Like the two following bundles it establishes associations for the sense of smell.
2. TheFornix (Figs. 51 and 52).—In each lateral half of the for nix there are four fasciculi: (a) The olfacto-hippocampalfasciculus, which rises in the olfactory triangle, anterior perforated sub stance and septum pellucidum and terminates in the hippo campus and uncus. There are two efferent fasciculi, of reflex function, in the fornix, which associate the hippocampal forma tion with the mammillary and habenular nuclei; (b) the hippo campo-mammillary fasciculus and (c) the hippocampo-habenular fasciculus; these rise in the hippocampal formation, and in the supracallosal and cingulate gyri; their terminations are indi cated by their names. The fibers from the supracallosal and cingulate gyri pierce the corpus callosum as the fibre perforantes of the fornix longus (Forel). All these may be considered associative in function, though they belong to afferent and effer ent paths (see p. 247). (d) The commissural bundle of the fornix connects the hippocampal formations; it is the commissura hippocampi (p. 1o8).
3. Fasciculus Occipito-frontalis Inferior.—The work of E. J. Curran has simplified certain systems of association fibers of the cerebrum. Curran shows that the uncinate and inferior longitudinal fasciculi, formerly considered separate systems, are but parts of one greater system extending through the basal portion of the hemisphere from the frontal to the occipital pole. This inferior occipito-frontal fasciculus, in its middle part, is compact and lies below the putamen and claustrum, in the base of the external capsule; in front, it radiates to the orbital and inferior frontal gyri; it establishes connections with the temporal and uncinate gyri in the middle region; and, posteriorly, it runs lateral to the optic radiation to reach the fusiform, lingual, cuneate and occipital gyri (Jour. Comp. Neurol. & Psychol., Vol. iv). The function of this bundle is uncertain. The fact that its occipital part is well developed in the chimpanzee and orang, but not present in the macaque monkey (Ferrier and Turner) throws little light upon it. The part of the inferior occipito-frontal fasciculus that arches over the lateral fissure, connecting the frontal with the temporal and limbic lobes, is the fasciculus uncinatus of the older descriptions, while that part which joins the temporal pole and gyri with the gyri about the occipital pole constitutes the fasciculus longitudinalis in ferior, as described in former editions of this book; the deeper and longer fibers, which reach the whole length of the hemi sphere and connect the uncinate and inferior longitudinal fasciculi into one system, were first demonstrated by Curran.
The tracing was accomplished by blunt dissection, an old-time method revived and successfully employed by Hubertus J. H. Hoeve (Figs. 99 and poo).
4. The superior longitudinal fasciculus (fasciculus longitud inalis superior, Fig. 97) is a sagittal bundle located beneath the convex surface of the hemisphere, just above the posterior ramus of the lateral fissure of the cerebrum. According to Cunningham, it runs just above and lateral to the putamen of the lentiform nucleus, external to the base of the corona radiata. It runs through the summit of the external capsule, as the infe rior occipito-frontal fasciculus runs through the base. Its fibers diverge at the posterior end of the lateral fissure and ra diate into the parietal, occipital and temporal cortex: some of them, arching around that fissure, run as far forward as the temporal pole. The superior longitudinal bundle joins the frontal cortex with the parietal, occipital and the external tem poral. It thus associates the psychic auditory and visual cen ters with the motor speech center; hence motor aphasia is the result of its interruption.
5. The Fasciculus Occipito-frontalis Superior (Foreli).—This is a large bundle of fibers formerly regarded as a part of the corpus callosum (Fig. 99). It is situated below the corpus callosum and, in equatorial sections of the brain, is found in the angle formed between the callosum and the internal capsule, just external to the lateral ventricle. It extends from the cor tex of every part of the frontal lobe to the cortex of the convex surface and lateral border of the occipital lobe. Posteriorly, the fibers diverge to form a fan-like sheet in which there is an intermingling of fibers from the corpus callosum (Cunningham); and that sheet enters into the external boundary of the inferior horn of the lateral ventricle and into the floor, lateral wall and roof of the posterior horn, hence the synonym, tapetum. The tapetum is lined by the ventricular ependyma and is separated from the inferior longitudinal bundle by the optic radiation. Its particular function is unknown. It has been recently suggested that the fibers of this bundle are projection fibers connected with the corpus striatum; but this is so at variance with the findings of Kinnier Wilson in apes that it should not be accepted without confirmation.