538 THE BRAIN 



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 inferior section of the rhomboidal fossa, in a definite 

 depression, the inferior fovea. To the lateral side of the 

 superior and inferior foveae and the intermediate part of the 

 sulcus limitans is the area acustica^ which is prolonged laterally 

 into the lateral recess where, in rare cases, a projection, the 

 tuberculum acusticum, appears on its surface. Below the 

 inferior fovea, between the trigonum hypoglossi medially and 

 the area acustica laterally, lies a depressed, grey -coloured, 

 triangular area called the a/a rinerea, which is separated from 

 the lower part of the floor, the area postrema, by a raised 

 bundle, the funiculus separans. Immediately above the in- 

 ferior fovea a number of ridges, the medullary stria (O.T. stria 

 acustica), cross the floor of the fossa. Laterally they cross 

 the restiform body, at the lateral border of the fossa, and 

 become continuous with the cochlear root of the acustic 

 nerve ; and medially they disappear into the median sulcus. 



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 

 latter, from above, is the frenulum veli ; and 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 

 epithelial ependyma and the obex. 



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- 

 lium at the lower end of the roof, is a thin layer of grey 

 matter, called the obex. Above, at the posterior medullary 

 velum, the tela becomes continuous with the pia mater on 

 the lower surface of the vermis of the cerebellum. Laterally 

 the tela is prolonged, on each side, posterior to the restiform 

 body, over the lateral recess, and it forms the stronger part 

 of the wall of that expansion. Between the medial part of 

 the tela chorioidea of the fourth ventricle and the pia mater 

 on the lower surface of the vermis of the cerebellum lies the 

 cisterna cerebello-medullaris (O.T. cisterna magna). 



