472 CIIORDATA 



The relation? of tliose nerves are the most |irim!live in the Tehlhyopsiiia 

 (ti'_;. 527I. Oi these, tliree (1. 11, and \11I) are pin-ely sensory, ^m'int; to the 

 olfactory or^jan, e\"e anJ ear res|ieeii\ ely. The third, fonrth and sixth are as 

 purelv motor, supplviuL; the e\"e muscles. 'I'lie remainder are mi.xed in char- 

 acter. The !:;lossophar\nL;eal ner\e is tvpicah This lea\es llie medulla and, 

 at the top of the lirsi u'ill cleft, divides into two branches, one of which (prr- 

 Irc'Hiilic) ^oes in from of the opcniuL;; the other i/>e.f/-/n'»;i;/;V) jiassing I ehind 

 it, both inner\"altni; llie muscles of the region and su|i]MyinL; a sense oru:an above 

 the gill openiiiL;. 'I'lie \'a^us, in the same wav, has pre- and post-trematic 

 blanches for the ]visierior uill clefts, a eondilion which leads to the \iew 

 that this is a compound ner\e. In addition the Nau'us sends a branch lo the 

 ftomach and heart. Tu the .\mniotes, with the development of hnpLjs, this 

 branch also sup|ilies the lunns, hence the name piieumoj^astric. The \aL;us 

 also Lcives off a ner\"e which supplies the sense organs of the lateral line of 

 the trunk. 



The facial nerve dixddes over the spiracle in the same way as the ninth o^■er 

 the first gill cleft. It also supjilies the lateral line organs of the head. The liflh 

 nerve also di\ides at the angle of the mouth, a branch going into each of the 

 jaws, a third going towards the tip of the head, ^^'ith the loss iif gills and 

 lateral line organs in the terrestrial vertebrates the sex'cnth and tenth ner\"es lose 

 the corresponding branches. The facial ner\'e, which in the Ichthyopsida is 

 largely sensory, becomes almost exclusively motor in the mammals, as a result 

 of the great development of the imtscles of the face, w hich are practically lacking 

 in tire lower forms. 



Since the head undoubtedlv consists of several coalesced body segments (at 

 least as many as there are visceral arches, and apparently morei, the (|uestion 

 arises wdiether the cranial ner\'es are as e\identlv segmental as are those ot the 

 trunk. To this is allied the further question whether Bill's La-d' that a mixed 

 nerve consists of dorsal sensory and ventral motor components is a]iplicable 

 here. In answering this question the olfactory and optic ner\es, arising from 

 the prosenceiihalon, must be excepted. The latter, especially, is not a peripheral 

 nerve, but is a tract connecting two parts of tlie brain, since the retina, as- we 

 shall see, is Init an evagination of a part of the brain towards the peri])herv. 

 With this cerebral character is connected the peculiarity of the crossing or 

 chiasma of the optic ner\e. In the teleosts the wdrole right nerve goes to the left 

 eye, the whole of the left to the right eye, the nerves cither siin]ily crossing or 

 passing through networks in each other. In most vertebrates only a part of 

 the fibres cross outside of the brain. If all of the fibres cross inside the brain 

 (as is the rule for true cranial nerves') the result is the appearance as in thccvclos- 

 tomes, that each eye is su]iplied by the ner\'e from its own side. 



Of the remaining nerves \yhich spring from the hind lirain and, ^vith the 

 exception of the trochlearis, from its side and base, the ocv\loniotor and abducens 

 (possibly also the trochleari.s) develop like ventral roots. The eye muscles 

 which they supply are the remains of the somatic musculature arising from the 

 myotomes, which has largely degenerated. All of the other cranial nerves (the 

 hypoglossus excepted) arise like dorsal roots, are sensory, and are ]irovided with 

 the equivalents of spinal ganglia {i;i-iii<-iilatc gaiit;!ion of the facial, Cassrriiui 

 gaiis^lion of the trigeminal, vagus and glos.sopharyngeal ganglia), but they also 

 contain numerous motor fibres. It is inii>ortant to note that the muscles in- 

 nervated by these motor fibres do not arise from the myotomes and hence do not 

 belong to the somatic musculature, but are a jiart of the 'visceral muscles,' 

 Now, since the dorsal roots in Amf>liioxiis contain motor fibres which sup]ily 

 visceral muscles, and visceral motor nerves occur in the dorsal roots of the 

 vertebrates, it is necessary to modify Bell's Law and to say that the ventral roots 



