Fossa Rhomboidea.—The floor of the fourth ventricle, the fossa rhomboidea, is, as indicated by its name, rhomboidal in outline. Its posterior part, bordered by the corpora restiformia, belongs to the myelencephalon; its middle part lies in the metencephalon; and its anterior part belongs to the isthmus. By means of a longitudinal furrow, sulcus medianus fossae rhomboideae, it is divided into symmetrical halves. Transversely coursing white bands, the striae medullares or striae acusticae, which run from the lateral recesses toward the mid-line, separate the pars superior from the pars inferior fossae rhomboideae. The part of the fossa included between the medullary striae constitutes the pars intermedia.
The striae medullares present many variations in their course and develop ment. They may be wanting or many, but are seldom identical in development and course on the two sides. Often they run obliquely outward and upward from the sulcus medianus.
The pars inferior of the ventricle deepens in its lower portion, presents several fields defined by furrows and, on account of its peculiar shape, is called the calamus scrifitorius. At the lower border of the pars inferior lies the obex, a thin white medullary sheet from which the taeniae ventriculi quarti pass laterally. Immediately in front of the obex, where the sulcus medianus sinks into the central canal of the spinal cord, is a small depression, the ventriculus Arantii. In the pars superior, the median sulcus widens into the fossa mediana. On each side of the median furrow, a flat ridge, the eminentia medialis, extends the entire length of the ventricular floor. This ridge is narrow in its lower part and forms a triangular field, the trigonum nervi hypoglossi, whose base is above at the striae medul lares and apex below, directed toward the ventriculus Arantii.
On careful inspection, two special divisions of this field are recognized, an outer broader part, the area plumiformis (Retzius), and an inner narrower one, the area medialis •trigoni nervi hypoglossi (Retzius). At the border between those two fields
are to be seen mostly short, obliquely coursing delicate furrows and folds, and likewise a thin feathery band. Such markings are often visible also at the lateral border of the trigonum hypoglossi. Retzius, therefore, named this lateral and broader field, " area plumiformis." In the upper part of the ventricular floor, the eminentia medialis is broader and pro jects more into the ventricle. The elevation is termed the colliculus facialis. Laterally, the eminentia medialis is defined by the sulcus limilans, which in the pars superior widens into the fovea superior, and in the pars inferior into the fovea inferior. Below the fovea inferior, and lateral to the trigonum hypoglossi, is seen a gray oblique triangular field, known as the ala cinerea, which begins pointed at the fovea inferior and broadens toward the lower border of the fossa rhomboidea.
In front of the posterior border of the fossa and behind the ala cinerea, lies a small gray mammillated field, the area postrema, that extends from the mid-line along the lower border of the ventricle forward and outward. A light narrow band, known as the funiculus separans, runs from the opening central canal outward and forward, between the area postrema and the ala cinerea.
The fovea superior is accompanied laterally by a bluish colored area, the locus caeru leus. The latter and the superior fovea exhibit small furrows and folds, rugae loci caerulei et foveae superioris, which may often be followed, for a considerable distance, for ward toward the isthmus and backward toward the recessus lateralis. To the outer side of the sulcus limitans, lateral to the fovea superior, 'the fovea inferior and the ala cinerea, the area acustica is seen as a flat elevation, which toward the recessus lateralis presents the tuberculum acusticum. The funiculus separans, above noted, courses toward the lower inner end of the area acustica and there disappears.