174 
Proceedings of Royal Society of Edinburgh. [sees. 
cerebellar peduncles. (The level of the section is at the decussation of these 
peduncles. ) 
Fig. 2. Section made almost immediately above the preceding one, through 
the lowest portion of the nucleus of the third nerve. The inferior extremity 
of the nucleus (III. i.n.) is seen lying upon the dorsal aspect of the posterior 
longitudinal fasciculus. At this level none of the root fibres are visible, but 
on the inner aspect there are seen fibres {i.o.f.) passing forwards to join the 
anterior fibres of the posterior longitudinal fasciculus. The two sets of 
anterior fibres converge towards each other in the shape of the letter Y. They 
appear to be genetically different from the fibres of the main part of the 
fasciculus, for fine medullated fibres pass from their anterior surface across the 
middle line, forwards, to disappear either in the superior cerebellar peduncle, 
or, what is more probable, to terminate in a nucleus situated on the inner side 
of each of these tracts, or perhaps in a group of medullated fibres lying still 
nearer the anterior surface. (These are not represented in the drawing. ) At 
this level there are no commissural fibres between the two nuclei, a fact which 
seems to warrant our separating this as a distinct portion of the nucleus, the 
inferior oculomotor nucleus. 
Fig. 3. Section is made at a level slightly above that of the preceding 
through that part of the nucleus at which intercommissural fibres (e.f.) begin 
to appear. An oval-shaped nucleus lies on either side dorsally to the posterior 
longitudinal fasciculus. Fine fibres form a rich network between the two nuclei. 
A large group of medullated fibres ( a.o.f '.) originates in the interior of the 
nucleus and passes inwards and forwards, internally to the Y-shaped portion 
of the posterior longitudinal fasciculus. The fibres which arise from the 
anterior surface of this Y-shaped portion pass forwards to the opposite side in 
a manner similar to that described in Plate II., but appeared now in the section 
from which the drawing was made, rather to end in a group of fibres imme- 
diately anterior and internal to the superior cerebellar peduncle. (In addition 
to these, other fibres were seen, after crossing the middle line, to bend back- 
wards and outwards and become continuous with the fibres known as the deep 
fibres of the corpora quadrigemina. ) This part of the nucleus (III. a.n.) forms 
the lower part of the anterior group. 
Fig. 4. The section is made through the inferior extremity of the median 
nucleus (III. w,.n.). The two anterior nuclei still continue to be united by 
intercommissural fibres. The anterior nucleus is distinctly increased in size. 
It forms a compact oval-shaped group slightly imbedded in the posterior longi- 
tudinal fasciculus. Fibres passing from its inner aspect forwards into the 
Y-shaped portion of the fasciculus are still well seen. In the median line, 
dorsal to the intercommissural fibres, is the lower extremity of the median 
nucleus (III. m.n.). At this level it has a very indefinite form and outline, 
and the cells are interspersed by a fine net-work of medullated fibres. Among 
the fibres of the outer portion of the posterior longitudinal fasciculus, motor 
cells are scattered in such numbers as to deserve a special name. I have 
accordingly termed these the external nucleus (III. e.n.). The posterior 
longitudinal fasciculus, both in its posterior or oblique and in its anterior or 
Y-shaped portion, shows a notable increase in the number of its fibres. Part 
of this is due to fibres entering its anterior aspect from the various sources 
already indicated, but more markedly to the increase of connections with the 
higher levels of nucleus on the same side. The condition of the vascular 
supply of the nucleus in the section from which the drawing was made is of 
