1906-7.] Experimental Lesions in Motor Cortex of Monkey. 291 
half of the medulla oblongata until the true decussation of the pyramids 
begins. The degeneration in the pyramid is dense, and is scattered 
uniformly over its whole area. The fine fibres have to a large extent 
disappeared, and have evidently terminated in the grey matter of the optic 
thalamus, substantia nigra and pons. 
The decussation of the pyramids is first observed in sections passing 
through a plane slightly below the level of the lower end of the inferior 
olivary nucleus, in the closed part of the medulla oblongata. The fibres 
leave the postero-mesial angle of the pyramid in bundles, and passing 
backwards, inwards and downwards, cross the raphe, interlacing with 
similar bundles of normal fibres from the sound side. They can be traced 
through successive sections, and are seen to pass obliquely downwards 
through the formatio reticularis until, at the junction of the bulb and 
spinal cord, they take up a position in front of the substantia gelatinosa 
Fig. 7. — Transverse section 
through lower part of medulla 
oblongata ; middle of pyra- 
midal decussation showing 
homolateral fibres passing to- 
wards side of lesion. 
of Rolando.* About midway between the beginning of the decussation 
and the junction of the medulla oblongata and cord (fig. 7) a few separate 
fibres can be seen to turn off from the bundles which are about to cross 
the raphe, and to pursue a course on the side of the lesion exactly similar 
to that just described for the crossed fibres. As the sections are followed 
downwards these homolateral fibres are given off from the degenerated 
pyramid in increasing numbers, and at the lower limit of the decussation 
the ratio of the crossed (heterolateral) to the uncrossed (homolateral) is 
about 40 to 1, roughly speaking. All that now remains of the degenerated 
anterior pyramid is a thin lamina of fibres lying along the border of the 
anterior median fissure in its posterior half. It can be traced well into 
the first cervical segment, where it disappears. It represents the direct 
(anterior) pyramidal tract in man and the anthropoid apes. In the first 
cervical segment there are thus three pyramidal tracts — crossed, direct 
* The whole of the lower half of the medulla oblongata was cut and mounted serially, 
not a single section being missed. 
