Metathalamus.—This embraces the medial and lateral gen iculate bodies which were described on p. 140, 164. Both contain thalamo-cortical and thalamo-tectal (thalamo-quadri geminate) neurones. The latter send their axones to the quadrigeminal colliculi through the brachium inferius and brachium superius; while the axones of the former enter into their respective radiations.
1. Optic Radiation (Gratioleti).—The optic radiation is a two-way funiculus composed of afferent special sense fibers and efferent, reflex fibers. The afferent fibers, thalamo-occipital fasciculus, rise in the lateral geniculate body and the pulvinar of the thalamus and carry the visual impulses through the inter nal capsule to the striated cortex along the calcarine fissure, where they evoke the sensation of sight. The efferent, occipito thalamic fibers rise from the large solitary pyramids of Meynert in the visual cortex; they descend through the optic radia tion to the lateral geniculate body and thalamus and, through the brachium superius, some continue to the superior quad rigeminal colliculus. In the superior colliculus contacts are formed with tecto-spinal neurones whose axones end in motor nerve nuclei, cranial and spinal. Destruction of the optic radiation on one side causes blindness in the opposite half of the visual fields, homonymous hemianopsia, without loss of optic reflexes.
2. The acustic radiation, like the optic, is a double funiculus of afferent and efferent fibers. The afferent, thalamo-temporal fibers rise in the medial geniculate body; they run through the internal capsule just behind the optic radiation, and terminate in the transverse and superior temporal gyri; there, the impulses from the cochlear nerve excite the sensations of sound. In the acustic cortex rise the efferent, temporo-thalamic fibers which have a reflex function. Traversing the acustic radiation in a descend ing direction, they reach the medial geniculate body and the greater number continue through the brachium inferius to the inferior quadrigeminal colliculus, the acustic reflex center. If the acustic radiation is destroyed on one side the result is deaf ness in the opposite ear.
The red nucleus (nucleus ruber) of the tegmentum is situated beneath the thalamus (Figs. 54 and 58). See p. 139. It is
a relay-station in the indirect afferent path, receiving the oppo site brachium conjunctivum cerebelli and, by its axones, the rubro-thalamic tract, continuing the path to the thalamus. It also receives efferent axones from the cerebral cortex (Beevor and Horsley) and globus pallidus and gives origin to one cen trifugal bundle of axones, the rubro-spinal tract, which after crossing over in the ventral decussation of the tegmentum (Forel's) descends, first, with the medial portion of the lateral fillet; second, through the lateral area of the medulla, and, third, through the lateral part of the spinal cord. Gradually diminishing, it disappears at the first lumbar segment. It ends in the gray crescent of the spinal cord. The red nucleus contains rubro-thalamic and rubro-spinal neurones. The rubro thalamic fasciculus terminates in the lateral nucleus of the thala mus; it continues the cerebellar path from the brachium con junctivum, though a part of the brachium continues without interruption to the thalamus. The rubro-spinal fasciculus ends in motor nuclei, cranial and spinal. The latter bundle, accord ing to Horsley, is a part of the coordinating mechanism for locomotion. The rubro-thalamic fasciculus belongs to the indi rect afferent path.
The nucleus hypothalamicus (Luysi) (Figs. 37 and 54) is a pigmented bi-convex mass of gray matter placed ventro-lateral to the red nucleus, and between it and the basis pedunculi. See p. 139. It is separated from the red nucleus by the zona incerta. It constitutes an important terminal nucleus for cer tain corticipetal fibers of the tegmentum and gives origin to others. Certain descending fibers from the striate body ter minate in this nucleus. The latter in part run through the tuber cinereum, just above the posterior border of the optic chiasma, and form the commissura superior (Meynerti), Gudden's commissure being called the commissura inferior. The hypo thalamic nucleus is closely associated with the globus pallidus of the lentiform nucleus; it is not connected with the thalamus. Besides receiving the afferent tegmental fibers, it receives the strio-hypothalamic fasciculus and its axones form the hypo thalamo-striate fasciculus, bundles which doubly connect the nucleus with the globus pallidus.