162 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



column). This group of cells lies at the inner angle of the 

 posterior column of gray matter (5, Fig. 76), and forms a column 

 usually described as extending from the middle lumbar to the 

 upper dorsal region. The axons from these cells pass to the 

 dorsal margin of the lateral funiculi on the same side to con- 

 stitute an ascending tract of fibers known as the tract of Flechsig, 

 or the fasciculus cerebellospinalis. 



General Relations of the Gray and White Matter in the 

 Cord. — Cross-sections of the cord at different levels show that 

 the relative amounts of gray and white matter differ considerably 

 at different levels, so that it is quite possible to recognize easily 

 from what region any given section is taken. At the cervical and 

 the lumbar enlargements the amounts of both gray and white 

 matter — that is, the total cross-area of the cord — show a sudden 



Fig. 74. — Curves to show the relative areas of the gray and white matter of the spinal 

 cord at different levels. — (Donaldson and Davis.) The Roman niimerals along the abscissa 

 represent the origin of the different spinal nerves. 



increase owing to the larger number of fibers arising at these levels. 

 The white matter, and therefore the total cross-area, shows also 

 a constant increase from below upward, due to the fact that in 

 the upper regions many fibers exist that have come into the cord 

 at a lower level or from the brain, those from the latter region being 

 gradually distributed to the spinal nerves as we proceed downward. 

 In the accompanying figure a curve is presented showing the cross- 

 area of the cord and the relative amounts of gray and white matter 

 at each segment. 



Tracts in the White Matter of the Cord, Methods of Deter- 

 mining. — The separation of the medullated fibers of the cord 

 into distinct tracts of fibers possessing different functions has 

 been accompUshed in part by the combined results of investiga- 

 tions in anatomy, physiology, and pathology. The two methods 

 that have been employed most frequently and to the best advan- 

 tage are the method of secondar}' degeneration (Wallerian degen- 

 eration) and the method of myelinization. The method of second- 

 ary degeneration depends upon the fact that, when a fiber is cut 

 off from its ceU of origin, the peripheral end degenerates in a few 

 days. If, therefore, a lesion, experimental or pathological, is made 



