The Tegmenta (Figs. 58, 63 and 64).—The posterior divisions of the pedunculi cerebri, which cover the other divisions, are in consequence called the tegmenta (tegmentum—a cover). They are united by a median raphe and fit ventrally into the con cavity of the substantia nigra. They are bounded by the lateral sulcus of the mid-brain on the free side, where each tegmentum presents the trigonum lemnisci, bounded by the sulcus lateralis in front; by the brachium inferius above; and inferiorly by the lateral fillet. Dorsally, the tegmenta fuse with the quadrigeminal lamina. Each tegmentum at the superior end blends with the thalamus, and helps to form the tegmental hypothalamic structures. Imbedded in that superior portion is the red nucleus (n. ruber) of the tegmentum (see tegmental hypothalamic region). Inferiorly, the teg menta are continued into the reticular formation of the pons.
The Cerebral Aqueduct (Aqueductus Cerebri, Sylvii, Figs. 17, 33 and 59).—The aqueduct is a very slender canal connecting the third and fourth ventricles. So it is the "iter a tertia ad quartum ventriculum." It is situated in the median line near the quadrigeminal lamina. It is 13 mm. long. In shape it is V-like, above; elliptical in the middle, with a vertical major axis; and T-form, below, where it joins the fourth ventricle. Its height varies between 1.7 mm. and 3 mm. (0.07 and 0.15 in.). Like other ventricles it is lined with ependyma. A layer of gray matter, thickest on the sides and floor, surrounds the aqueduct of the cerebrum. This is the stratum griseum centrale, which gives rise to some of the fibers of the posterior commissure. The stratum griseum centrale is continuous with the gray matter of the fourth ventricle. It is composed largely of gelatinous gray substance and possesses few medullated fibers and cell-bodies. The fibers constitute the system of the central gray substance called the fasciculus longitudinalis dorsalis (Schtitzi), which is located in the floor of the cerebral aqueduct close to the ependyma. The dorsal longitudinal bundle of Schutz rises in the stratum griseum centrale and its nucleus tegmenti dorsalis of the reticular formation; it descends through the ventricular gray substance of the pons and the medulla into the spinal cord; and it is said to terminate in the motor nuclei of cranial nerves and in other nuclei. Its function is in doubt, but it probably belongs to an olfactory reflex mechanism.
In the ventral part of the stratum griseum centrale are cell-groups forming the nuclei of the oculomotor and trochlear nerves; while in the extreme lateral part of the stratum, the mesencephalic nucleus of the trigeminal nerve is located.
Nuclei of the Oculomotor and Trochlear Nerves (Figs. 59 and 6o) .—Both nuclei together extend the entire length of the aqueduct, and the oculomotor is prolonged into the wall of the third ventricle where it receives a bundle of fibers from the opposite optic radiation and optic tract. The nucleus of the third nerve (n. oculomotorius), is a nucleus of origin, a genetic nucleus. It is composed of two distinct parts, a su perior visceral part and an inferior somatic part. The visceral part lies in the lateral wall of the third ventricle, directly above the mammillary body. It innervates smooth muscle inside the eye. Lateral and dorsal to it lies the nucleus of the posterior commissure (nucleus of the medial longitudinal bundle, nucleus of Darkschewitsch). The somatic part of the oculomotor nucleus is situated ventral to the superior quadrigeminal colliculus. It includes a long lateral group of cell-bodies belonging to the nerve of the same side and a median group, which contributes to both nerves. It supplies striated muscles. The two oculomotor nuclei are associated across the median plane by decussating dendrites. From them the root fibers run forward through the red nucleus and substan tia nigra and issue from the oculomotor groove. The nucleus of the fourth nerve (n. trochlearis) is a single oval mass of cell bodies situated ventral to the inferior colliculus of the corpora quadrigemina. This is also a genetic nucleus. The root fibers of the fourth nerve, trochlear, proceed dorsally and caudalward from the nucleus. They decussate with the fibers from the opposite nucleus in the superior medullary velum, from which they emerge on either side of the frenulum. They then continue in the opposite nerve around the side and over the anterior surface of the mid-brain. This is the only nerve that decussates en masse between the genetic nucleus and the point of exit from the brain.
The nucleus of the mesencephalic root of the trigeminal nerve is composed of large cell-bodies scattered in the extreme lateral part of the stratum griseum centrale, from the highest level of the mid-brain down to the pons. There is no break be tween this nucleus and the chief motor nucleus of the fifth nerve under the locus cxruleus. The axones of these large cell bodies run downward through the nucleus, accumlating gradu ally until they form a distinct crescentic strand, which joins the chief motor root of the same side. This is the usual de scription and it is supported by Otto May and Sir Victor Horsley; but the studies of J. B. Johnston cast some doubt upon its correctness.