The part of the brainstem above the pons is the mid-brain. Features of its dorsal surface are the upper part of the roof of the fourth ventricle, the superior cerebellar peduncles, these bulges, the inferior and superior colliculi, and in the mid-line the pineal body. The fourth cranial nerve, the trochlear, emerges from the dorsum of the midbrain.
The mid-brain spreads out into these two massive columns, the cerebral peduncles, which connect the brainstem to the cerebrum.
Here are the cerebral peduncles in the intact brain. They're largely hidden by the lower parts of the cerebral hemispheres, the temporal lobes. To see the cerebral peduncles better, we'll look at a brain in which the temporal lobe, and the cerebellum have been removed.
Here are the cerebral peduncles again. Here on the outside of the cerebral peduncle are the medial geniculate body, and the lateral geniculate body, which gives rise to the optic tract. Between the cerebral peduncles the third cranial nerve, the oculomotor, emerges.
We'll return to the intact brain. Here are the two oculomotor nerves. Here are the two optic tracts. They meet at the optic chiasm. From the optic chiasm the two optic nerves emerge. They're the second cranial nerves.
Here's the brain in situ with the right cerebral hemisphere removed. Here's the corpus callosum, which joins the two cerebral hemispheres, here's the divided cerebral peduncle, here's the midbrain. Here's the floor of the middle cranial fossa. Here's the optic nerve, running forwards beneath the dura toward the optic canal. Here's the oculomotor nerve, here's the trochlear nerve.
The ventral aspect of the brain passes upwards to here, then turns a corner and continues forwards into a complicated area that we'll look at later in this section.