3. The stratum griseum centrale of the mid-brain (Figs. 58, 62 and 64) surrounds the cerebral aqueduct (Sylvii). This gray matter begins in the lateral wall of the third ventricle. It extends through the mid-brain and is continuous with the gray substance in the floor of the fourth ventricle. Besides the nuclei of the third, fourth and a part of the fifth cerebral nerves and the nucleus tegmenti dorsalis, it contains scattered cell-bodies of variable size and shape which give origin to the true commissural fibers of the posterior commissure. The central gray substance of the mid-brain, together with its nucleus tegmenti dorsalis, receives the peduncle of the mammillary body and a part of the mammillo-tegmental fasciculus, and it originates the dorsal longitudinal bundle of Schutz, which descends to the genetic nuclei of the pons and medulla.
Oculomotor Nucleus (Figs. 59 and 6i).—The nucleus of the third cerebral nerve (nucleus nervi oculomotorii) is an elongated mass of gray substance in the ventral part of the stratum gri seum centrale, which extends from the lateral wall of the third ventricle down to the level of the transverse groove between the quadrigeminal colliculi. The nuclei are placed somewhat obliquely; at the lower end they fuse in the median plane. As was stated in the section on the mid-brain (p. 150) the oculomotor nucleus is composed of a visceral and a somatic part; the former innervates smooth muscles within the eye and the latter sup plies striated muscles outside the eyeball.
The greater number of axones of this nucleus run forward into the nerve of the same side; but those from the median nest go into both nerves, and a small bundle from each nucleus descends with the medial longitudinal bundle to the colliculus facialis, where it joins the facial nerve and through that nerve supplies the muscles of facial expression above the orbit. There is un certainty concerning the origin of this latter bundle; it may rise in the superior part of the facial nucleus.
Trochlear NUcleus (Fig. 62).—The nucleus nervi trochlearis is a small oval mass of cell-bodies situated at the level of and anterior to the inferior colliculus of the corpora quadrigemina. It is in the ventral part of the stratum.griseum centrale like the oculomotor nucleus. It is a purely somatic nucleus, supplying one striated muscle. Unlike the third, the axones from the nucleus of the fourth cerebral nerve run backward and issue from the posterior surface of the brain-stem at the isthmus; they are peculiar also in that the axones decussate before their emergence (Fig. 56).
The nuclei of the oculomotor and trochlear nerves receive fibers from the cerebral cortex through the pyramidal tract and other motor tracts of the internal capsule and thus obtain their voluntary motor and inhibitory impulses. It is probable also that the third nucleus receives fibers, through the medial longi tudinal bundle, from the opposite abducent nucleus, and that the part of the nucleus which receives these fibers supplies the medial rectus muscle of the eye. For the purpose of reflex
both the oculomotor and trochlear nuclei receive fibers from the anterior tecto-spinal and medial longitudinal bundles, from the mammillary peduncle and the mammillo-tegmental fasc iculus and, perhaps, from the cerebellum through the brachia conjunctiva.
Trigeminal nucleus of the mid-brain is a very small nucleus situated in the extreme lateral part of the central gray matter. It is continuous with the pontine nucleus of the fifth, located under the locus czeruleus, and is merely the superior end of the motor nucleus of the trigeminal. It gives origin to the mesen cephalic root of the fifth nerve, which descends to the pons and there joins the main motor root. In.its course downward the mesencephalic root runs between the cental gray matter and the brachium conjunctivum cerebelli. It contains a few ascend ing fibers from the sensory root (reflex).
Otto May and Sir Victor Horsley, in Brain, Vol. 33, support this view of the mesencephaliC nucleus of the trigeminal nerve, but the work of J. B. Johnston should be examined. By an abundance of comparative evidence Johnston revives the view formerly held, that the mesencephalic root joins the sensory root of the trigeminal nerve, not the motor. Johnston's views are as follows: The mesencephalic nucleus lies in the dorsal zone (the afferent zone) of the neural tube; it is a part of the neural crest included in the dorsal lamina of the tube and is analogous to a ganglion; its cells are similar to those of a spinal ganglion in man and to the giant cells in the spinal cord of fishes, being pear-shaped and fusiform bipolars; the fibers contributed to the mesencephalic root are the coarse dendritic processes, while the slender processes, the axones, terminate within the tectum; the mesencephalic root joins the main sensory root of the tri geminal nerve and, probably, is sensory in function (Jour. Comp. Neurol. and Psychol., Vol. 19).
Lesions of these cerebral nerve nuclei are apt to involve the tracts of the tectum and tegmentum. If so, the result is paraly sis of the nerves on the same side and hemianmsthesia, hemi ataxia, loss of taste (?) and deafness on the opposite side.
The white matter of the cerebrum is composed, in the adult condition, of medullated fibers; the medullation begins in the fourth month, in utero, and is continued for a considerable time after birth (Flechsig). Within the cortical substance the myelin sheaths continue to be laid down until late in life (Kaes, McMurrich). The cerebral fibers form three definite systems: i. Projection, or peduncular fibers.
2. Transverse, or commissural fibers.
3. Association fibers.