THE CEREBRAL NERVES 361 



cephalic flexure (Figs. 359 and 364). The fascicles converge, form the 

 trunk of the nerve, and' end in the premuscle masses of the eye. The 

 nerve eventually supplies all of the extrinsic muscles of the eye save the 

 superior oblique and external rectus. A branch also passes to the ciliary 

 ganglion. In the chick embryo, bipolar cells migrate along the fibers of 

 the oculomotor nerve to take part in the development of the ganglion. 

 The ciliary ganglion of human embryos is derived entirely from the semi- 

 lunar ganglion of the trigeminal nerve. 



4. The Trochlear Nerve fibers take their origin from neuroblasts of the 

 basal plate, located just caudal to the nucleus of origin of the oculomotor 

 nerve (Fig. 364). They are directed dorsally, curve around the cerebral 

 aqueduct, and, crossing in its roof, emerge at the isthmus (Fig. 339 A). 

 From their superficial origin, each is directed ventrally as a slender nerve 

 which connects with the anlage of the superior oblique muscle of the eye 

 (Fig. 359). 



6. The N. Abducens takes origin from a nucleus of cells in the basal 

 plate of the myelencephalon, located directly beneath the fourth neuro- 

 mere of the floor of the fourth ventricle (Figs. 359 and 364). The con- 

 verging fibers emerge ventrally at a point caudal to the future pons, and, as 

 a single trunk, course, cranially, mesial to the semilunar ganglion, finally 

 ending in the anlage of the external rectus muscle of the eye. Vestigial 

 rootlets of the abducens and hypoglossal nerve tend to fill in the gap be- 

 tween these two nerves, according to Bremer and Elze. 



HI. THE VISCERAL MIXED NERVES 



The nerves of this group, the trigeminal, facial, glossopharyngeal, and 

 vagus complex (vagus plus the spinal accessory), are mixed in function. 

 The trigeminal nerve, beside its visceral nerve components, contains also 

 numerous somatic sensory neurons which supply the integument of the 

 head and face. 



The facial, glossopharyngeal, and vagus nerves are essentially visceral 

 in function. Their sensory fibers, chiefly of the visceral type, supply the 

 sense organs of the branchial arches and viscera. A few somatic sensory 

 fibers, having the same origin and course in the myelencephalon, supply 

 the adjacent integument. 



5. The Trigeminal Nerve is largely sensory. Its semilunar ganglion, a 

 derivative of the ganglion crest, is the largest of the whole nervous system, 

 but very early is distinct from the other cerebral ganglia (Fig. 358). 

 It arises laterally at the extreme cranial end of the hind-brain. Central 

 processes from its cells form the large sensory root of the nerve that enters 

 the wall of the hind-brain at the level of the pontine flexure (Fig. 359). 

 These fibers fork and course cranially and caudally in the alar plate of the 



