536 



CHORD AT A. 



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 human anatomy 

 as follows: I, N. olfactorius; II, N. opticus; III, N. oculomotoiius; 

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

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



FIG. 570. Diagram of cranial nerves (shark), a, alveolaris ; ft, buccalis ; c, cere- 

 brum ; cb, cerebellum; ct, chorda tympani ; e, ear ; er, external rectus muscle ; 

 /, inferior rectus muscle ; g, Gasserian ganglion ; h, hyoid cartilage; hm, hyoman- 

 dibular ; i, internal rectus muscle ; 10, inferior oblique muscle ; j, Jacobson's 

 commissure ; I, lateralis of vagus ; m, mouth ; me, Meckel's cartilage: md, mandi- 

 bularis ; mar, maxillaris superior; n, nose ; o, optic lobes; op, ophthalmicus profun- 

 dus; os, ophthalmicus superficialis; p, pinealis; pi, palatine ; po, posttrematic 

 branches; j?\ pretrematic branches; pn, pneumogastric (intestinal) of vagus; 

 ptg, pterygoquadrate; s, spiracle; so, superior oblique muscle; sr, superior rectus 

 muscle; f, 'twixt brain; I-X, cranial nerves: 1-5, gill clefts. 



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

 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, 

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

 amphibia. 



Since the head undoubtedly consists of several coalesced body seg- 

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

 more), the question arises whether the cranial nerves are as evidently seg- 

 mental as are those of the trunk. To this is allied the further question 

 whether Bell's Law that a mixed nerve consists of dorsal sensory, and 

 ventral motor components is applicable here. Both problems have been 

 much discussed in recent years, but as yet the final answers have not been 

 given. It is probable that the present cranial nerves, the optic and olfac- 

 tory excepted, have arisen by manifold rearrangements of segmental 

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

 without considerable modification, since many cranial nerves (facialis, 

 trigemenus, etc.) contain motor fibres, although they are formed like 

 dorsal roots. 



