362 



THE PERIPHERAL NERVOUS SYSTEM 



myelencephalon. The caudal fibers constitute the descending spinal tract 

 of the trigeminal nerve, which extends as far caudal as the spinal cord 

 (Fig. 364). The peripheral processes separate into three large divisions, 

 the ophthalmic, maxillary, and mandibular rami, and supply the integu- 

 ment of the head and face, and the epithelium of the mouth and tongue. 

 The motor fibers of the trigeminal nerve arise chiefly from a dorsal 

 motor nucleus that lies opposite the point at which the sensory fibers enter 

 the brain wall (Fig. 364). In the embryo these fibers emerge as a sepa- 



Gang. jugular e N. 10 



Accessory root ganglia 



Gang, superius N. 9 



Gang, petrosum N. 9 



N. glossopharyngeus 



Gang, nodosum, N. 10 



N. laryng. sup. 



Gang. Fronep. 

 N. accessorius 



'ad. dors. 



Inter-gang, bridge 



N. liypoglossuf 1 



.' J TV. vagus 

 FIG. 363- Reconstruction of the cerebral nerves of an embryo of 10.2 mm. (Streeter). X 16.7. 



rate motor root, course along the mesial side of thesemilunar ganglion, and, 

 as a distinct trunk, supply the premuscle masses which later form the 

 muscles of mastication. From the chief motor nucleus, a line of cells 

 extending cranially into the mesencephalon constitutes a second source of 

 origin for motor fibers. In the adult, the motor fibers form a part of the 

 mandibular division of the nerve. 



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

 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 acous- 

 tic 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 



