The dorsal surface of the brain stem, and particularly that of the medulla and pons, is obscured by the cerebellum. When this is removed, the bilateral swellings caused by the ascending cuneate and gracile fasciculi can be seen, as well as the corresponding tubercles, which are the swellings caused by their nuclei. Dorsal to the olives are the inferior cerebellar peduncles, which climb to the lateral aspect of the fourth ventricle and then swing into the cerebellum between the middle and superior cerebellar peduncles. The inferior cerebellar peduncle receives fibers in the stria medullaris, a tract from the hypothalamic arcuate nucleus. The stria medullaris fibers pass dorsally through the midline of the medulla and cross the floor of the fourth ventricle.
The floor of the fourth ventricle (also called the rhomboid fossa) is in part the dorsal surface of the pons; the dorsal surface of the pons (also called the tegmentum of the pons) forms the rostral half of the floor of the ventricle, and is divided longitudinally by a medial sulcus into two symmetrical halves. The ventricle is broadin the middle and narrows caudally to the obex, the most caudal end of the fourth ventricle, and rostrally towards the aqueduct of the midbrain. Caudally, the ventricle narrows into two triangles or trigones. Beneath the medial area of the ventricle are several motor nuclei; the rostral ends of both the vagal and hypoglossal nuclei lie beneath these trigones. There is a swelling at the lower end of the medial eminence, the facial colliculus, which is formed by fibers from the motor nucleus of the facial nerve. The roof of the fourth ventricle is tent-shaped and extends upwards towards the cerebellum. The roof is formed rostrally by the superior cerebellar peduncles and by a sheathcalled the superior medullary velum. The rest of the roof consists of another sheath, the inferior medullary velum, which is often found adhering to the underside of the cerebellum. The sheath may be incomplete, creating a gap called the median aperture of the fourth ventricle or the foramen of Magendie, which constitutes the main communication between the ventricular system and the subarachnoid space. The lateral walls of the fourth ventriclesare provided mainly by the inferior cerebellar peduncles. There are recesses in the lateral walls, which extend around the medulla, and these open ventrally as the foramina of Luschka, through which cerebrospinal fluid can enter the subarachnoid space.
The dorsal surface of the midbrain is defined by four rounded swellings: the superior and inferior colliculi (the corpora quadrigemina). The colliculi make up the roof or tectum, and define the length of the dorsal surface, round 1.5 cm. The inferior colliculus is mainly a relay nucleus in the transmission of auditory impulses en route to the thalamus and cerebral cortex. The superior colliculus mediates control of voluntary eye movements and the head in response to visual and other forms of stimuli. The lateral surface of the midbrain is formed principally by the cerebral peduncle. Parts of the epithalamus (see, the habenular nuclei and the stria medullaris are seen ostral to the midbrain. The third ventricle of the diencephalon and the pineal body are also shown.