Cerebellum and the Fourth Ventricle

The lateral boundaries are formed on each side from above down by the superior cerebellar peduncle, the inferior cerebellar peduncle, and the cuneate and gracile tubercles.

Roof of Fourth Ventricle. The upper and lower parts of the V-shaped roof are formed by the superior and inferior medullary vela, which are thin laminae of white matter between the superior and inferior cerebellar peduncles. The lower part of the inferior velum has a median aperture (foramen of Magendie); cerebrospinal fluid escapes through this opening and the lateral aperture (foramina of Luschka) into the subarachnoid space. Because these are the only communications between the ventricular and subarachnoid spaces, their blockage can produce hydrocephalus.

The lower part of the roof and the posterior walls of the lateral recesses are invaginated by vascular tufts of pia mater, which form the T-shaped choroid plexus of the fourth ventricle.

The floor of the fourth ventricle is rhomboid shaped and is divided into symmetric halves by a vertical median sulcus. Its upper (pontine) and lower (medullary) parts are demarcated by delicate transverse strands of fibers, the striae medullares of the fourth ventricle.

On each side of the median sulcus is a longitudinal elevation, the medial eminence, lateral to which runs the sulcus limitans. Its superior part is the locus ceruleus, colored bluish-gray from a patch of deeply pigmented nerve cells. Also lateral to the upper part of the medial eminence is a slight depression, the superior fovea, and just below and medial to this fovea is a rounded swelling, the facial colliculus, which overlies the nucleus of the abducens (VI) nerve and the facial (VII) nerve fibers encircling it; the motor nucleus of the facial nerve lies more deeply in the pons. Inferolateral to the superior fovea is the upper part of the vestibular area, which overlies parts of the nuclei of the vestibulocochlear (VIII) nerve.

The lower (medullary) part of the medial eminence overlies the twelfth cranial nerve nucleus and is termed the hypoglossal trigone. Lateral to it is a slight depression, the inferior fovea, which, together with the neighboring vagal trigone, overlies parts of the dorsal nuclei of the glossopharyngeal and vagus nerves. Lateral to the inferior fovea is the lower part of the vestibular area, overlying parts of the vestibular nuclei of the vestibulocochlear nerve. On a deeper plane, parts of the trigeminal, solitary tract, and ambiguus nuclei also underlie the floor of the fourth ventricle. Some of the nuclei mentioned, such as the dorsal vagal and ambiguus nuclei, as well as others located in the nearby reticular formation, are concerned with cardiovascular, respiratory, metabolic, and other important functions, and are regarded as vital centers. Any lesion in this relatively small area of the brain may produce disastrous results.

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