LONGITUDINAL FIBERS OF PONS The longitudinal fibers of the pons are arranged in two dis tinct groups, viz., the basilar or anterior and the dorsal or posterior (Figs. 113-118).
The basilar longitudinal fibers are situated in the pars basilaris pontis (Figs. 113-118). Four fasciculi make them up. They are the four efferent tracts of the basis pedunculi. The fronto-pontal, temporo-pontal and intermediate tracts termi nate chiefly in the nucleus pontis. The pyramidal fibers run from the middle three-fifths of each basis pedunculi down through the basilar transverse layer of the pons and the pyra mids of the medulla oblongata. Together with the above cerebro-pontal tracts they form a thick bundle on either side of the median line, which presses down the superficial trans verse fibers and produces the sulcus basilaris. The nucleus pontis, one on either side, is situated among the pyramidal fibers. The pyramidal tracts diminish in size during their descent because of the fibers which leave them to decussate and end in the nuclei of motor cerebral nerves.
The Dorsal Longitudinal are contained in the pars dorsalis pontis in the formatio reticularis (Figs. 113– 118). They are in and dorsal to the corpus trapezoideum and lie in the floor of the fourth ventricle where they are intermingled with the reticular gray substance. They do not form a compact layer but are collected into a number of dis tinct strands of which the larger are visible to the naked eye in Weigert-Pal sections. The dorsal longitudinal fibers are mingled with many transverse and oblique fibers, and thus there is produced the netlike arrangement suggesting the name, formatio reticularis. The formatio reticularis of the pons is continued down from the tegmentum of the mid-brain and comprises the tegmental region of the pons. The gray matter in the meshes of this network which is continued up from the medulla contains the nuclei of the fifth, sixth and seventh cerebral nerves and a part of the nucleus of the eighth nerve, and also the nuclei of the formatio reticularis, viz., the nucleus centralis superior, medius and inferior, and the nucleus lateralis medius. In the formatio reticularis are the bundles or tracts of fibers that constitute the dorsal longitudinal fibers of the pons. These tracts are as follows: medial fillet, lateral fillet, ventral spino-cerebellar fasciculus, spino-tectal fasciculus, spino thalamic fasciculus, medial longitudinal fasciculus, gustatory fasciculus, spinal tract of the trigeminal nerve; mesencephalic root of the trigeminal nerve, tecto-spinal fasciculi-anterior and lateral, reticulo-spinal fasciculi-anterior and lateral, thalamo-olivary fasciculus, thalamo -spinal fasciculus, rubrof spinal fasciculus and the dorsal longitudinal fasciculus o Schiitz in the gray substance. The brachium conjunctivum
cerebelli is partly buried in the dorsal region of the pons, so its position and relations should be noted here (for descrip tion see p. 287). The ventral spino-cerebellar fasciculus and the spinal tract of the trigeminal (or fifth) nerve are the only tracts not already considered in our study of the tegmental region of the mid-brain, p. 152.
I. The medial fillet (lemniscus medialis, Figs. 113-118) is a large bundle of fibers that runs through the pons next the median plane. In the lower part of the pons it lies within and dorsal to the trapezoid body. Its origin is found on the opposite side in the nucleus funiculi gracilis and nucleus fu niculi cuneati and in the terminal nuclei of common sensory cerebral nerves (Fig. 125). It conducts impulses of the tactile, static and muscular senses. In the mid-brain it gives off the superior fillet (lemniscus superior) which terminates in the superior quadrigeminal colliculus. The medial fillet ends in the lateral nucleus of the thalamus (Fig. 54). Interrup tion of the medial lemniscus causes ataxia on the opposite side.
2. Lateral Fillet (lemniscus lateralis).—The lateral fillet forms a link in the special sense, auditory path (Fig. 119). As stated on `p. 297 it is but the longitudinal continuation of the corpus trapezoideum and the medullary striae. It takes form near the middle of the pons, where the fibers of the trapezoid body bend upward to a longitudinal direction; and it runs just lateral to the medial fillet (Figs. 113 and 115). Very soon it be comes separated from the medial fillet by the brachium con junctivum of the cerebellum. It runs dorso-medially over the conjoined brachium to the inferiot colliculus of the corpora quadrigemina, where a few of its fibers end; but the greater number are continued through the brachium inferius to the medial geniculate body. The chief origin of the lateral fillet is found in the opposite cochlear nuclei, though some of its fibers rise in the nucleus of the corpus trapezoideum, the superior olivary nucleus, and the nucleus of the lateral fillet, which constitute partial relays in the auditory path. It is also true that a few fibers enter the lateral fillet from the cochlear nuclei and nerve of the same side; they are supposed to decussate near or in the quadrigeminal bodies and terminate in the opposite inferior colliculus. Destruction of the lateral fillet causes deafness in the opposite ear.