IV. VERTEBRATA 



471 



but connected with it by a nerve, it lies in a special cavity in tlie parietal bones, 

 which occurs not only in recent but in fossil forms. Above this eye the skin 

 may be transparent. 



Most of the nerves which come from the brain {cranial nerves) arise 

 between the mid brain and the spinal cord, especially from the medulla 

 oblongata. The olfactory and optic nerves are an exception, the one 

 arising from the cerebrum, the other from the 'twixt brain, but ijoth, and 

 especially the optic, differ so much from the peripheral nerves that they 

 can hardly be classed with them. Development shows that the optic 

 nerve is a part of the brain. Following custom, however, and including 

 these two, the pairs of cranial nerves may be enumerated in the terms of 



Fig. 527. — Diagram of cranial nerves (shark), a, alveolaris; 6, buccalis; c, cere- 

 brum; cb, cerebellum; ri, chorda tympani; e, ear; er, external rectus muscle;/, inferior 

 rectus muscle; g, Gasserian ganglion; h, hyoid cartilage; hm, hyomandibular; ;', internal 

 rectus muscle; jo, inferior oblique muscle; j, Jacobson's commissure; /, lateralis of vagus; 

 ■m, mouth; mc, Meckel's cartilage; md, mandibularis; mx^ maxiUaris superior; ?7, nose; 

 o, optic lobes; op, ophthalmicus profundus; os, ophthalmicus superficialis; /», pinealis; 

 pi, palatine; po, posttrematic branches; pr, pretrematic branches; pn, pneumogastric 

 (intestinal) of vagus; ptg, pter\'goquadrate; s, spiracle; so, superior oblique muscle; sr, 

 superior rectus muscle; t, 'twixt brain; I-X, cranial nerves; 1-5, gill clefts. 



human anatomy as follows: I, A^. olfactorius; II, N. opticus; III, N. 

 oculomotorius; IV, N. trochlearis (patheticus); V, A^. trigeminus; VI, 

 N. abducens; VII, N. facialis; VIII, N. acusticus; IX, TV", glossopharyn- 

 geus; X, A^. vagus (jmetinwgastricus) ; XI, N. accessorius; XII, N. hy- 

 poglossus. To these must be added the recently discovered A^. termmalis, 

 first found in fishes and recently in man. It arises from the cerebrum and 

 supplies the wall of the nasal organ. The accessorius in fishes and am- 

 phibia is a part of the vagus; the hypoglossus, strictly speaking, belongs 

 to the spinal nerves and only secondarily is associated with the cranial 

 nerves, which explains its course, outside the skull, in cyclostomes and 

 amphibia. 



