The strio-fugal tracts are intermingled with the ventral stalk of the thalamus. They comprise four sets of fibers, viz., the strio-thalamic, strio-rubral, strio-hypothalamic and strio-nigral, which rise in the globus pallidus and terminate, respectively, in the thalamus, nucleus ruber, nucleus hypothalamicus and substantia nigra.
The temporo-pontal tract extends from the temporal cortex downward to the nucleus of the pons; perhaps a few fibers go to the motor nuclei of cranial nerves. The temporo-pontal path is situated inferior to the posterior end of the lentiform nucleus and behind a frontal plane cutting the mammillary bodies.
The retro-lentiform part of the internal capsule is that part of the superior lamina which bends down behind the lentiform nucleus and joins the inferior lamina. In the retro-lentiform region the special sense tracts are located, the acustic, the optic and probably the gustatory.
The acustic radiation is the most inferior of the three special sense tracts. It is composed of thalamo-temporal fibers (audi tory), which run from the medial geniculate body to the transverse and superior temporal gyri, and of temporo-thalamic fibers (reflex), which, in an inverse direction, connect those gyri with the medial geniculate body and the inferior quadrigem inal colliculus.
Above the acustic radiation in the retro-lentiform part of the capsule, the optic radiation is located. It is also a two way funiculus. It connects the thalamus with the visual cortex along the calcarine fissure in the occipital lobe.
The gustatory tract has not been definitely located. Prob ably it is situated anterior to the optic radiation, between that and the parietal stalk of the thalamus. In accord with Sir Victor Horsley's tracing of the gustatory path up. through the mid-brain to the dorso-medial part of the lateral nucleus of the thalamus, the radiation should rise in the thalamus and run through the capsule to the taste cortex, probably, in the gyrus cinguli just behind the splenium of the corpus callosum.
The parietal stalk of the thalamus is the common sensory tract of the capsule. It is located in front of the special tracts in the occipital part of the superior lamina. Anteriorly, its fibers intermingle with the posterior fibers of the pyramidal tract. The corticipetal fibers of the parietal stalk rise in the
thalamus and end almost wholly in the posterior central gyrus; a few go to the anterior central and perhaps to the middle of the gyrus cinguli. They carry all kinds of common sensory impulses excepting those giving rise to pleasure and pain. The latter impulses induce their appropriate sensations in the thalamus (Head and Holmes).
The pyramidal tract, or cerebrospinal tract, is the voluntary motor tract contained in the internal capsule. It is situated in the genu and the adjacent portion of the pars occipitalis, anterior to the parietal stalk of the thalamus and between the thalamus and the lentiform nucleus. By its position in the capsule the pyramidal tract is divided into a genicular and an occipital part. Both parts rise in the motor cortex of the anterior central and paracentral gyri, and terminate in con nection with the nuclei of all motor nerves; the part in the genu goes to the motor nuclei of cranial nerves, the occipital part to the motor nuclei of spinal nerves. The pyramidal tract con ducts voluntary motor impulses and inhibitory impulses.
Among the fibers of the pyramidal tract, behind the genu of the capsule, there is a small tract described by von Monakow, which rises in the opercular region of the frontal lobe and ter minates in the red nucleus. It is the cerebrorubral tract (tractus cerebrorubricus).
The pars frontalis of the internal capsule contains two tracts and a large number of internuncial fibers connecting the nuclei of the striate body.
The internuncial fibers freely associate the caudate nucleus and the putamen of the lentiform nucleus, and the caudate and globus pallidus. It is through the thalamus that the striate body is connected with the cerebral cortex (A. S. Kinnier Wilson).
Fronto-pontal Tract.—The upper part of the fronto-pontal tract and the frontal stalk of the thalamus are intermingled with one another in the frontal part of the capsule. The fronto pontal tract rises in the posterior and middle parts of the three frontal gyri; it descends to the nucleus of the pons and per haps to the motor nuclei of cranial nerves.