The fourth ventricle is an expansion of the central canal of the medulla oblongata. It is a roughly tent-shaped cavity filled with cerebrospinal fluid (CSF), situated beneath the cerebellum, above the pons and above the rostral half of the medulla. Laterally, the ventricle is bounded by the superior and inferior cerebellar peduncles.
The roof or dorsal surface of the fourth ventricle consists of a sheet of white nonnervous tissue called the inferior medullary velum. A layer of pia mater covers the inner lining or ependyma. Situated at the caudal part of the roof is an opening, the median eminence or foramen of Magendie, which connects the subarachnoid space and the interior of the ventricle. The ventricle communicates with the subarachnoid space also through two lateral openings called the foramen of Luschka . Situated caudally above the ventricular roof is a double layer of pia mater, called the tela choroidea, which lies between the cerebellum and the ventricular roof. The tela choroidea is highly vascularized, and its blood vessels project through the roof of the caudal part of the ventricle to form the choroid plexus. The choroid plexus together with others situated in the lateral and third ventricles produce the CSF.
The floor or rhomboid fossa of the fourth ventricle is formed by the rostral half of the medulla and the dorsal surface of the pons, and is divided longitudinally into symmetrical halves by the median sulcus. The floor is raised because of the nucleus and the fiber tracts that run beneath it in the pons and medulla. Thus, there is a slight swelling in the floor, the facial colliculus, caused by the fibers leaving the motor nucleus of the facial nerve as they arch over the abducens nucleus. Although not shown here, there are other nuclei situated immediately belowthe floor of the fourth ventricle. These include the vagal and hypoglossal nuclei, together with their fiber connections.
The ventricle is filled with fluid, and if it is overfilled, as can occur through abnormal production of CSF, hydrocephalus may result. This is a clinically significant increase in the volume of CSF, due to the obstruction of the foramina of the roof of the fourth ventricle, or to displacement of the medulla by a tumor, or adhesions of tissues through meningitis, or through the presence of a congenital septum. The increase in fluid increases pressure on the nuclei and fiber tracts immediately below the floor of the fourth ventricle, and can result in autonomic and motor disturbances such as cardiac, respiratory, and vasomotor problems.
Tumors may arise in the ependymal lining of the fourth ventricle, or in the pons, or the vermis of the cerebellum and may spread to the fourth ventricle. Alternatively tumors of ependymal origin may invade the cerebellum, causing locomotor disturbances.