1889-90.] Dr Alexander Bruce on Third Cranial Nerve. 175 
interest. One artery ran close to the median plane from before backwards into 
the median nucleus, while another, placed laterally to the above, ran to each 
of the anterior nuclei. This appearance is not shown in the drawing, owing to 
want of space. 
Fig. 5. The section is made through the centre of the median nucleus 
( m.n .). This is seen to occupy the median plane, and to have the shape of 
a long spindle tapering more gradually towards its posterior than towards 
its anterior extremity. It has on either side a bundle of medullated fibres 
arising among its cells and running forwards towards the Y-shaped portion 
of the posterior longitudinal fasciculus. Some of these fibres evidently arise 
from this latter structure, while others as certainly pass through it and enter 
into the innermost root fibres of the nerve. The anterior nucleus (III. a.n.) 
is seen lying upon the posterior surface of the innermost three-fourths of the 
posterior longitudinal fasciculus. Behind the anterior group lies a second 
smaller almost circular group of nerve cells, the postero-external (III. p.e.n.), 
of a similar shape and size. This lies behind the outermost fourth of 
the posterior longitudinal fasciculus. From its anterior surface a bundle 
of fibres passes inwards and forwards between the median nucleus and the 
anterior group, insinuating itself between the lateral fibres of the median 
nucleus and fibres arising from the internal surface of the anterior group. 
Most of these fibres from the postero-external nucleus seem to terminate in 
the Y-shaped portion of the posterior longitudinal fasciculus of the same side. 
It is possible that some of them may join the innermost root fibres of the 
third nerve, but it is certain that none of them cross the middle line to enter 
the nerve of the opposite side. The root fibres from this nucleus pass through 
the underlying portions of the posterior longitudinal fasciculus and enter the 
outermost part of the nerve. A small group of nerve cells (external nucleus, 
e.n.) lies on the outer side of the posterior part of the posterior longitudinal 
fasciculus. In the wedge-shaped space, left between the posterior part of the 
median nucleus on the inner side and the posterior nucleus with its ventral 
fibres on the outer, lies a large nucleus ( p.i.n .), to which I have given the name 
of the postero-internal or pale nucleus. It is characterised by its cells being 
(1) considerably smaller than those of the lateral groups, (2) staining less 
intensely by Weigert’s method, (3) by the cells being surrounded by large 
lymph spaces, and (4) by their having a comparatively slight intercellular 
network of medullated fibres. The external nucleus (III. e.n.) is seen as a 
small nearly circular group of cells partly imbedded and partly external to the 
outermost portion of posterior longitudinal fasciculus ( p.l.f, .). 
Fig. 6. A vertical transverse section made above the level of the median 
nucleus. The anterior nuclehs (III. a.n . ) is greatly reduced in size, being rather 
less than half the size of the same nucleus in fig. 5. The number of the cells 
contained in it is diminished in a much higher proportion. The postero-external 
nucleus, on the other hand, is slightly increased in size. It maintains the same 
relationship to the posterior longitudinal fasciculus as in the previous section. 
The two postero-internal or pale nuclei ( p.n .) have the form of long spindles or 
ovoids, and are separated from each other by an interval about a third of their 
width. The nucleus and its cells present the same distinctive characteristics 
as have been described under fig. 5. Fine medullated fibres pass from the 
anterior aspect of the three groups of nuclei towards the innermost portions of 
the posterior longitudinal fasciculus. Some undoubtedly enter the roots of 
the nerve, others become connected with the fasciculus. 
