1889 - 90 .] Dr Alexander Bruce on Third Cranial Nerve. 171 
in virtue of its relation to the median nucleus. Of these three, the 
lowest (fig. 3) lies below the level of the median nucleus, and has a 
highly developed system of commissural fibres; the intermediate 
(fig. 4, and fig. 5, III. a.n.) lies at the same level as the median 
nucleus, and has also, at least in its lower portion, a well-developed 
commissural system; while the upper division lies at a higher plane 
than the median nucleus, and has extremely scanty commissural 
connections with its fellow (figs. 6 and 7). The anterior nucleus 
attains its largest dimensions at its lowest extremity (figs. 3 and 4), 
the point at which the commissural system is most highly developed. 
It gradually diminishes in size as it ascends. It is seen on longi- 
tudinal transverse sections to be imperfectly segmented from what 
I have termed the inferior nucleus ; this is of somewhat smaller 
size than the adjacent portion of the anterior group. The upper 
extremity of the nucleus under consideration is seen to fade gradu- 
ally away, its volume and the number of cells diminishing almost 
pari passu. It terminates altogether at a slightly lower plane than 
does a group of cells to be afterwards described as the superior or 
small-celled nucleus. 
B. The postero-lateral group is formed of motor cells of the same 
size and type as those of the anterior group. It lies on the posterior 
surface of the longitudinal fasciculus, near its outer extremity. Its 
length is considerably less than that of the anterior group. Below, 
it begins slightly above the inferior extremity of the median nucleus ; 
while above, it is replaced by the superior or small-celled nucleus. 
It begins, below, as a small circular group of nerve cells, distinctly 
separated from the anterior group (fig. 5, p.l.n.), and gradually 
increases in size upwards, attaining its maximum near its upper 
extremity (fig. 6, p.l.n.). I have given the name of external nucleus 
(fig. 5, III. l.n.) to a group situated external to the middle and 
upper part of the postero-lateral group, partly between, and partly 
outside the longitudinal fasciculus. This is much shorter than the 
postero-lateral nucleus, lying opposite its middle third. 
C. The median nucleus (fig. 4, and fig. 5, m.n.) lies in the middle 
line opposite the intermediate portion of the anterior group. Its 
upper portion (fig. 5, m.n.) has the shape of an elongated spindle 
tapering anteriorly to a sharp point, while the posterior end is 
somewhat rounded. A line drawn transversely between the centres 
