The Mid-Brain

The Mid-BrainThe mid-brain is a short stem which serves mainly as a conducting path between the fore- and hind-brains. Its upper or dorsal part consists of the corpora quadrigemina; the remainder forms the crura cerebri or peduncles of the brain.

The original cavity of the mid-brain vesicle is represented by a fine canal, the iter or aqueduct of Sylvius, which is continued behind into the fourth ventricle, and opens in front into the third ventricle.

External Appearances. - On the upper surface are four small rounded eminences - the corpora quadrigemina - separated from one another by a crucial groove. They are arranged in pairs: two superior or anterior associated with the sense of sight, and two inferior or posterior associated with the sense of hearing.

Passing outwards from the side of each is a rounded strand of fibres, the brachium. Each superior brachium terminates partly in a small elevation - the external geniculate body - and is partly continuous with the optic tract. Each inferior brachium runs into a little globular swelling - the internal geniculate body (Fig. 55).

The inferior and lateral surfaces of the mid-brain are composed of the crura cerelbri. Each crus is made up of two parts: an upper portion designated the tegmentum, and a lower, the crusta. A groove on the surface - the lateral sulcus - indicates the division.

The tegmenta are continuous from side to side of the mid-brain, but the crustae of opposite sides are separated from one another by a deep interpeduncular grove. From slight, furrows or sulci (oculomotor sulci) on the sides of this groove the third cerebral nerves emerge. Where the crura adjoin the pons they lie close to one another, but as they pass forwards they diverge, leaving a triangular interpeduncular space.

Internal Structure. - In a transverse section across the mid-brain the aqueduct of Sylvius can be seen near the superior surface (Fig. 56).

Grey Matter. - The important masses of grey matter of this region may be arranged in four groups: (1) the grey matter surrounding the aqueduct of SyTlvius; (2) the grey nuclei of the quadrigeminal bodies; (3) the substantia nigra; (4) the red nucleus.

(1) In the grey matter of the floor of the aqueduct of Sylvius the nuclei of the trochlear (iv), of the oculomotor (iii), and of a portion of the trigemin€tl (v) nerves are located.

(2) Around the cells of the inferior corpora quadrigemina, terminate many of the fibres of the lateral fillet, a band of fibres derived from certain nuclei of the auditory nerve in the medulla and pons. From the cells of the inferior corpora quadrigemina fibres pass out by the inferior brachia into the tegmenta and run up to the fore-brain.

The fibres which enter the superior quadrigeminal body are derived from nerve cells of the retina; they run in the superior brachium. Many of the emerging fibres pass out through the superior brachium to the fore-brain; others, after decussating, are continued down through the pons and medulla to the spinal cord.

(3) The substantia nigra is a deeply pigmented mass of grey matter which extends across the crus from the lateral sulcus to the oculomotor sulcus, and separates the tegmentum from the crusta.

(4) The red nucleus lies within the tegmentum. In it the fibres of the superior cerebellar peduncle terminate after decussation. A few of the fibres originating from the cells of the red nucleus pass up to the fore-brain, but the majority decuc,sate and run down to the spinal cord.

White Matter. - The continuations of the important tracts already noticed in the medulla and pons can all be traced in the mid-brain. The crustae consist practically entirely of motor fibres which originate in the cortex of the fore­brain.

In the tegmentum the fillet lies above the substantia nigra: the outer portion of it the lateral fillet-enters the inferior quadrigeminal body; the remainder, named the mesial fillet, runs on to the fore-brain. The posterior longitudinal fasciculus is situated above the fillet and just below the grey matter of the aqueduct of Sylvius. From and to it pass fibres connecting the various nuclei in its vicinity.