The brain Part 2 Midbrain (Mesencephalon)

The midbrain is a constricted portion of the brain connecting the forebrain and the hindbrain.

It consists of a pair of cerebral peduncles and the tectum which has two pairs of rounded masses which are called corpora quadrigemina.

There is a tunnel in between these two structures called the cerebral aqueduct which connects the third and fourth ventricle.

The cerebral peduncles are large bundles of nerve fibers from the cortex to the spinal cord and brain stem. The cerebral peduncles are hidden under the temporal lobes and can be seen by either removing the temporal lobe or by pulling the temporal lobe away.

They converge as they enter the pons. The area between the two peduncles is known as the interpeduncular fossa and it contains a layer of grayish substance called the posterior perforated substance. Blood vessels pass through this to supply the brain.

On a cross section each peduncle appears to be divided by a darkly pigmented layer known as substantia nigra into dorsal (towards the back of the body) and ventral (towards the front of the body) parts. The dorsal part is called tegmentum and the ventral part called the base.

There is a mass of neurons in the tegmentum called the red nucleus. This is concerned with the control of movements.

The two upper masses of neurons of the tectum are called superior colliculi and the two lower ones called inferior colliculi. The superior colliculi are concerned with reflex actions of the eye while the inferior colliculi are involved in the reflex movements in response to auditory stimuli.

The nucleus of the III cranial nerve is situated at the level of the superior colliculus just ventral to the cerebral aqueduct. The nucleus of the IV cranial nerve is situated in the tegmentum at the level of the inferior colliculus.

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