599 



former fusion occurs before the fibres emerge above the dorsal border 

 of the lateral rootlet of the ophthalmic. 



The Buccal Nerve. — This large nerve (2) passes upwards 

 and backwards and splits into its two rootlets: 1) a slightly larger, 

 external and somewhat posterior (= the lateral rootlet — 2"); and 

 2) an internal and somewhat anterior (= the dorsal rootlet — 2'). 

 The latter lies immediately anterior to the dorsal rootlet of the ex- 

 ternal mandibular. The former bends backwards, and its fibres, inter- 

 lacing with the fibres of the lateral ophthalmic rootlet, form with it 

 the greater bulk of the lateral lateral line root. The dorsal rootlet 

 fuses with the dorsal rootlet of the external mandibular and passes over 

 into the dorsal lateral line root as above described. Its anterior 

 fibres are diffusely spread out so as to assist in obliterating the tri- 

 angular space which would otherwise be formed by the crossing of 

 the three lateral line nerves. 



The Ophthalmic Nerve. — This also large nerve (1) courses 

 backwards and downwards and splits into two almost equal large root- 

 lets. The dorsal rootlet (T) bends upwards, and, following the ventro- 

 posterior margin of the restiform body, passes over into the dorsal 

 lateral line root. Some of its ventral fibres also assist in obliterating 

 the triangular space mentioned above. The lateral rootlet {1") con- 

 tinues the course of the main nerve, and, interlacing finally with the fibres 

 of the lateral rootlet of the buccal, as already described, goes over 

 with the latter into the lateral root of the lateral line system. 



The discrete nature of the fifth, seventh and lateral line nerves 

 makes Chimaera a very unique fish as regards its cranial nerves, and 

 it is to be presumed that such a simple condition is more primitive 

 than the complex fusions and interminglings that obtain in other fishes. 

 This separation may however be purely secondary, just as the form 

 of the brain of Chimaera undoubtedly is, but on the other hand the 

 vagus is also in a very simple and unfused condition in Chimaera, and 

 in fact the same may be said of its cranial nerves generally. We 

 think it by no means unlikely that careful analysis of the fifth and 

 seventh nerves in the light of the component-theory will reveal much 

 the same conditions as those described above in other cartilaginous 

 fishes. This is indeed suggested by the excellent description of the 

 roots of these nerves in Raja given by Stannius^), and also by the 

 scheme of the cranial nerves devised by Ewart-). 



February 24, 1906. 



1) Op. cit., p. 29. 



2) Trans. R. Soc. Edinburgh, Vol. 37, Part 1, 1892. 



