THE CEREBRAL NERVES 359 



in the development of the ganglion. The ciliary ganglion of human embryos is 

 derived entirely from the semilunar ganglion of the trigeminal nerve. 



4. The Trochlear Nerve fibers take 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. 321 A). From their superficial origin they 

 are directed ventrally as a slender nerve which connects with the anlage of the 

 superior oblique muscle of the eye (Fig. 341). 



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

 the myelencephalon located directly beneath the fourth metamere of the floor of 

 the fourth ventricle (Figs. 341 and 345). 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. 



III. 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 ganglion crest, but 

 very early is distinct from the other cerebral ganglia (Fig. 340). 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 which enters the wall of the hind- 

 brain at the level of the pontine flexure (Fig. 341). 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 epithelium of the 

 mouth and tongue. The central fibers fork and course cranially and caudally 

 in the alar plate of the myelencephalon. The caudal fibers constitute the des- 

 cending spinal tract of the trigeminal nerve, which extends as far caudalward as 

 the spinal cord (Fig. 345). 



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

 nucleus which lies opposite the point at which the sensory fibers enter the brain 

 wall (Fig. 345). In the embryo these fibers emerge as a separate motor root, 

 course along the mesial side of the semilunar ganglion, and as a distinct trunk 



