NERVOUS SYSTEM OF FISHES. 191 



in the Plagiostomi it is reduced to a few thin shining aponeurotic 

 bands closely adherent to the inner surface of the cartilaginous walls 

 of the cranium and spinal canal ; such traces of dura mater are more 

 feeble and indistinct in Osseous Fishes, in which no proper continu- 

 ous fibrous membrane can be distinguished fi'om the inner periosteum 

 of the walls of the cerebro-spinal cavity : no curtains of dura mater 

 divide the cerebral from the acoustic compartments of the cranium in 

 the Osseous Fishes. 



KERVES. 



The head is short and obtuse in the embryo fish ; the ganglionic 

 centres of the olfactory nerves are always originally developed in 

 close contiguity with the prosencephalon ; they govern the develop- 

 ment of the rhinencephalic arch ; and, as this advances in the elonga- 

 tion of the skull, and recedes from the jorosencephalic arch, either the 

 brain is co-elongated, the rhinencephalon retaining its primitive relation 

 with its vertebra, and the prolonged crura occupying the narrow 

 interorbital tract of the cranial cavity ; or, the rhinencephalon 

 retains its primitive juxtaposition with the prosencephalon, and the 

 olfactory nerves ai'e prolonged through the interorbital space, per- 

 forate or traverse a notch in the prefrontals, and expand, as a resolved 

 plexus, upon the pituitary plicated sac. 



The rhinencephalon accompanies its vertebra and recedes from the 

 rest of the brain in Sal/no, Ci/prinus proper, Brama, Tinea, Gadiis, 

 Lota, Hippoglossus, Clupea, Belone, Lucioperca, Cobitis, the Plectog- 

 nathi, and Plagiostomi ; it retains its primitive contiguity with the 

 prosencephalon in Perca, Scomber, Esox, Pleuronectes, Blennius, 

 Anguilla, Cyclopterus, Gasterosteus, Eperlanus, Leuciscus, Cottus, 

 Trigia, Amblgopsis, Echeneis, the Ganoidei and Lepidosiren. The 

 condition of this difference would be an interesting subject of enquiry. 



As the crus of the rhinencephalon is formed not only of fibres con- 

 tinued from the pi'osencephalon, but also, and in some fishes cliiefiy, 

 of distinct white and grey tracts traceable along the base of the 

 mesencephalon, in part as far back as the prepyramidal bodies, so the 

 origin of the olfactory nerve has been described as cliaracterised by the 

 same complexity and extent ; and it is true tliat in some instances, 

 where the rhinencephalon is in contact with the prosencephalon, a 

 small portion of the true olfactory nerve may be distinctly traced, 

 e. g. in the Perch, backwards as far as the mesencephalon : just as we 

 find in some fishes. Sturgeon, e. g., a portion of the optic nerve trace- 

 able as far back as the cerebellum, and in the Eel to the hypoaria, 

 and not exclusively terminating in the optic lobe. Most of the cha- 

 racteristics of origin and course atti'ibuted in works of Comparative 



