FASCICULUS CEREBRO-SPINALIS IN THE RAT 413 



The pyramidal fasciculi in the white rat take a very light stain 

 with the Weigert methods, appearing under low magnification as 

 grayish blue areas clearly marked off from the remainder of the 

 white substance which stains a deep blue. On the other hand, 

 the pyridine-silver technique causes these tracts to stand out 

 from the rest of the cord because of the dark brown color which 

 they assume. Since the remainder of the white substance stains 

 a very light brown, the contrast is striking and could be equalled 

 only by the most fortunate Marchi preparations. This contrast 

 is equally evident in the decussation and after the pyramidal 

 tracts have assumed their position on the ventral surface of the 

 medulla. 



Since nowhere in the literature are to be found altogether sat- 

 isfactory figures and descriptions of the position and shape of 

 the pyramidal tracts at different levels of the rat's medulla and 

 spinal cord, it seems desirable again to go over these purely topo- 

 graphical features before taking up the finer structure. 



TOPOGRAPHY 



The changes in shape, size, and position of the tract at various 

 levels can best be described in connection with figures 1 to 7, 

 Figure 1 was drawn from a section through the upper end of the 

 decussation of the pyramids. At and above this level in the 

 medulla the pyramids are situated on either side of the anterior 

 median sulcus, but do not project ventrally as they do in the human 

 brain. Fibers can be seen detaching themselves from the pyr- 

 amids and running backwards to decussate as small bundles, or 

 as individual fibers. On reaching the gray substance they 

 spread our rather diffusely in the form of small branching bundles, 

 many of whose fibers end within the medulla at the level of their 

 decussation. 



Where the decussation is at its height (fig.2) the crossing bundles 

 are of large size. They run backward at some distance from the 

 central canal, and are gathered together on the dorsal surface of 

 the gray substance into two large fasciculi. These are at first 

 some distance apart, but approach each other in the lower part 

 of the medulla. Figure 3 represents the lowest level of the 



