PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 147 



Transverse section through medulla oblongata (fig. 6). The sec- 

 tion here described passes through the point where the central 

 canal opens into the fourth ventricle. The pyramid has now a 

 plano-convex outline the posterior surface being flattened against 

 the inferior oUvary nucleus which lies behind. The long axis 

 runs from the middle hne obhquely outward and forward. The 

 whole area is studded with black dots and some fibers can be 

 traced for a very short distance into the olivary nucleus but no 

 indication of fine degeneration is visible in its gray matter. The 

 inner angle of the pyramid is now becoming pointed and some 

 fibers can be seen streaming out from this angle toward the 

 median raphe but none cross as yet at this level. 



Transverse section of medulla oblongata through middle of inferior 

 olivary nucleus (fig. 7). The degenerated fibers coming off 

 from the inner angle of the pyramid can now be followed in 

 small bundles across the raphe. They are at first directed back- 

 ward toward the posterior longitudinal fasciculi in front of the 

 central canal and cross the raphe very obliquely. When almost 

 in contact with these fasciculi they turn suddenly outward and 

 then curve backward through the central gray matter entering 

 the funiculus cuneatus in which they bend caudalward. In their 

 passage through the gray matter they form bow-shaped curves 

 with the concavities toward the central canal. At this level 

 the transversely cut fibers in the funiculus cuneatus form a thin 

 zone lying against the posterior aspect of the gray matter. 



Transverse section of medulla oblongata through middle of pyra- 

 midal decussation {fig. 8). At a slightly lower level than that out- 

 lined in figure 7, the degenerated fibers are crossing in large num- 

 bers and follow practically the same course as already described 

 in the last section. The pyramid itself is now considerably 

 diminished in size and the area of degeneration in the funiculus 

 cuneatus has broadened antero-posteriorly and also moved some- 

 what towards the middle line but it has not yet invaded the 

 funiculus gracilis. 



Transverse section of medulla oblongata through the lower end of 

 inferior olivary nucleus (fig. 9). At this level the decussating fibers 

 are much reduced in number and cross the raphe less obliquely 



