THE FOURTH VENTRICLE. 317 



The superior triangle of the floor presents the coiliculus 

 facialis, superior fovea, locus caeruleus and a part of the acustic 

 area. 



The coiliculus facialis (Fig. 86), the superior extremity 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. 96). External to 

 it and in front of the striae medullares is a small fossa, the fovea 

 superior. 



Fovea Superior (Fig. 86). The fovea superior is near the lateral 

 wall of the ventricle and marks the location of the facial nucleus 

 (seventh) and 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 

 caeruleus. 



The locus ccsruleus (Fig. 86) continues to the superior angle 

 of the ventricle. The blue color is due to the substantia ferrugi- 

 nea, a pigmented layer of cell-bodies underlying the ependyma. 

 The principle motor nucleus of the trigeminal nerve (fifth) lies 

 beneath the superior part of the locus caeruleus (Fig. 96). 



Inferior Triangle of the Ventricular Floor. It presents: 

 The trigonum hypoglossi, fovea inferior, ala cinerea and eminentia 

 cinerea, and most of the area acustica (Fig. 86). 



The hypoglossal triangle (Fig. 86) 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 (Fig. 96). External to the trigonum hypo- 

 glossi and inferior to the striae medullares is the inferior fovea, 

 which forms the apex of the ala cinerea. 



Ala Cinerea (Trigonum Vagi, Fig. 86). 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, eminenlia cinerea, which is directed toward the clava. The 



