ENCEPHALON OF FISHES. 285 



olfactory nerve never forms a ganglion before spreading upon 

 the olfactory capsule ; the rhinencephalic crus, when prolonged 

 to the capsule, always expands into a ' tuberculum olfactoriura,' 

 or rhinencephalon, before it transmits the true olfactory nerves to 

 the capsule. In other words, the olfactory nerve conveys im- 

 pressions to a proper centre or lobe, which, in Fishes, may be 

 situated close to the capsule, or close to the rest of the brain, 

 and the length of its crus will be inversely as that of the nerve. 

 The olfactory lobes or rhinencephala are serially homologous with 

 the optic lobes. As to the prosencephalon, since this does not 

 immediately receive or transmit any nerve, it resembles in this 

 important character the cerebellum, and proceeds, even in the 

 present class, to be developed to a degree beyond the ganglions 

 of any special nerves or organs of sense. 



The more special homology of the prosencephalic lobes, 

 under their normal proportions and solid structure in Osseous 

 Fishes, with the parts of the complex and fully developed prosen- 

 cephalon in Mammals, will be made manifest as we trace the 

 progress of that complication synthetically. Cuvier had already, 

 by the opposite course of analysis, reduced the hemispheres 

 in birds to the ' corpora striata,' with their commissures and a 

 thin supraventricular covering. ' Le corps cannele,' he says, 

 ' forme a lui seul presque tout l'hemisph&re.' 1 But he failed 

 to recognise the homology of the prosencephala in Fishes. 

 Since Arsaki's time 2 their homology with the cerebral lobes of 

 Reptiles, Birds, and Mammals has been generally recog- 

 nised. Girgensohn 3 says they may well be compared with 

 the ' corpora striata ; ' but he notes the important difference, 

 that, whereas these ' transmission ganglia ' {durchcjangsknoteii) 

 give passage to the radiating fibres of the cerebral crura in 

 their course to other parts of the cerebrum in Mammals, those 

 fibres terminate in the solid prosencephala of Fishes. The 

 establishment of the lateral ventricles in the prosencephala of the 

 Plagiostomes and Lepidosiren also show them to be something 

 more than ' corpora striata.' 



It now becomes important to note the mode of establishment 

 of these cerebral ventricles : they are not formed by the super- 

 addition of a layer or film of neurine overlapping parts answerable 

 to the solid hemispheres in other Fishes, but are either central 

 excavations, as in the elongated prosencephala of the Lepi- 

 dosiren, fig. 186, Iv. or they are deep fissures towards the under 

 part, as in the coalesced hemispheres of the Shark ; Avhence I 



1 CC. t. ii. 1799, p. 162. 2 nil. 1813. 8 lxiii. p. 155. 



