Midbrain - The midbrain is the short portion of the brain between the pons and the cerebral hemispheres. The dorsal portion of the midbrain (the tectum) contains the superior and inferior colliculi. The superior colliculi are associated with the optic system. The superior quadrigeminal brachium extends laterally from them and conects with the lateral geniculate body. The inferior colliculi are associated with the auditory system.
The reticular activating System - A portion of central cephalic brain stem and adjacent areas composed of reticular formation, subthalamus, hypothalamus and medial thalamus has been shown in animal experiments to be essencial for iniciation and maintenance of alert wakefulness. The reticular activating system may be considered to be essential for arousal from sleep, wakefulness, alerting or focusing of attention, perceptual association and directed intrsopection; its impaired function may be associated with anesthesia and comatose states. This system can be stimulated en masse by stimulation of all peripheral sense organs. Upon excitation, a sleeping animal arouses, and electrocortical tracings change from a sleeping to a walking pattern.
The reticular formation of the tegmentum of the medulla and pons refers to the diffuse primitive system of interlacing fibers and nerve cells forming the central core of the brain stem.