Houser, Zhe Neurones of a Selachian. 123 
Section VJ. Tue MIpBRAIN. 
The midbrain of Mustelus has retained to a marked degree 
the features characteristic of this cerebral vesicle in its primitive 
condition. Its ventricle has been so little encroached upon by 
nervous matter that the name agueduct of Sylvins would not 
be applied as a descriptive term were it not made necessary by 
usage in higher vertebrates. The aqueduct is produced later- 
ally into a pair of spacious recesses, each of which occupies the 
interior of an optic lobe, while the general cavity communicates 
freely through narrower extensions with the third ventricle in 
front and the fourth ventricle behind (Fig. 1, m.). The optic 
lobes are two thin-walled bodies of dome-like form, separated 
from each other by a conspicuous median furrow, and the pair 
taken together are somewhat broader than the base of the mid- 
brain on which they rest. The anterior divisions of the cere- 
bellum lie upon and partially conceal the optic lobes from the 
dorsal aspect. The base of the midbrain is a direct continua- 
tion anteriorly of the great fibre-system of the oblongata and 
metencephalon, showing but slight diminution in size. 
The microscopical anatomy of the midbrain is really quite 
complicated, owing to the diversity of the terminations and 
connections which are established here. The base is a crowded 
highway between the different parts of the encephalon. The 
fibres of the optic nerve have their termination. in the dorsal 
midbrain, the tectum mesencephali, with a nexus of intrinsic 
neurones and fibres of more distant origin associated with them. 
Finally, the aqueduct of SyLvius is bordered by nervous matter 
which is phylogenetically distinct from either the base or the 
tectum, and this has undergone special development at certain 
points as the nuclei of important motor neurones. — 
I. The Tectum Mesencephat. 
The tectum mesencephali lies as an investing cap upon the 
central gray matter which surrounds the aqueduct of SyLvius. 
Its situation here is fraught with significance, for it represents 
an addition to the more ancient nervous structures made neces- 
