536 



CnoiiDATA. 



Develo^Jmeiit shows that the optic nerve is a joart of tlie brain. 

 Following custom, however, and including these two, the pairs of 

 cranial nerves may be enumerated in the terms of human anatomy 

 as follows: T, N. olfactorius; II, N. opiticus; III, IS", oculomotorius; 

 IV, N. trochlearis (patheticus); V, N. trigeminus; VI, X. abducens; 

 VII, N. facialis; VIII, N. acusticus; IX, X. glossopharyngeus; 



Fig. 570. — Diagram of cranial nerves (shark), o, alveolaris ; h, buccalis ; c, cere- 

 brum ; c6, cerebelluin; tf, chorda tympani ; e, ear ; ^r, external rectus muscle ; 

 ./", inferior rectus muscle : g, Gasseri'an ganglion ; /*, hyoid cartilage; hm, hyoman- 

 dibular ; /, internal rectus muscle ; io, inferior oblique muscle ; j, Jacobson's 

 commissure ; I, lateralis of vagus : »), mouth ; mc, Meckel's cartilage: md. mandi- 

 bularis ; rtix, maxillaris superior; ", nose ; o, optic lobes; o;i, ophthalmicus profun- 

 dus; OS, ophthalmicus supertlcialis ; p^ pinealis; />/, palatine : po, posttrematic 

 branches; 7>r, pretrematic branches; pn, pneumogastric (intestinal) of vagus; 

 ]'tg, pterygoquadrate; s, spiracle; so, superior oblique muscle; sr, superior rectus 

 muscle; ^ 'twixt brain; /-A', cranial nerves; i-;";, gill clefts. 



X, X. vagus (pneumogastricus), XI, N. accessorius: XII, X. 

 hypoglossus. The accessorius in fishes and amphibia is a part of 

 the vagus; the hypoglossus, strictly speaking, belongs to the spinal 

 nerves and only secondarily is associated with the cranial nerves, 

 whicli explains its course, outside the skull, in cvclostomes and 

 amphibia. 



SiTieo tlio head niidoubtedly consists of several coalesced Ixxly seg- 

 ments (at least as many as there are visceral arches, and apparently 

 more), the qnestion arises whether the ci'ani;d nerves are as evidently seg- 

 mcnt;il as are those of the trunk. Tn this is ;illied the further question 

 whether Bell's L;iw that a niixeil nerve consists of dorsal sensory, and 

 ventral motor components is applic;ible here, llotli problems have been 

 mncli (liscnsseil in recent years, l)ut as yet the fin;il iinswers have not been 

 given. It is proljal)le tliat the present cranial nerves, the optic and olfac- 

 tory excepted, have arisen l)y manifold rearrangements of segmental 

 nerves. On the other hand it seems impossible to accept Bell's Law here 

 without considerable modilicalion, since many cranial nerves (facialis, 

 ti'igcmenus, etc.) contain motor fibres, allhougli they are formed like 

 dorsal roots. 



