Midbrain

 Midbrain is middle part of the brain present between diencephalon and pons varolli. A narrow passage called Iter runs through its length and connects third ventricle to fourth. Consists of corpora quadrigemina  and crura cerebri. Corpora quadeigemina is formed by two pairs of lobes present in dorsal thick wall of midbrain. Upper pair called superior colliculi control and coordinate movement of head in response to visual stimulus. Inferior colliculi controls and co-ordinate head movement in response to auditory stimulus. Crura cerebri is a pair of thick bands of anterior, longitudinal nerve fibres. These fibres connect cerebrum, cerebellum and spiral cord and also function as relay centre. They also from RAS network which controls muscle tone.