1906-7.] Experimental Lesions in Motor Cortex of Monkey. 289 
(fig. 4) the degenerated fibres are found to be included for the most part 
in the middle three-fifths of the crusta, roughly speaking, but they are 
not limited to this region. Both in the mesial fifth (fronto-pontine) and 
in the external fifth (temporo-pontine) are to be found many scattered 
fibres ; they become fewer and fewer as one passes away from the area of 
dense degeneration in the middle part of the crusta, and only disappear 
entirely in the very outer part of the external fifth. The degeneration 
is most concentrated in the middle portion (about the middle third) of the 
crusta, and gradually shades off laterally and mesially from this area. 
Many fibres pass backwards from the posterior aspect of the crusta ; some 
end in the grey matter of the substantia nigra, others can be followed 
beyond this into the tegmentum. There is no evidence of any distinct 
Fig. 4. — Transverse section of mesencephalon, showing de- 
generation in crusta following destruction —A, of entire 
motor cortex ; B, of leg area ; C, of arm area ; D, of part of 
face and arm areas. 
bundle of fibres leaving the crusta to join either the mesial or lateral fillet, 
as has been described by Hoche, Barnes, Spitzka, and others in the human 
subject. 
In the uppermost regions of the pons, where the pyramidal tract begins to 
be subdivided into bundles of varying size by the transverse cerebellar fibres, 
the degeneration affects all the bundles, but not uniformly. There are several 
small bundles lying ventro-laterally to the main mass, and flattened antero- 
posteriorly ; these and the lateral portions of the larger bundles show only 
slight degeneration. It is densest in the central and larger bundles, and 
fades away both in the mesial and in the lateral directions from these, 
but the degenerated fibres are more numerous in the mesial than in the 
lateral bundles. It is noteworthy that none of the bundles, even in the 
higher levels of the pons, are free from degeneration following a lesion 
strictly confined to the motor cortex, neither the mesial (fronto-pontine) 
nor the lateral (temporo-occipito-pontine) bundles. They contain a mixture 
of normal and degenerated fibres, in which the former predominate. 
VOL. xxvii. 19 
