VAN 121: Veterinary Gross Anatomy-II (2+2)

CORPORA QUADRIGEMINA

• These are four rounded eminences, which form the upper part of the midbrain and lie under posterior part of the cerebral hemispheres.
• They consist of two parts of colliculi (anterior and posterior) separated by a transverse groove.
• The anterior colliculi are larger and placed higher in level than the posterior and are known as nates.
• They are gray in colour, hemispherical and separated by a shallow furrow.
• A wide groove intervenes between them and thalami. The posterior colliculi are smaller and paler called the testes.
• They are limited behind by a transverse furrow on either side from which emerges the fourth pair of nerves.
• The anterior colliculi are connected by an indistinct band of fibres called the superior brachium to the lateral geniculate body.
• The posterior colliculi are connected to the medial geniculate body by a band of white fibres called the inferior brachium.
• The posterior colliculi also receive the lateral lemniscus or fillet seen laterally as a rounded narrow band passing upward and forward from under the pons.
• The anterior colliculi are visuo-spinal reflex centres concerned with movements of musculature of eyes, head, neck, and also of the limbs necessitated by visual impulses. It receives visual impulses from the optic tract through the superior brachium; its efferent fibres form the tecto-spinal tract.
• The posterior colliculi are audito-spinal reflex centres concerned with similar reflexes in response to auditory impulses conveyed to them by the lateral lemniscus. Efferent fibres form the tecto-spinal tract.