8 4 
MAMMALIAN ANATOMY 
like cavity of the diencephalon, the third ventricle, from which the 
foramen interventriculare leads on each side laterally into the 
lateral ventricles. Note the line along the surface of each optic 
thalamus marking the attachment of the chorioid plexus (the 
taenia thalami). On the ventral and lateral surfaces of the thala¬ 
mus note that the optic tract spreads out over the surface, a part 
of its fibers passing on to reach the geniculate body, which appears 
as a slight elevation in the angle between the thalamus and the 
midbrain, while still others enter the superior colliculus of the 
corpora quadrigemina. 
In the mesencephalon note that the exposed surface is wholly 
external surface. The four eminences of the corpora quadrigemina 
are known as the colliculi, the two superior colliculi being larger 
and nearer the middorsal line, while the inferior colliculi are more 
laterally located. The trochlear nerves may now be plainly seen 
as they emerge from the dorsal surface of the brain posterior to the 
inferior colliculi. Note the large size of the cerebral peduncles 
which form the floor of the mesencephalon, and their connection 
anteriorly with the base of the cerebrum which they reach by 
passing medial to the optic thalami. Note their convergence 
posteriorly to disappear within the pons. 
In the rhombencephalon note the thin roof of the rhomboid 
fossa or fourth ventricle. If this thin roof has been removed, the 
whole floor of the rhomboid fossa is exposed to view, bordered 
laterally by the cut surfaces of the bundles of fibers which enter 
the cerebellum, and by the lateral funiculi of the medulla. 
Anteriorly the fossa closes in beneath the corpora quadrigemina to 
form the aquaeductus cerebri, and posteriorly it closes in similarly 
to form the canalis centralis of the spinal cord. In its floor may 
be seen bordering the longitudinal sulcus numerous paired eleva¬ 
tions which indicate the location of certain of the masses of gray 
substance or “nuclei'’ of the medulla. From one of these the 
bundles of fibers which make up the acoustic nerve may be seen to 
extend transversely beneath the epithelial lining of the fossa, to 
issue from the surface at the lateral margins. 
