THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 147 



tens very closely agrees with Hyrtl's of L. paradoxa in the points with 

 which we are now concerned. He finds, contrary to Owen, that the 

 four recti muscles " may clearly be distinguished," though there are 

 no ohliqui. " Special nerves to these muscles (the third, fourth, and 

 sixth) were not found ; " but the ophthalmic division of the fifth is 

 described as giving off in the orbit " ciliary and oculo-motor nerves," 

 which, however, do not appear to have been traced to their distri- 

 bution. 



Gegenbaur 1 simply quotes Hyrtl to the effect that all three eye- 

 muscle nerves are represented by branches of the fifth ; which, how- 

 ever, is a wider and more positive statement than Hyrtl really made. 

 Stannius 2 also states, on Hyrtl's authority, that the eye-muscle 

 nerves have no independent roots ; and Huxley 3 notes that in Lepido- 

 siren " the three motor nerves of the eyeball are completely fused 

 with the ophthalmic division of the fifth," a condition which he is dis- 

 posed to view as the most primitive arrangement met with among 

 vertebrates. 



In considering what importance is to be attached to this often- 

 quoted exception to the general rule, we have first to notice that we 

 are dealing with animals in which the eyes are " very small " and 

 " feebly developed ; " secondly, that the eye-muscles are so small that 

 their very existence was not only overlooked, but expressly denied, by so 

 competent an anatomist as Professor Owen; thirdly that the two anato- 

 mists, Hyrtl and Humphry, who have described these muscles, agree 

 in saying that the recti muscles are alone present, a condition clearly 

 not fully realised by those who state, on Hyrtl's authority, that the 

 fourth nerve is, like the third and sixth, represented by a branch of 

 the fifth ; fourthly, that in neither of the cases mentioned were the 

 nerves actually traced into the muscles in question. 



To these points we must add one, urged with great force by 

 Schwalbe, 4 and which acquires much weight from the cases of Fetro- 

 myzon and Lepidosteus already considered, viz. that a sufficiently care- 

 ful examination of the brain has not been made to render us certain 

 as to the alleged absence of independent roots of origin for such of 

 the eye-muscle nerves as may be present. 



The importance of Schwalbe's warning is strikingly exemplified by 



1 Gegenbaur, Eexanchus, p. 549. 



a Stannius, Das peripherische Nervensysiem, p. 18. 



8 Huxley, Anatomy of Vertebrated Animals, p. 73, note. 



1 Schwalbe, Das Ganglion Oculomotorii, p. 72. 



