242 COMPARATIVE ANATOMY 



to the remaining gill-clefts. Thus these nerves form a series 

 correlated with the branchial apparatus. The maxillary and 

 mandibular divisions of the fifth nerve may be regarded as 

 pretrematic and post-trematic branches in relation- to the 

 mouth, and there is embryological evidence, which cannot be 

 detailed here, for regarding the mouth as a modified pair of 

 gill-clefts, the lower jaw being the modified branchial arch 

 supporting the hinder wall of the cleft, and the upper jaw an 

 outgrowth of that arch. The fifth nerve, then, belongs to 

 the same series as the seventh, ninth and tenth, and is 

 essentially a segmental nerve related to an important segmental 

 organ — viz. a gill-cleft. The first, second, and eighth nerves, 

 whatever their primitive relations may have been, are now 

 nerves of special sense related to the three pairs of sense 

 capsules. But what of the third, fourth, and sixth nerves 

 supplying the muscles of the eyeball ? There is embryological 

 evidence to show that they also belong to a series of segmental 

 nerves, and the muscles which they supply are the relics of three 

 pairs of primitive head segments or myotomes. The first 

 embryonic myotome gives rise to the anterior, superior, and 

 inferior rectus, and to the inferior oblique muscle ; the second 

 to the superior oblique and the third to the posterior rectus. 

 The third, fourth, and sixth nerves, then, are the segmental 

 nerves of the three anterior myotomes of the primitive head, 

 but they do not represent the whole of the segmental nerves. 

 There is evidence to show that they are the ventral or motor 

 roots, and the ophthalmic and main divisions of the fifth and 

 the seventh nerves are the corresponding dorsal roots proper 

 to those segments. Similarly the ninth and tenth nerves do 

 not appear to represent whole segmental nerves but only their 

 dorsal roots, the vagus belonging to several segments. It is 

 not surprising that, in a structure so complex and so far 

 changed from its primitive condition as the vertebrate head, 

 there should still be much uncertainty and conflict of opinion 

 as to the amount and nature of the changes undergone, but 

 the statements given above may be said to be so fairly estab- 

 lished that they may legitimately find a place even in an 

 elementary treatise. It is beyond doubt that the vertebrate 

 head was segmented, and the head of the embryo dogfish is 

 still segmented. Thus we are able to institute a comparison 

 between the segmented anterior end of the headless Amphi- 



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