CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
741 
where it passes into the aqueduct of Sylvius. Tire anterior end is continuous directly 
with the posterior extremity of the ventricle, the widest part being immediately 
behind the crura cerebelli ad medullam, thus differing from the arrangement in the 
Teleostei, in which the anterior end forms a distinct chamber beneath the cerebellum, 
communicating with the posterior part by a narrow passage. Behind the cerebellum 
the fourth ventricle is covered by the processes of the pia mater. Busch'"' terms it 
the arachnoid, but adds that it differs essentially from the arachnoid of the human 
brain, inasmuch as it abounds in numerous vessels. He speaks of a third membrane 
as the pia mater dipping into all the cavities. It does not appear from my dissections 
to be different, but both seem to be the same membrane. 
The walls of the fourth ventricle (figs 1, 2, 3, and 5) are formed on each side by an 
elongated and anteriorly-folded ridge which becomes merged into the lateral parts of the 
cerebellum. These may be looked upon as the restiform bodies. On the floor (figs. 1 and 
5) may be seen two longitudinal cylindrical prominences separated by a furrow occupy¬ 
ing the central line ; these are the funiculi teretes. They indicate the position of the 
ventral longitudinal columns, which may be considered to correspond to the anterior 
pyramids. Busch f also considered them to be so, but in the figure he makes them 
extend as visible protuberances quite up to the posterior commissure, an arrangement 
which was not found in my specimens, although they are traceable thus far, but are 
not apparent on the surface. Laterally the floor is occupied by a row of hemispherical 
beadlike tubercles, which form the origins for the roots of the vagus. Immediately 
behind the crura cerebelli ad medullam the funiculi teretes disappear, and the place of 
each is taken by a shallow longitudinal trough, the external margin of which corre¬ 
sponds to the external limit of the ventral longitudinal column. Towards the anterior 
end of the crura a deep furrow appears in the central line of the floor of the ventricle ; 
it extends into the aqueduct of Sylvius and gradually deepens until it joins the third 
ventricle. In the optic lobe the aqueduct of Sylvius becomes somewhat enlarged and 
forms a ventricle of a concavo-convex form, the concavity being directed upward ; near 
to its anterior end a passage, the continuation of the furrow on the floor above 
mentioned, leads into the third ventricle beneath the posterior commissure. 
The third ventricle, or thalamencephalon, is not closed in superiorly by nervous 
tissue, but gives entrance to processes from the pia mater which intrude through its 
roof to form the choroid plexus. It communicates by a gutter-shaped passage with 
the ventricle of the cerebrum, which ventricle at the posterior end is single, the 
foramen of Monro, but at the anterior end becomes double, the lateral ventricles ; 
each lateral ventricle communicates by a small round foramen with a passage leading 
into the peduncle of the olfactory lobe; this passage passes forward in the tuberosity 
situated on the outer side of the cerebrum, close to the outer wall, and soon becomes 
a mere slit, which enlarges when the peduncle is free of the cerebrum, and takes on a 
* Op. cit,, p. 20, tab. iii., fig. 8. 
t Op. cit., p. 23. 
