7. The gustatory fasciculus (Figs. 115-118) runs upward through the pons close to the fourth ventricle. Imbedded in the gelatinous gray substance in the medulla, it ascends just ventral to it in the pons between the gray substance and the thalamo-olivary fasciculus. It rises in the nucleus of the soli tary tract and terminates in the lateral nucleus of the thalamus, as stated on p. 161.
8. The roots of the trigeminal nerve are both present in the upper half of the pons; below the middle only the spinal tract is found. The spinal tract of the trigeminal nerve runs along the border of the reticular formation just lateral to its own nucleus and ventro-medial to brachium conjunctivum and restiform body. It is composed of T-branched axones which enter the pons as the sensory root of the trigeminal nerve; its origin is within the semilunar ganglion. The as cending rami terminate in the mesencephalic nucleus of the fifth nerve, while the descending rami end in the nucleus of the spinal tract of the trigeminal nerve all the way down to the second cervical segment of the cord.
The mesencephalic root of the trigeminal nerve has been described on p. 239. Where both roots are present, the mesencephalic root and its nucleus are medial to the spinal tract. The mesencephalic root bends forward at the middle of the pons and enters the trigeminal nerve.
9. The tecto-spinal tracts are the anterior and the lateral, described on pp. 155, 156 and 233.
Anterior Tecto-spinal Fasciculus.—This ocular-reflex bundle is 'continued from the mid-brain down through the pons in nearly the same relative position. Diverging a little from the medial longitudinal bundle as it descends through the pons, it is located in the formatio reticularis a short distance ventro lateral from it. The anterior tecto-spinal bundle can be recognized in normal adult tissue only in the dorsal tegmental decussation (Meynerti) of the mid-brain; lower down it can be distinguished from the surrounding tissues by degeneration and medullation but in no other way (Fig. 113). Having given fibers to the oculomotor and trochlear nuclei above, it sends fibers to the nucleus of the abducent nerve at this level, and perhaps to other pontine nuclei (see Mid-brain, p. 155).
The lateral tecto-spinal fasciculus descends from the tectum of the mid-brain on the side of its origin; it is a direct tract. It proceeds through the pons in the lateral part of the reticular formation, in company with the thalamo-spinal and rubro spinal tracts. It terminates in motor nuclei (Figs. 115-118).
io. The reticulo-spinal fasciculi, described in the mid-brain (p. 154-5) are largely reinforced in the pons by the middle lateral and the three central nuclei of the reticular formation, all of which are located in the pons. They are association tracts between the reticular nuclei of the brain-stem and the gray matter of the cord (Figs. 115-118).
The anterior reticulo-spinal fasciculus, the uncrossed tract, is a part of the medial longitudinal bundle. Its course is along the median plane just in front of the ventricular gray substance.
The lateral reticulo-spinal fasciculus is made up of crossed axones from the reticular nuclei which, upon entering the tract, divide into short ascending and long descending rami. By degeneration it can be traced through the lateral part of the reticular formation of the pons; it is not a compact tract.
Rubro-spinal Fasciculus of Monakow.—In the we have traced this tract from the red nucleus through the ventral tegmental decussation (Foreli) to the opposite side where it mingles with the lateral fillet down near the isthmus (Figs. 59 and 6o). It occupies the same position in the upper half of the pons; it is close to the posterior surface of the corpus trapezoideum in the lower part of the pons (Figs. 113-118). In the gray crescent of the spinal cord the rubro-spinal tract ends. Its function is coordination of the movments of locomo tion (Horsley).
A number of axones from the red nucleus, both crossed and uncrossed, terminate in the reticular formation of the pons and medulla, the fasciculus rubro-reticularis; others are said to end in the nucleus of the lateral fillet, the fasciculus rubro-laquearis.
12. The thalamo-olivary fasciculus (or olivary fasciculus) is regarded by many as an ascending tract, but the weight of evidence at present is in favor of a descending course. Rising in the thalamus (?) and descending through the mid-brain it enters the center of the reticular formation of the pons, hence the name central tegmental tract (Fig. 115). It runs dorsal to the medial fillet and the corpus trapezoideum in the lower part of the pons, just medial to the superior olivary nucleus. It terminates in the inferior olive. Through the pons and upper medulla, it is visible as a large and loose fasciculus.
13. The Fasciculus bundle descends with the rubro-spinal tract through the lateral part of the reticular formation in the pons and medulla to the lateral column of the spinal cord (J. S. Collier). It terminates in the gray substance of the cord, probably giving off collaterals to corresponding nuclei in the brain-stem (Figs. 115-118).
14. The dorsal longitudinal bundle of Schutz ( fasciculus longitudinalis dorsalis, Figs. 115 and xi 7) descends through the ventricular gray substance of the pons, just beneath the medial eminence of the ventricular floor. Rising in the central gray matter and nucleus tegmenti dorsalis of the mid-brain it is said to establish connections with all cranial nerve nuclei (Villiger). It constitutes important links in the olfactory reflex mechanisms.