If you remember the superior cerebellar peduncles converging towards the midbrain, you will recognise them deccusating around 'A' in this section. This puts the level of this section in the caudal midbrain. Their main fibres are on their way to the red nucleus and beyond to the thalamus.

Where do these fibres come from? What is their destination?

The area dorsal to the black line is the tectum - here formed by the inferior colliculi. The black line passes through the aqueduct.

The area between the black and the white lines is the tectum. Largely occupied by the deccusating fibres mentioned above, the lemnisci have been pushed outwards in the sweeping curve enclosed by the white outline - which is just as good, for these fibres will terminate in the thalamus. In the dorsolateral part of the tegmentum one would also expect the mesencephalic nucleus of V.

In the grey matter around the aqueduct, ventral to the cavity is the area of nucleus of IV. The remaining "periaqueductal grey" is well-known for many other features, not seen here.

The most ventral part is the cerebral peduncle, with the substantia nigra (better seen in a rostral section) separating it from the tegmentum.

The peduncles contain more than the pyramidal tract fibres. The pyramidal tract fibres are in the middle part of the peduncle (at 'C'). Lateral and medial to these are the fibre masses coming from vast areas of the cerebral cortex ('B' and 'D'), destined to end in the pontine nuclei.

Ypu will encounter a number of terms describing this area : cerebral peduncles, crus cerebri and the basis pedunculi. The fine distinction is both semantic and topographical. Knowing the location of the pyramidal and corticopontine fibres and the substantia nigra is more relevant!