164 



Basic Structure of Vertebrates 



IV. Trochlear, mainly somatic motor, its fibers coming from a 

 motor center located ventrally in the region of junction of the mesen- 

 cephalon and the metencephalon, and supplying the dorsal oblique 

 muscle. It may include some proprioceptive fibers. The trochlear 

 nerves are the only nerves which emerge from the dorsal surface of the 

 brain (Figs. 136, 138A). At the region of emergence, between the optic 

 lobes and the cerebellum, the right and left nerves cross, forming a 

 trochlear chiasma. 



V. Trigeminal, so called because in all vertebrates it has three 

 main divisions (Fig. 147 'A) :a sensory ophthalmic extending above the 

 eye to the skin on the upper side of the snout or the forehead ; a sensory 

 maxillary to the skin on the side of the snout and the region of the 

 upper jaw and to the upper teeth; and a mixed mandibular supplying 

 skin, teeth, and muscles of the lower jaw, and the tongue. Most of the 

 sensory fibers of the nerve arise from cells in the large semilunar 



(Gasserian) ganglion situated on the 

 base of the nerve just external to its place 

 of emergence from the medulla. But a 

 unique feature of the trigeminal nerve is a 

 sensory ganglionic center located inside the 

 brain — in the roof of the mesencephalon. 

 The trigeminal motor center is in the 

 lateral column of the medulla. 



VI. Abducent, a somatic motor 

 nerve (but containing some proprioceptive 

 fibers) emerging from the medulla ventrally 

 and behind the trigeminal, and supplying 

 the posterior rectus muscle of the eye- 

 ball (Fig. 148). 



VII. Facial, a mixed nerve which 

 may have two or three ganglions. Its 

 motor component comes from a center in 



the medulla posterior to that of the abducent nerve. Of all the cranial 

 nerves, its distribution is most complex (Fig. 1475). 



In fishes and wholly aquatic amphibians, an important part of the 

 facial nerve is concerned with the lateral-line organs, a system of 

 sense-organs peculiar to these groups. A single lateral-line organ, a 

 neuromast, is a cluster of elongated epidermal cells, at whose bases 

 are the terminal fibrils of afferent nerve-fibers belonging to cells situ- 

 ated in a ganglion of the facial nerve (Fig. 149). Usually these neuro- 

 masts occur on the walls of canals which lie deep in the skin but are 

 lined by epidermis. At intervals, the canal opens to the exterior by small 

 pores. The lumen of the canal is filled by mucus. The longest of these 



Fig. 149. Sense-organ of 

 lateral line of the amphibian, 

 Diemyctylus (aquatic form). 

 (c) Cone-cells; (s) spindle- 

 cells. (After Kingsbury. 

 Courtesy, Kingsley: "Com- 

 parative Anatomy of Verte- 

 brates," Philadelphia, The 

 Blakiston Company.) 



