CHAP, i.] THE SPINAL CORD. 873 



the brain through the structures called the pyramids of the bulb, 

 which we shall study later on. These pyramids cross over or 

 decussate as they are about to pass into the cord, forming what is 

 known as the decussation of the pyramids, and the tract of fibres 

 in question shares in this decussation. Hence this tract is called 

 the crossed pyramidal tract or more simply the pyramidal tract. 



A smaller, less conspicuous descending tract occupies the 

 median portion of the anterior column (Fig. 100, d. P.) This 

 is not only much smaller but also much more variable than the 

 crossed pyramidal tract, is not present in 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 pyramid 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 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. 100, desc. I.) as an elongated area stretching from the py- 

 ramidal tract towards 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 pyra- 

 midal tract, which is to a much greater extent composed 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. 100, a?), 

 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 " descending comma tract." But the degeneration 

 reaches a short way only, below the section or injury, and the 

 group of fibres thus degenerating can hardly be considered as 

 forming a tract comparable to the other tracts. The area 

 probably represents fibres of the posterior root which take a 

 descending course soon after their entrance into the cord. 



II. Ascending tracts, that is to say, tracts in which the 

 degeneration takes place above the section or injury. 



A conspicuous ascending tract of a curved shape (Fig. 100, 

 C. b.) occupies the outer dorsal part of the lateral column lying 



