xm 



PHYLUM CHORDATA 



101 



The fourth or trochlear nerve (Figs. 788, IV, and 796, IV.) arises from 

 the dorsal surface of the brain at the junction of the mid-brain 

 with the medulla oblongata. It is a very small and purely motor 

 nerve, supplying only the superior oblique muscle of the eye. 



The fifth or trigeminal nerve (Fig. 788, 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 ganglion (g. gn.), which maybe incompletely 

 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 



.<t.r 



sym 



br., 



FIG. 788. 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. h. hyomandibular 

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

 1 5, branchial branches ; X. c, cardiac branch ; X. g, gastric branch ; X. I, lateral branch; 

 XI, accessory ; XII, hypoglossal. au. auditory organ ; br. 1 7, branchial clefts ; cblm. cere- 

 bellum ; c. gn. ciliary ganglion ; c. h. cerebral hemispheres ; A. dorsal branch of spinal nerve : 

 d. r. dorsal root ; e. eye ; gn. d. r. ganglion of dorsal root ; in. b. mid-brain ; med. 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 3, ventral branches of spinal nerves ; sym. sympathetic 

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



divides into two branches, a superficial (7. o. s.) and a deep (V. o. p.), 

 the former present in Fishes only : the latter in some Fishes a 

 semi-independent nerve given off separately from the dorsal part of 

 the trigeminal ganglion. The ophthalmic is purely sensory, and 

 supplies the skin in the neighbourhood of the mouth and certain 

 parts in the orbit. The maxillary nerve (V. m.x.) is also sensory : 

 it supplies the parts in relation with the upper jaw, including the 

 teeth. The mandibular nerve (V. md.} is partly sensory, partly 

 motor : it supplies the muscles of the jaws, the skin and teeth of 

 the lower jaw, and sends off a gustatory nerve or nerve of taste to 

 the epithelium of the tongue in the higher forms. The ophthalmic 

 nerve is connected by a branch with the ciliary ganglion. 



The sixth or abducent (Figs. 788, VI, and 796, VI.) is a small motor 



