THE GRAY MASSES OF THE RHOMBENCEPHALON.
In the floor of the trigonum lemnisci, in the isthmus, lies the nucleus lemnisci.
The pons includes a larger ventral portion, the pars basilaris ponlis, and a smaller dorsal one, the pars dorsalis ponlis. These two divisions are readily seen in a cross section. The basal part exhibits numerous transversely coursing white fibre-strands that continue laterally into the pontile crura or middle cerebellar peduncles. In the lower part of the basilar division, between the thin white fibre-bundles, grayish lamellae repre sent the cross-sections of the tracts of fibres, which descend from the cerebral peduncles, pass through the entire pons and continue to the medulla oblongata and the spinal cord. These are the pyramidal tracts, the fasciculi longitudinales fiyramidales. The fibrae fiontis superficiales are seen as transversely coursing fibres that pass ventral to the pyram idal tracts, while the fibrae ponlis profundae run dorsal to or partly through the pyramidal strands. The pontile nuclei, nuclei pontis, are small masses of gray substance lying scattered between the bundles of fibres.
The pars dorsalis pontis, also termed the tegmentum pontis, appears gray in transverse sections. It contains the following nuclei : The nucleus nervi abducentis, within the colliculus facialis, ' The nucleus nervi facialis, The nucleus motorius et sensibilis nervi trigemini, The nucleus tractus spinalis nervi trigemini, The nuclei nervi acustici, within the area acustica, embracing :— Nucleus medians nervi cochleae, Nucleus dorsalis Nucleus medialis Nucleus lateralis (Deiters) Nucleus superior (Bechterew) nervi vestibuli,Nucleus n. vestibularis spinalis The nucleus olivaris superior, The nucleus corporis trapezoidei, The nuclei reticulares tegmenti.
Within the cerebellum (Fig. 87), in addition to the cortex or substantia corti calis covering the entire surface, special gray masses are found within the corpus medullare. In the medial part of the hemisphere lies the nucleus dentatus, which appears as a much plicated lamella of gray substance with a medially directed opening, the hilus nuclei dentati. Within the worm, the roof-nucleus, nucleus fastigii
or nucleus tecti, lies on each side of the mid-line. Between the nucleus fastigii and the nucleus dentatus, two additional centres are found, the nuclei globosi, small gray masses lateral to the roof-nucleus, and the nucleus emboliformis, medial to the dentate nucleus.
Within the medulla oblongata, in the fasciculus gracilis within the clava, lies the nucleus fasciculi gracilis, while in the fasciculus cuneatus, in its position correspond ing to the tuberculum cuneatum, lies the nucleus fasciculi •cuneati. The tuberculum cinereum corre sponds to the nucleus tractus sfii nails nervi trigemini. • Within the olive are found the nucleus oliva ris inferior, with the nuclei of the two accessory olives, the nucleus olivaris accessorius ventralis and dorsalis. The nuclei arcuati lie ventral to the pyramidal tracts, while within the lateral columns are the nuclei laterales.
The floor of the trigonum hypoglossi contains the nucleus nervi hypoglossi. Close to the latter but within the floor of the ala cinerea, is the sensory nucleus of the vagus nerve, which anteriorly is continuous with the like nucleus of the glossopharyngeal nerve. In this region, medial to the ala cinerea, the motor nucleus dorsalis of the glossopharyn geal and vagus nerves appears as a small group of cells. The nucleus tractus so/hard occupies the elongation of the sensory nucleus of the glossopharyngeal and vagus nerves. Somewhat lateral, but more deeply placed, lies the nucleus ventralis or nucleus ambiguus of the ninth and tenth nerves. The caudal prolongation of the nucleus ambiguus con tains the elongated nucleus nervi accessorii, whose spinal part reaches into the ventral horn of the spinal cord. The nerve-cells within the formatio reticularis, occurring scattered or in small groups, constitute the nucleus of the formatio reticularis.
The more important of these nuclei are discussed at greater length in connection with the Fibre-Tracts. The positions of the nuclei of the cerebral nerves are diagram matically shown in Figs. 88 and 89.