186 LECTURE VIII. 



Girgensohn (lxiii. p. 155.) says they may well be compared with the 

 * corpora striata ; ' but he recognises the important difference, that, 

 whereas these ' transmission ganglia ' (durchgangsknoten) 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 highly important to note the mode of establish- 

 ment 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 exca- 

 vations, as you perceive in these elongated prosencephala of the 

 Lepidosiren ^fig. 54. lv\ or they are deep fissures towards the under 

 part, as in the coalesced hemispheres of the Shark ; whence I con- 

 clude that the solid prosencephalon of Osseous Fishes is not a mere 

 representative of a basal ganglion forming the floor of the ventricle 

 of the hemispheres in the higher Vertebrata, where such ganglion is 

 a medium of transmission or source of accession to the cerebral 

 fibres ; but that the fish's prosencephalon is the seat of the terminal 

 expansion of the radiating medullary fibres of the cerebral crura. 

 Dissection of the recent brain shows (as in jig. 51. p) that these 

 fibres, besides being blended with grey matter, as in the corpora 

 striata, are thickly covered with a layer of the same grey and highly 

 vascular neurine of which the hemispheric convolutions in Mammals 

 are chiefly formed ; and it is most interesting to perceive on the 

 superficies of the solid prosencephalon in many fishes the fore- 

 shadowing of the convolutions, which are not fully established until 

 the Mammalian type is attained. The prosencephalon of the fish is 

 far, indeed, from being a miniature model, but it may be regarded as 

 representing the potential germ, of the complex cerebral hemispheres 

 of man. 



The average proportional weight of the brain to the rest of the 

 body in Fishes is as 1 to 3000. A certain size seems to be essential 

 to the performance of its functions, as a recipient of the impressions 

 from the organs of sense ; and it does not, therefore, vary in different 

 species so as to accord precisely with the general bulk of the body. 

 The size of the optic lobes, e. g. has a more constant and direct re- 

 lation to that of the eyes, which soon acquire their full development. 

 We find the entire brain proportionally greater in young than in old 

 fishes : it acquires its full size long before the termination of the 

 growth of the fish, if this has a fixed period. But as the head must 

 grow with the growth of the fish, provision for occupying the in- 



