THE CEREBRAL NERVES 



363 



7. The Facial Nerve is largely composed of efferent motor fibers which supply 

 the facial muscles of expression. In 10 mm. embryos these fibers arise from a 

 cluster of neuroblasts in the basal plate of the myelencephalon located beneath 

 the third rhombic groove or neuromere (Fig. 364). The fibers from these cells 

 course laterally, and emerge just mesial to the acoustic ganglion. The motor trunk 

 then courses caudally and is lost in the tissue of the hyoid visceral arch, tissue 

 which later gives rise to the muscles of expression (Fig. 359). The sensory fibers 

 of the facial nerve arise from the cells of the geniculate ganglion, which are in turn 

 derived from the ganghon crest (Streeter). This ganghon is present in 7 mm. 



Ntict luntor n X (amblguui) 



Nucl, n iiypnglossi 



N. accessorlua -, 



Fig. 364. — Reconstruction of the nuclei of origin and termination of the cerebral nerves in an embryo 

 of 10 mm. The somatic motor nuclei are colored red (Streeter). X 30. 



embryos (Fig. 358), located cranial to the acoustic ganglion. The centrally 

 directed processes of the geniculate ganglion enter the alar plate and form part 

 of the sohtary tract. The peripheral fibers in part course with motor fibers in 

 the chorda tympani, join the mandibular branch of the trigeminal nerve, and end 

 in the sense organs of the tongue. Other sensory fibers form later the great 

 superficial petrosal nerve, which extends to the spheno-palatine ganglion. 



The motor fibers of the facialis at first course straight laterad passing cra- 

 nial to the nucleus of the abducens. The nuclei of the two nerves later gradually 

 shift their positions, that of the facial nerve moving caudad and laterad, while the 

 nucleus of the abducens shifts cephalad. As a result, the motor root of the 



