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pus striatum, but to some extent, also, in the cerebrum ; the deep portion, made up of the fasciculi teretes and posterior pyramids and accessory fibers from the cerebellum, constitute the sensory tract (the tegmentum), which terminates in the optic thalamus and cerebrum.
Function.-The crura are conductors of motor impulses and sensory impressions; the gray matter, the locus niger, assists in the coördination of the complicated movements of the eyeball and iris, through the motor oculi communis nerve. They also assist in the harmonization of general muscular movements, section of one crus giving rise to peculiar movements of rotation and somersaults forward and backward.
CORPORA QUADRIGEMINA. The Corpora Quadrigemina are four small, rounded eminences, two on each side of the median line, situated immediately behind the third ventricle, and beneath the posterior border of the corpus callosum.
The anterior tubercles are oblong from before backward, and larger than the posterior, which are hemispherical in shape; they are grayish in color, but consist of white matter externally and gray matter internally.
Both the anterior and posterior tubercles are connected with the optic thalami by commissural bands named the anterior and posterior brachia, respectively. They receive fibers from the olivary fasciculus and fibers from the cerebellum, which pass upward to enter the optic thalami.
The corpora geniculata are situated, one on the inner side and one on the outer side of each optic tract, behind and beneath the optic thalamus, and from their position are named the corpora geniculata interna and externa ; they give origin to fibers of the optic nerve.
Functions.—The Tubercula quadrigemina are the physical centers of sight, translating the luminous impressions into visual sensations. Destruction of these tubercles is immediately followed by a loss of the sense of sight; moreover, their action in vision is crossed, owing to the decussation of the optic tracts, so that if the tubercle of the right side be destroyed by disease or extirpated, in a pigeon, the sight is lost in the eye site side, and the iris loses its mobility.
The tubercula quadrigemina as nerve centers preside over the reflex movements which cause a dilatation or contraction of the iris, irritation of the tubercles causing contraction, destruction causing dilatation. Removal of the tubercles on one side produces a temporary loss of power of the opposite side of the body, and a tendency to move around an axis is mani
of the oppo
fested, as after a section of one crus cerebri, which, however, may be due to giddiness and loss of sight.
They also assist in the coördination of the complex movements of the eye, and regulate the movements of the iris during the movements of accommodation for distance.
CORPORA STRIATA AND OPTIC THALAMI.
The Corpora Striata are two large ovoid collections of gray matter, situated at the base of the cerebrum, the larger portions of which are imbedded in the white matter, the smaller portions projecting into the anterior part of the lateral ventricle. Each striated body is divided, by a narrow band of white matter, into two portions, viz.:
1. The caudate nucleus, the intraventricular portion, which is conical in shape, having its apex directed backward, as a narrow, tail-like process.
2. The lenticular nucleus, imbedded in the white matter, and for the most part external to the ventricle; on the outer side of the lenticular nucleus is found a narrow band of white matter, the external capsule; and between it and the convolutions of the island of Reil, a thin band of gray matter, the claustrum ; the corpora. striata are grayish in color, and when divided present transverse striations, from the intermingling of white fibers and gray cells.
The Optic Thalami are two oblong masses situated in the ventricles posterior to the corpora striata, and resting upon the posterior portion of the crura cerebri. The internal surface projecting into the lateral ventricles is white, but the interior is grayish, from a commingling of both white fibers and gray cells. Separating the lenticular nucleus from the caudate nucleus and the optic thalamus is a band of white tissue, the internal capsule.
The internal capsule is a narrow, bent tract of white matter, and is, for the most part, an expansion of the motor tract of the crura cerebri. It consists of two segments, an anterior, situated between the caudate nucleus and the anterior surface of the lenticular nucleus, and a posterior, situated between the optic thalamus and the posterior surface of the lenticular nucleus. These two segments unite at an obtuse angle, which is directed toward the median line. Pathological observation has shown that the nerve fibers of the direct and crossed pyramidal tracts can be traced upward through the anterior two-thirds of the posterior segment, into the centrum ovale, where, for the most part, they are lost; a portion, however, remain.