THE CEREBRAL NERVES 361 



muscles of the eye save the superior oblique and external rectus. A branch is also 

 supphed 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 semilunar 

 ganghon 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. 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 neuromere of the floor of 

 the fourth ventricle (Figs. 359 and 364) . The converging 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 between these two nerves, according to Bremer and Elze. 



m. 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 trigem- 

 inal nerve, beside its visceral nerve components, contains also numerous somatic 

 sensory neurones which supply the integument of the head and face. 



5. The Trigeminal Nerve is largely sensory. Its semilunar ganglion is the 

 largest of the whole nervous system and is a derivative of the ganghon crest, but 

 very early is distinct from the other cerebral ganglia (Fig. 358). It arises later- 

 ally at the extreme cranial end of the hind-brain. Central processes from 

 its cells form the large sensory root of the nerve which 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 myelencephalon. The caudal 

 fibers constitute the descending spinal tract of the trigeminal nerve, which extends 

 as far caudad as the spinal cord (Fig. 364). The peripheral processes separate 

 into three large divisions, the ophthalmic, maxillary, and mandibular rami, and 

 supply the integument of the head and face and the epithehum of the mouth and 

 tongue. 



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



