156 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



for the view that the posterior roots may contain some efferent fibers. 

 Some of the groups of tract cells have been given special names, 

 such, for instance, as Clarke's column (columna vesicularis). This 

 group of cells lies at the inner angle of the posterior horn of gray 

 matter (5, Fig. 71), 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 

 columns on the same side to constitute an ascending tract of fibers 

 known as the tract of Flechsig, or the dorsal or direct cerebellar tract. 

 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 



Whife matter. Gray matter. Enrire section. 



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80 

 60 

 40 

 SO 



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^Composite curves based on 4 Cases. 



ffVHHinni n in r? y va w xm ix x xi xu i u ju kyi nmnffj 



Fig. 69. Curves to show the relative areas of the gray and white matter of the spinal 

 cord at different levels. (Donaldson and Davis.) The Roman numerals 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 accomplished 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 secondary degeneration (Wallerian degen- 



