The colliculus facialis (Fig. 112) in the superior part of the eminentia medialis, is located next the median groove. It is produced largely by the genu of the facial nerve. Beneath it is the nucleus of the abducent (sixth) nerve (Fig. 133). Lateral to it and above the strim medullares is a small fossa, the fovea superior.
Fovea Superior (Fig. 2).—The fovea superior is near the lateral wall of the ventricle and marks the location of the facial nucleus (seventh) and ventro-medial to that the salivary nucleus of the intermediate nerve which are deeply seated in the pons. Running upward along the wall of the ventricle from the superior fovea is the sulcus limitans. It is a blue floored groove in the pons called locus cxruleus.
The locus cceruleus (Fig. 112) continues to the superior angle of the ventricle. The blue color is due to the substantia fer ruginea, a pigmented layer of cell-bodies underlying the ependyma. The principal motor nucleus of the trigeminal nerve (fifth) lies beneath the superior part of the locus ceruleus but is not formed by the substantia ferruginea (Fig. 133).
Inferior Triangle of the Ventricular Floor.—It presents The trigonum hypoglossi, fovea inferior, ala cinerea and emi nentia cinerea, and most of the area acustica (Fig. 112).
The hypoglossal triangle (Fig. 112) is produced by the inferior half of the eminentia medialis. Its apex is in the inferior angle of the ventricle and forms one nib of the calamus scriptorius; its base looks upward and is situated under the medullary striae. The twelfth nerve rises from the column of cells whose upper one-half is covered by it and it also covers the nucleus intercalatus (Streeter) (Fig. 133). Lateral to the trigonum hypoglossi and inferior to the stria medullares is the inferior fovea which forms the apex of the ala cinerea.
Ala Cinerea (trigonum vagi, Fig. n2).—The vagus triangle is of a darker color than the ventricular floor around it, hence the name ala cinerea. The inferior fovea forms the depressed and superiorly directed apex of the ala; its floor rises inferiorly to the base, eminentia cinerea, which is directed toward the clava. The nucleus ale cinerem, with nucleus cardiacus in its middle part, and the nucleus tractus solitarii, two nearly parallel columns of cell-bodies, 15 mm. in length, are in part covered by the ala cinerea (Fig. 133). These nuclei are located in the gelatinous gray substance near the ventricle. Deep in the formatio reticularis lies the nucleus ambiguus, a large column of cells 2 cm. long; a portion of its upper half is covered by the ala cinerea. The nucleus salivarius is also located in the reticular formation; it lies in the upper medulla ventral to sulcus limitans.
Area Postrema.—Below the ala cinerea and between it and the tamia ventriculi quarti there is a small fusiform strip of the ventricular floor which Retzius has called the area postrema. An oblique stria separates it from the base of the ala cinerea.
The area acustica occupies the lateral angle of the ventricular floor (Fig. 112). It is partly in the superior triangle, but chiefly in the inferior. It is an irregular triangle: its base is on the sulcus limitans, its apex lies in the lateral recess of the ventricle and its sides are formed by the twnia and the restiform body. The acustic triangle is crossed by the medullary strim. A slight eminence, the tuberculum acusticum, makes the lateral angle of the acustic area most prominent. Beneath the acustic area are the vestibular nuclei of the auditory nerve; also the lateral part of the cochlear nucleus, which is found in the acustic tubercle (Fig. 133).