Olfacto-mesencephalic Fasciculus (Basal Bundle of Wallen berg).—This bundle rises in the cortex of the olfactory tract. It terminates in the tuber cinereum, mammillary body, teg mentum of mid-brain, pons, medulla and cord.
The fasciculus mammillaris princeps with its two divisions the mammillo-thalamic and mammillo-tegmental fasciculi (p. 86); the pedunculus corporis mammillaris (p. 86); and the habenulo-peduncular fasciculus (p. 147, 222) are described on the pages indicated.
The inter pedunculo-tegmental fasciculus rises from the inter peduncular nucleus (ganglion) and terminates in the stratum griseum centrale, the nucleus tegmenti dorsalis and nucleus teg menti profundus, where the dorsal longitudinal bundle of Schutz and the reticulo-spinal tracts arise and continue to vari ous motor nuclei.
The exact origin of the cortical fillet (Figs. 93 and 94) has not been entirely determined, but it is known to rise chiefly in the lateral nucleus of the thalamus. The ventral stalk of the thalamus (Fig. 93) runs through the internal capsule in the inferior lamina. Its afferent fibers end in the globus pallidus. The ventral stalk of the thalamus can no longer be considered a part of the cortical fillet; as its afferent fibers end in globus pallidus which has no direct connection with the cortex (see p. 218). From the anterior end of the thalamus streams a great pencil of fibers, called the frontal stalk (Fig. 93). It mingles to a small extent with the fibers of the pyramidal tract, but runs chiefly through the frontal part of the internal capsule. Its termination is in the caudate nucleus and the posterior and middle parts of the three frontal gyri.
The parietal stalk issues from the lateral surface of the thala mus higher up than the ventral stalk and mingles with the py ramidal fibers in the superior lamina of the internal capsule. Its location is principally in the posterior third of the occipital part of the capsule (Figs. 93 and 94). Its sensory fibers terminate in the posterior central gyrus and the contiguous part of the paracentral gyrus; these are small and medium-sized fibers (Sachs). A greater number of fibers (of medium caliber with a few large and small fibers interspersed) end in front of the central sulcus—in the fronto-parietal operculum, the anterior central and superior frontal gyri and the middle of the gyrus cinguli. The fibers of this larger group are not sensory, as their impulses do not excite sensations; they have a reflex or automatic function, and probably exert a controlling influence over motor discharges from the cortex. The sensory fibers of the parietal stalk carry impulses to the receptive sensory cortex, where the proper sensations are evoked. According to
Head and Holmes, they form five functional tracts which trans mit all kinds of common sensory impulses except those of pain and pleasure (see p. 225).
Three special sense fasciculi traverse the internal capsule, the gustatory, the optic and the acustic or auditory. Recently, the gustatory fasciculus has been traced by Otto May and Sir Victor Horsley (Brain, Vol. 33, p. 186). It rises in the nu cleus of the solitary tract (nucleus of Nageotte). Ascending beside the central gray substance of the mid-brain, dorsal and lateral to the medial longitudinal bundle, it enters the internal medullary lamina of the thalamus and terminates in the dorsal third of the nucleus lateralis. The gustatory radiation of the internal capsule, probably located between the optic radiation and the parietal stalk of the thalamus, continues the taste path from thalamus to cortex. Auditory impulses run through the lateral fillet and the brachium inferius to the medial genicu late body; while optic impulses run directly to the lateral genicu late body and the pulvinar without passing through the mid brain at all. Within the internal capsule the gustatory tract cannot positively be located; but the acustic and visual paths are well known.
The acustic radiation (Figs. 93 and 74) continues the auditory path from the medial geniculate body through the retrolenti form part of the internal capsule, to the transverse temporal gyri and the third and fourth fifths of the superior temporal gyrus (Barker). Interruption of these fibers produces deafness in the opposite ear, which is complete because the acustic path above the inferior quadrigeminal colliculi is wholly crossed. There are some corticifugal temporo-thalamic fibers in the acustic radiation; they are probably reflex in function.
The optic radiation (Figs. 49, 75, 93 and 94) rises in the lateral geniculate body and in the pulvinar of the thalamus. It con tinues the visual conduction path through the retrolentiform region of the internal capsule to the cortex of the lingual and cuneate gyri. Half-blindness in the same side of both retinae results from section of the optic radiation. The corticifugal occipito-thalamic fibers in the optic radiation are believed by Campbell to be axones of the solitary giant cells (Meynert's) in the occipital cortex. They run through the lateral geniculate body and brachium superius to the superior colliculus of the corpora quadrigemina, where they end in contact with the neu rones of the anterior tecto-spinal bundle. Their function is reflex.