THE GRAY MASSES OF THE MID—BRAIN.
Surrounding the aquaeductus cerebri is the central gray substance, stratum gris eum centrale. At the bottom of this stratum, at the level of the superior colliculi, lies the oculomotor nucleus, which joins the upward prolongation of the small nucleus nervi trochlearis (Figs. 88 and 89). Lateral, at the edge of the central gray substance, lies the small nucleus radicis decendentis nervi trigemini. The nucleus of the posterior commissure and posterior longitudinal bundle is located in advance of that of the oculo motor nerve. Ventral and lateral to the central gray substance, the formatio reticularis spreads out. Between the basis pedunculi and the tegmentum lies the substantia nigra, which extends upwards as far as the hypothalamus, while between the substantia nigra and the central gray substance is located the red nucleus, the nucleus rtiber or nucleus tegmenti, which appears round in cross-sections (Fig. 207).
As small nuclei of the tegmentum, the ganglion dorsale tegmenti and the ganglion profundum mesencephali laterale et mediale are to be noted. The ganglion dorsale is a small round nucleus lying behind the trochlear nucleus, while the ganglion profundum is lodged within the formatio reticularis, ventro-lateral to the nuclei of the oculomotor and trochlear nerves.
The anterior quadrigeminal body is covered by the stratum zonale and contains the stratum griseum colliculi superioris; the posterior body encloses the centrally placed nucleus colliculi inferioris.
Within the posterior part of the substantia perforata posterior, towards the front border of the pons, scattered nerve-cells constitute the ganglion interpedunculare of Gudden.