and upper thoracic regions, and has been called the " descend- 

 ing " comma tract. But the degeneration reaches a short way only below 



