= Solution
\b[The <fourth ventricle> is the cavity of the hindbrain between the dorsal surfaces of the <pons> and upper <medulla oblongata> anteriorly and the <cerebellum> posteriorly.] It is continuous above with the <cerebral aqueduct> and below, at the <obex>, with the <central canal of the spinal cord>. Its floor is diamond-shaped when viewed from behind, while its roof rises dorsally towards the <fastigium>.
The floor, the <rhomboid fossa>, consists of an upper pontine triangle and a lower medullary triangle. Its upper apex points towards the <cerebral aqueduct>; its lower apex is the <obex>; the two lateral angles continue into the <lateral recesses of the fourth ventricle>. A median groove divides it into right and left halves. On each side, the <medial eminence> lies medial to the <sulcus limitans>, which separates predominantly motor territories medially from sensory territories laterally. The <striae medullares of the fourth ventricle> cross the floor near the junction of its pontine and medullary portions; their extent varies.
Several surface features identify deeper relations. In the pontine part, the <facial colliculus> overlies the <abducens nucleus> and the internal looping course of fibres of the <facial nerve>; it is not the exposed <facial motor nucleus>. The upper part of the <sulcus limitans> has the <superior fovea>, and near its rostral lateral region lies the <locus coeruleus>. Laterally, the <vestibular area> overlies the <vestibular nuclei>. In the lower triangle, the <hypoglossal trigone> overlies the <hypoglossal nucleus>, and the <vagal trigone>, lateral to it, overlies the <dorsal motor nucleus of the vagus>. The <area postrema> lies still farther caudolaterally, separated from the <vagal trigone> by the <funiculus separans>. These are landmarks and underlying relations, not claims that the <cranial nerve nuclei> lie loose within the cavity.<Neuroanatomy, Fourth Ventricle (StatPearls)>[Introduction; Structure and Function; Surgical Considerations: floor and relations]{ref}
The upper lateral boundaries are formed by the <superior cerebellar peduncles>. The lower lateral boundaries relate to the <inferior cerebellar peduncles> and adjacent dorsal medullary margins. The lateral recess on each side curves around the <inferior cerebellar peduncle> towards its external opening near the <flocculus>.
The roof has superior and inferior slopes. Superiorly the thin <superior medullary velum> spans the <superior cerebellar peduncles>, with the <lingula of the cerebellum> on its dorsal surface. Inferiorly, the roof relates to the <inferior medullary velum>, <nodule of the cerebellum> and thin <tela choroidea>. Vascular folds of the <tela choroidea> carry the <choroid plexus of the fourth ventricle>, with median and lateral extensions. The cavity has an <ependymal> lining; the <choroid plexus> has its specialized epithelial covering and produces <cerebrospinal fluid>.
There are three important exits to the <subarachnoid space>: one <median aperture of the fourth ventricle> through the inferior roof, opening towards the <cisterna magna>, and two <lateral apertures of the fourth ventricle> at the ends of the <lateral recesses of the fourth ventricle>. These must be distinguished from the inlet through the <cerebral aqueduct> and the inferior continuity with the <central canal of the spinal cord>. Most <cerebrospinal fluid> leaving the <fourth ventricle> reaches the <subarachnoid space>, not the narrow adult <central canal of the spinal cord>.
The three-dimensional arrangement explains the method of examination: reflecting the <cerebellum> and opening the delicate roof exposes the <rhomboid fossa>, but destroys part of the structure one is trying to describe. Studying an intact roof, then the exposed floor, and using a median section to locate the <fastigium> avoids interpreting a dissected cavity as naturally open behind. The modern nomenclature is useful here; not every fine landmark or physiological interpretation in this account should be read as an assertion about what an 1867 examiner required.<Morphometric Assesment of the External Anatomy of Fourth Ventricle and Dorsal Brainstem in Fresh Cadavers>[Gross-dissection study: dorsal brainstem, rhomboid fossa and fourth-ventricular approaches]{ref}
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