418 S. WALTER RANSON 



STRUCTURE 



We turn now from the consideration of the tract as a whole to 

 the characteristics of the individual fibers and shall learn why 

 the tract stains so intensely with the pyridine-silver technique 

 and so faintly with the Weigert methods. In pyridine-silver 

 preparations of the spinal cord all the axons are stained ; the larger 

 ones are yellow, while the smaller ones are dark brown or black. 

 The other elements of the white substance (such as myelin sheaths, 

 neuroglia and blood vessels) are stained faintly or nor at all. 

 Nearly all of the axons in the pyramidal fasciculus (fig. 8, a) 

 are very small and these closely packed dark brown axons give 

 the characteristic brown color to the fasciculus as a whole. This 

 contrasts sharply with the structure seen in the remainder of the 

 white substance of the cord (fig. 8, b) where the large, light yellow 

 axons, surrounded by thick unstained rings of myelin give rise 

 to a lighter color and a more open structure. There are a few 

 medium-sized axons in the pyramidal tract and a f^w of the very 

 fne ones in the other fasciculi of the cord. Figures 8 and 9 were 

 taken from the fourth thoracic segment of the spinal cord at the 

 border of the pyramidal fasciculus. 



As has been said, the pyrimidal tract takes a light grayish blue 

 stain in Weigert preparations (fig. 5). It contains many very 

 fine medullated fibers and a few of medium size (fig. 9, a). The 

 myelin sheaths of the pyramidal axons are thinner than the 

 sheaths on axons of the same size in other regions of the cord. 

 Some are so thin and faintly stained that they are just recogniz- 

 able. The medullated fibers do not occupy all the space in the 

 tract but are separated from each other by unstained spaces. 

 When we compare the Weigert and the pyridine-silver prepara- 

 tions of the same level of the cord we see that the axons in the 

 pyramidal fasciculus (fig. 8,a) are much more numerous than the 

 myelin sheaths (fig. 9,"a) and that the axons are more closely 

 packed together. In any given section, therefore, many of the 

 axons are without myelin sheaths. This is susceptible of two 

 interpretations: either many of the pyramidal fibers are entirely 

 non-medullated ; or the myelin sheaths of the pyramidal fibers are 



