l86 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



rectus and the inferior oblique muscles. The trochlear nerve 

 leaves the dorsal surface of the hinder margin of the mid-brain and 

 goes to the superior oblique muscle. The abducens nerve comes 

 from the inferior surface of the medulla and supplies the lateral rectus 

 muscle. In most vertebrates these nerves are readily traced, but not 

 infrequently the abducens is united near its origin with the fifth 

 nerve, while in Petromyzon it has not been found, and it is said that 

 all of the eye-muscle nerves are absent from the adult Bdellosloma. 



The peculiarities and inequalities of the distribution of the three eye- 

 muscle nerves receive their explanation in the development of the eye muscles. 

 These arise from three myotomes (p. 133 and fig. 145), one somite forming three 

 of the rectus and the inferior oblique muscles, the other two somites forming one 

 muscle each. With the division of the myotome into four parts, the oculo- 

 motor nerve continued its distribution to each of the derivatives. 



The trochlear nerve is peculiar among all motor nerves in that it leaves 

 the dorsal surface of the central nervous system. R is farther peculiar in having 

 a chiasma in the dorsal surface of the mid-brain, from which its fibres extend 

 downward to the ganglion cells in the floor of the mid-brain. The eye-muscle 

 nerves are the only somatic motor nerves in the head of the lower vertebrates. 



V. Nervus Trigeminalis. — The trigeminal is one of the largest 

 cranial nerves. It arises (fig. 193, V) from the antero-lateral angle 

 of the myelencephalon (dorsal zone) and bears a large semilunar or 

 Gasserian ganglion near its origin. This may he either within or 

 outside the skull. In the higher vertebrates the nerve divides just 

 beyond the ganglion into three main trunks, the ophthalmic, maxil- 

 lary, and mandibular nerves, whence its name. In the lower verte- 

 brates the maxillary and mandibular are united for some distance. 



In the fishes there are two branches of the ophthalmic nerve, 

 an ophthalmicus superficialis of the fifth and an ophthalmicus pro- 

 fundus. Both are purely sensory. In the higher vertebrates 

 (tetrapoda) only the profundus persists. The chief distribution of 

 the superficialis is the skin of the top of the head and the tip of the 

 snout. The profundus goes between the eye muscles and the eye, 

 sends branches to the eyelids and conjunctiva, and extends to the 

 mucous membrane of the nose. In its course it is connected with the 

 ciliary ganglion, of problematic relationships, which receives also 

 sympathetic fibres and sends nerves to the iris and the ciliary muscle 

 of the eye. 



The superior maxillary nerve runs along the margin of the upper 

 jaw, supplying the face, the lower side of the orbit and the teeth. 



