DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 829 



C. Mandibularis 



The mandibular ramus is composed of general sensory (afferent) fibers 

 with cell bodies lying in the mesencephalic nucleus of the fifth nerve (see 

 figure 356A). Associated with these sensory fibers are motor fibers (generally 

 spoken of as special visceral motor fibers) distributed to the muscles of mas- 

 tication. The latter muscles arise from mesoderm associated with the first or 

 mandibular visceral arch. During development the motor fibers arise from 

 a localized mass of neuroblasts lying in the pons of the metencephalon (see 

 figure 356A), and they emerge from the ventro-lateral aspect of the pons and 

 grow out toward the mandibular arch. Later they become associated with the 

 sensory fibers observed above. 



Summary of functional components: ( 1 ) General somatic afferent (sensory) 

 fibers, of the proprioceptive variety, originating in mesencephalic nucleus of 

 the fifth nerve (fig. 35 6 A, B), (2) special visceral efferent (motor) fibers to 

 muscles of mastication from motor nucleus noted above. 



VI. Abducens 



The word abducens means to lead away, or draw aside. It is applied to the 

 sixth cranial nerve because it innervates the lateral rectus muscle of the eye- 

 ball whose function is to pull the eye away or outward from the median line. 

 It is composed almost entirely of somatic efferent (motor) fibers whose origin 

 is within a nucleus lying in the caudo-ventral area of the pons (fig. 356A). 

 In the embryo, neuroblasts in this area grow outward from the ventro-lateral 

 wall of the pons and forward into the developing premuscle mass of the 

 external (lateral) rectus muscle. 



Summary of functional components: (1) Somatic efferent fibers, (2) gen- 

 eral somatic afferent fibers, i.e. proprioceptive fibers from the external rectus 

 muscle. 



VII. Facial 



In higher vertebrates this nerve is composed largely of motor fibers of the 

 special visceral variety innervating the musculature derived from the hyoid 

 visceral arch. As indicated previously (Chap. 16) the muscle tissue of this 

 arch forms the facial (mimetic) and platysma musculature of mammals and 

 the posterior belly of digastric and stylohyoid muscles. In fishes muscle tissue 

 is restricted to the region of the hyoid arch and is concerned with movements 

 of this arch. The motor fibers distributed to the hyoid arch of fishes are located 

 in the hyomandibular branch of the facial nerve (see figure 3571). Aside 

 from these special visceral motor fibers, sensory fibers are present whose cell 

 bodies lie within the geniculate ganglion of the facial nerve. The sensory fibers 

 which innervate some of the taste buds on the anterior two-thirds of the 

 tongue in mammals are special visceral afferent fibers coursing in the chorda 

 tympani nerve, whereas those along the pathway of the facial nerve are 



