104 



ZOOLOGY 



the diencephalon, just in front of the infundibulum. It differs 

 from all the other nerves in being originally a hollow out-pushing 

 of the brain, containing a prolongation of the diacoele (see Fig. 786). 

 It supplies the retina or actual organ of sight, and is therefore 

 a purely sensory nerve. 



The third or oculomotor nerve (III.) arises from the crus cerebri 

 or ventral region of the mid-brain. In its course is a ganglion, the 

 oculomotor or ciliary ganglion (c. gn^. It supplies four out of the 

 six muscles of the eye-ball (see below, Fig. 787), viz, the superior, 

 inferior, and internal recti, and the inferior oblique (Fig. 787, III.), 

 as well as the ciliary muscles and muscles of the iris in the 

 interior of the eye. It is therefore a purely motor nerve. 



Hhe fourth qt trochlear nerve (Figs. 779 and 787, IV.) arises from 

 the dorsal surface of the posterior extremity of the mid-brain 



br.r 



Fig. 779. — Diagram of the cerebral and anterior spinal nerves of a Craniate. I, olfactory 

 nerve ; II, optic ; III, oculomotor ; IV, trochlear ; V. trigeminal ; V. o. s. superficial ophthal- 

 mic branch; V. o. p. deep ophthalmic; VI, abducent; VII, facial; VII. 7(, hyomandibular 

 branch ; VII. p^ palatine branch ; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; X. br. 

 1 — 5, branchial branches ; X. e, cardiac branch ; X. .f/, gastric branch ; X. J, lateral branch ; 

 XI, accessory ; Xlf, hypoglossal, ait. auditory organ ; br. 1 — 7, branchial clefts ; cblm, cere- 

 bellum ; c. </rt. ciliary ganglion ; c. h. cerebriil hemispheres ; d. dorsal branch of spinal nerve ; 

 d. r. dorsal root ; e. eye ; rin, d. r. ganglion of dorsal root ; m. b. mid-brain ; nied. obi. medulla 

 oblongata ; mth. mouth ; na. olfactory sac ; o. I. olfactory bulb ; pn. b. pineal body ; pn. e. 

 pineal eye ; sp. c. spinal cord ; sp. 1 — S, ventral branches of spinal nerves ; sym. sympathetic 

 nerve ; sym. gn. sympathetic ganglion ; v. r. ventral root. 



It is a very small and purely motor nerve, supplying only the 

 superior oblique muscle of the eye. 



The fifth or trigeminal nerve (Fig, 779, V.) is of great size and 

 wide distribution. It arises from the side of the medulla, fre- 

 quently by two roots, a dorsal and a ventral, thus resembling in 

 its origin a spinal nerve. Near its origin it enters a ganglion, 

 the trigeminal or Gasserian ganfflion (g. gn), which may be partly 

 divided into two parts, an antero-dorsal and a postero-ventral. 

 The trunk of the nerve early divides into two principal branches, 

 the ophthalmic and the mandibular (V. md.): the latter sends off 

 a maxillary nerve (V. mx.), and we thus get the three divisions to 

 which the name trigeminal is due. The ophthalmic nerve frequently 

 divides into two branches, a superficial (V. o. s.) and a deep (V. o.p), 



