764 
MR. A. SANDERS ON THE ANATOMY OF THE 
“ nucleus rotundus,” also by bundles of fibres which are derived from the region 
slightly in advance of the origin of that commissure ; these latter cords pass down in 
front of the “nucleus rotundus,” close to its anterior edge, forming a border thereto. 
The whole of these fibres stream out from the lower edge of the “nucleus rotundus,” 
and disappear in the posterior part of the hypoarium. 
Deep origins of the Cerebral Nerves. 
The origin of the olfactory nerve (fig. 1, n. 1) has been described in the account of 
the histology of the lobi olfactorii. 
The optic nerves (figs. 3 and 4, n. 2) are derived from three sources. A few of the 
inferior fibres come up from the anterior end of the hypoarium. The superior fibres 
are derived from the three outer layers of the tectum lobi optii ; in some sections the 
fibres for this nerve form an additional distinct stratum of longitudinal fibres inter- 
calated between the first and second layer ; this shows more particularly towards the 
middle of each tectum. 
The third root is formed by fibres, which arise in the interior of the torus semi- 
circularis, take a direction outwards towards its external edge, and form a bundle 
which passes through the crura cerebri, and being intercalated between them and the 
outer margin of the torus, descend towards the anterior inferior end of the tectum, 
where it joins the optic tract. 
The motores oculorum (figs. 1, 2, and 5, n. 3) are derived from a group of medium- 
sized cells, situated beneath the floor of the aquaeductus Sylvii immediately behind its 
entrance into the ventricle of the optic lobe ; from these cells the fibres pass down 
close to the middine towards the ventral surface of the medulla, where they pass 
through the commissura ansulata ; they then emerge from the angle between the 
medulla and the hypoarium, having received some fibres from the opposite side of the 
commissure. This nerve has two separate origins : the main origin from the ganglion 
on its own side, above described, and a smaller one from the region of the crura lobi 
optici of the opposite side. 
The trochleares (fig. 6, n. 4) arise from the anterior end of the cerebellum on a level 
with the base of the valvula cerebelli, where each nerve decussates with its fellow of 
the opposite side, and becomes intermingled with the transverse commissure of the cere- 
bellum above the aqueduct ; the nerve then passes down the margin of the medulla, 
outside the crura cerebelli, and emerges behind the posterior end of the optic lobe. 
The trifacial (figs. 2, 7, 8, and 9, n. 5) has three distinct points of origin, of which 
two belong to the anterior part of its root, and one to the posterior part. The ante- 
rior root is made up of two distinct species of fibres ; the ventral part of the root is 
composed of fine slender, while the dorsal half has large, coarse, double contoured 
fibres. The regions of origin whence these are derived are placed at some distance 
apart ; the latter come in a nearly straight line from a large ganglion situated beneath 
