492 THE BRAIN 



portion of the fossa a rounded eminence, the colliculus facialis, 

 appears on its surface ; below the colliculus the eminentia 

 medialis narrows rapidly, and its terminal, tapering portion 

 is called the trigonum hypoglossi. The medial eminence is 

 bounded laterally by a sulcus, the sulcus limitans. In the 

 upper region, along the lateral border of the sulcus limitans, 

 is a narrow bluish-tinted area called the locus cceruleus ; 

 the colour of that area is due to a subjacent collection of 

 pigmented cells which constitute the substantia ferruginea. 

 Opposite the colliculus facialis the sulcus limitans expands 

 into a shallow fossa, the superior fovea. The lower end of 

 the sulcus limitans terminates, in the upper part of the in- 

 ferior section of the rhomboidal fossa, in a definite depression, 

 the inferior fovea. To the lateral side of the superior and 

 inferior foveae and the middle part of the sulcus limitans is 

 the area acustica, which is prolonged laterally towards the lateral 

 recess, and, in rare cases, a prominence, the tuberculum acusti- 

 cum, appears on its surface. Below the inferior fovea, be- 

 tween the trigonum hypoglossi medially and the area acustica 

 laterally, lies a depressed, grey-coloured, triangular area called 

 the ala cinerea, which is separated from the lower part of the 

 floor, the area postrema, by a raised bundle, the funiculus 

 separans. Immediately above the inferior fovea a number of 

 ridges, the medullary striae, (O.T. strice. acusticcz], cross the floor 

 of the fossa transversely. Laterally, they cross the restiform 

 body, at the lateral border of the fossa, and become con- 

 tinuous with the cochlear root of the acoustic nerve ; and, 

 medially, they disappear into the median sulcus (Fig. 195). 



The roof of the fourth ventricle is formed, in the upper area, 

 by the medial parts of the brachia conjunctiva and the inter- 

 vening anterior medullary velum. Descending upon the 

 velum, from above, is the frenulum veli ; issuing from it, in 

 the same region, are the rootlets of the trochlear nerves. 

 The lower part of the upper portion of the roof is covered 

 by the lingula of the cerebellum. The roof of the inter- 

 mediate section of the ventricle is the white matter of the 

 vermis of the cerebellum, and the roof of the lower part is 

 reduced to the- lining epithelial ependyma with the posterior 

 medullary velum, and the obex (see p. 493). 



The tela chorioidea of the fourth ventricle is the layer of 

 pia mater which covers and strengthens the epithelial roof of 

 the lower part of the cavity. Between it and the epithe- 



