THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 141 



notably from the accounts of all other writers whom I have been able 

 to consult. 



Concerning the above accounts, it appears that they can be reduced 

 to two sources — (1) the description given in 1838 by Schlemm and 

 d' Alton, which I have assumed to be the source from which Owen, 

 Huxley, and Gimther obtain the accounts given in their text-books 

 quoted above, because their descriptions, which are very brief, agree 

 exactly with that of Schlemm and d' Alton, and add nothing to it; and 

 (2) Gegenbaur's description in 1871, which must be independent, in- 

 asmuch as it does not quite agree with Schlemm and d' Alton's. 

 According to Gegenbaur, the only peculiarity is that the sixth nerve is 

 not independent but a branch of the fifth, which supplies the rectus 

 inferior as well as the rectus externus ; while, according to Schlemm 

 and d' Alton, three of the muscles — the rectus inferior^ rechis externus, 

 and obliqiius inferior — are supplied by the fifth nerve ; and, in addition 

 to this, the third and fourth nerves unite together, a point which 

 Gegenbaur does not notice. 



The dissection is a difficult one, on account of the small size of the 

 nerves concerned ; and additional evidence from direct observation is 

 necessary before we can decide wdiether either of the above descrip- 

 tions is perfectly correct. 



There are, however, certain points of considerable importance which 

 concern not only Petromyzon, but many other forms as well, and may 

 be conveniently dealt with here. 



Both the third and fourth nerves are distinctly stated to have in- 

 dependent roots of origin, and to arise from the normal situations in 

 the brain ; and this being the case, I wish to point out that the ana- 

 tomical arrangement of the nerves would probably be more correctly 

 described by saying that the third nerve, though having a separate 

 root of origin, becomes connected with the fifth, so that in the adult 

 some of its branches appear to be derived from the fifth ; than by 

 saying, with Huxley, Stieda, and Gunther, that the muscles in question 

 are supplied by branches of the fifth. 



Fischer long ago adopted this view. He describes, on Schlemm and 

 d' Alton's aiithority, the condition of the nerves in Petromyzon in these 

 words : — " Genus Petromyzon duas Oculomotorii ostendit partes, 

 alteram liberam, parisqne quarti quoque continentem fibras, alteram 

 cum Trigemino conjunctam ; " * and Stannius gives still clearer ex- 

 1 Fischer, op, cit., p. 47, note 1. 



