Cross Section at the Level of the Genu of the Facial Nerve
Beneath the floor of the rhomboid fossa lies the magnocellular nucleus of the abducensnerve (A1) and, ventrolaterally to it, the nucleus of the facial nerve (A2). Thevisceroefferent superior salivatory nucleus (A3) is seen between the abducens and fa-cial nuclei. The lateral field is occupied by the sensory terminal nuclei of the vestibularnerve and the trigeminal nerve, namely, the medial vestibular nucleus (Schwalbe’s nu-cleus) (A4), the lateral vestibular nucleus (Deiters’ nucleus) (A5), and the spinal nu-cleus of the trigeminal nerve (A6).
The fibers of the facial nerve bend around the abducens nucleus (A1) and form the fa-cial colliculus (A7). We distinguish an as-cending limb (A8) and, cranial to the il-lustrated section, a descending limb. The apex is the internal genu of the facial nerve (A9). The fibers of the abducens nerve (A10) descend through the medial field of the teg-mentum. The medial longitudinal fasciculus (AB11) is seen medially and the posteriorlongitudinal nucleus (Schütz’s bundle)(AB12) dorsally to the abducens nucleus. Deep in the pontine tegmentum run the central tegmental tract (AB13) and the spinothalamic tract (A14). Secondary fibersof the auditory pathway collect from the anterior cochlear nucleus as a wide fiber bundle, the trapezoid body (AB15); they cross ventral to the medial lemniscus (A16) to the opposite side where they ascend in the lateral lemniscus (B17). They synapse in part in the adjacent nuclei of the trapezoid body, namely, the anterior nucleus of thetrapezoid body (A18) and the posterior nu-cleus of the trapezoid body (superior olive)(AB19). The spinal tract of the trigeminalnerve (A20) lies in the lateral field.
The pontine bulb is formed by the transversepontine fibers (A21). They are corticopon-tine fibers, which synapse in the pontine nu-clei (A22), and pontocerebellar fibers, whichare postsynaptic and extend to the cerebel-lum in the medial cerebellar peduncle (brachium pontis) (A23). Embedded in the middle of the longitudinally cut fiber bundles is the transversely cut fiber bundle of the pyramidal tract (AB24).