234 Relations of the Frontal Lobe in the Monkey 
passing forward from the lesion into the frontal lobe, and backward into 
the centrum semi-ovale, to the cortex of both central convolutions in 
about equal amount, and backward among the fibers of the cingulum just 
beneath the cortex of the mesial surface (Fig. 11). Somewhat lower, but 
still in the level of the external wound, the degeneration in the pre-central 
convolution greatly exceeded that in the post-central. There was consider- 
able degeneration in the anterior fourth of the corpus callosum, cor- 
responding closely in antero-posterior extent to the tissue alluded to 
above as injured by the clot. It doubtless comes from that area, as the 
cut made by the knife could injure but very few callosal fibers. The 
majority of the degenerated fibers entering the upper levels of the anterior 
segment of the internal capsule are probably projection fibers having their 
cells of origin in that portion of the cortex of the pre-central convolution 
which hes within the pre-central sulcus just anterior to the external 
wound. A study of the serial sections shows that only a small number of 
these fibers—those having the longest course—can be motor fibers, and 
even that is open to question. Some of those fibers from the upper levels 
of the anterior segment enter the thalmus by passing mesial to the genu, 
others by passing through the genu; other fibers, mostly from slightly 
lower levels of the anterior segment, enter the anterior portion of the 
posterior segment in bundles that pass through the lenticular nucleus. 
These bundles leave the anterior segment at nearly a right angle, plunge 
into the lenticular nucleus and wind around the fibers of the genu to 
enter the posterior segment of the internal capsule. After a short down- 
ward course with the capsular fibers, some leave the capsule and enter 
the thalamus, where, together with those already described as entering 
the thalamus directly from the anterior segment, they break up into fine 
filaments, some of which probably end in either the external or anterior 
nucleus of the thalamus. A good many of these filaments, however, can 
be traced inward and backward to the ganglion habenule, where some of 
them appear to terminate about the cells of origin of the fasciculus retro- 
flexus of Meynert. 
In the level in which the ganglion habenule disappears from the sec- 
tion and gives place to the upper levels of the superior colliculus the 
passage of degenerated fibers from the lesion through the anterior 
segment of the internal capsule has practically ceased and the remainder 
of the degeneration in the posterior segment passes downward with the 
capsular fibers, but is gradually forced backward by the entrance of new 
fibers, toward the middle portion of the posterior segment. In this level 
also there is the beginning of a copious fine degeneration running back- 
