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The cellular lining of the ventricular system and central canal of the spinal cord.

Reference entry for the fourth ventricle. Its account of fluid passage is supplemented in the solution by distinguishing the principal subarachnoid exits from continuity with the narrow central canal of the spinal cord.

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.^[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.^[ref]