The colliculus inferior of the corpora quadrigemina is elon gated transversely (Fig. 56). It is joined to the medial genicu late body by an oblique ridge, called the brachium inferius and it forms the termination of two ridges that approach it from below, due to the lateral fillet, and the brachium conjunctivum of the cerebellum. Its surface is made up of medullated fibers continuous with the lateral fillet and brachium inferius, which forms the stratum zonale; gray substance, called the nucleus of the inferior colliculus, constitutes its deep portion. This nucleus receives a part of the lateral fillet and brachium inferius, and gives rise to a portion of the brachium inferius and tecto spinal fasciculi; it forms a partial relay in the acustic path and a reflex acustic center.
Brachium Superius (Figs. 55 and 56).—It connects the lateral geniculate body with the superior quadrigeminal colliculus; but, excepting its extreme posterior end, it is buried in the substance of the thalamus. Its course is anterior and medial to the medial geniculate body. The brachium superius is composed chiefly of efferent fibers from the optic radiation, a tract partially relayed in the lateral geniculate body; it probably contains also a few optic fibers from the lateral root of the optic tract and a few from the stratum griseum of the superior quadrigeminal colliculus. Optic
reflex impulses are conducted by the brachium superius.
Brachium Inferius (Figs. 55 and 56).—The inferior arm, connecting the inferior quadrigeminal colliculus and the medial geniculate body, is visible through its whole length. It forms the superior boundary of the triangle of the lateral fillet; the lateral fillet and the sulcus lateralis of the mid-brain form the other two sides. Two strands of afferent fibers and one of efferent fibers make up the brachium inferius. The former are the lateral fillet (most of it) and the axones of the nucleus col liculi inferioris; these terminate in the medial geniculate body and form a stage of the acustic path. The efferent fibers in the brachium inferius are axones of the acustic cortex which descend through the acustic radiation to the medial geniculate body and continue through the brachium inferius to the inferior colliculus. .