566 HANDBOOK OF PHYSIOLOGY. 



and considerable light has been by these means shed upon the conduction 

 of impulses to and from the nervous system by the study of the results of 

 section of different parts of the central nervous system, and of the spinal 

 nerve-roots. Thus we have not only embryological evidence mapping 

 out different tracts, but also confirmatory pathological and experimental 

 observations. 



The tracts which have been made out are the following: 



(a) Of descending degeneration. 



(i.) The crossed pyramidal tract (fig. 352,7). This tract is situated 

 to the outer part of the posterior cornu of gray matter. It is found 

 throughout the whole length of the spinal cord; at the lower part it ex- 

 tends to the margin of the cord, but higher up it becomes displaced 

 from this position by the interpolation of another tract of fibres, to be 

 presently described, viz., the direct cerebellar tract. The crossed 

 pyramidal tract is large, and may touch the tip of gray matter of the 

 posterior cornu, but is separated from it elsewhere. In shape on cross- 

 section it is somewhat like a lens, but varies in different regions of the 

 cord, and diminishes in size from the cervical region downward. 

 The tract is particularly well marked out, both by the degeneration and 

 the embryological methods. The fibres are supposed to pass off as they 

 descend, and to join the various local nervous mechanisms of nerve cells 

 and their branchings which are represented in the cord. The tract of 

 degeneration may be traced upward beyond the cord, in a way to be 

 presently described. The fibres of which this tract is composed are 

 moderately large, but are mixed with some that are smaller. 



(ii.) The direct or uncrossed pyramidal tract (fig. 352, 10). This 

 tract is situated in the anterior column by the sides of the anterior 

 fissure. ' It is smaller than (i.), and is not present in all animals, 

 though conspicuous in the human cord and in that of the monkey. It 

 can be traced upward to the cerebral cortex, and downward as far as 

 the mid or lower thoracic region, where it ends. 



(iii.) Antero-lateral descending tract (fig. 353,9). An extensive 

 tract, elongated but narrow, and reaching from the crossed to the direct 

 pyramidal tract. It is a mixed tract, since not all of its fibres degenerate 

 below the lesions. 



(iv.) Comma tract (fig. 352, 3) is a small tract of fibres which degen- 

 erate below section or injury of the cord. Its presence has been demon- 

 strated in the cervical and thoracic regions. It is supposed to consist of 

 the descending collaterals of the posterior nerve-roots as they pass into 

 the postero-external columns. 



(t>) Of ascending degeneration. 



(i.) Postero-median column (fig. 352,1). This tract degenerates up- 

 ward on injury or on section of the cord, as well as on section of the 

 posterior nerve roots. It exists throughout the whole of the cord from 

 below up, and can be traced into the bulb. It consists of fine fibres. 



