286 ANATOMY OF VERTEBRATES. 



conclude 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 Vertebrates, 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 

 fig. 178, 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 

 interesting to perceive on the superficies of the solid prosence- 

 phalon in many fishes the foreshadowing of the convolutions, 

 which are not fully established until an advanced Mammalian 

 grade is attained. The prosencephalon of the fish is far from 

 being a miniature model, but it may be regarded as the potential 

 representative, 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. In a chub (Leuciscus Cy- 

 prinus) weighing 842 scruples, the brain, exclusive of the olfactory 

 lobes, weighed one scruple ; in a carp ( Cyprinus Carpio), weigh- 

 ing 11,280 grains, the brain weighed 14 grains; in a lamprey 

 weighing 750 grains, the brain weighed half a grain. 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 pre- 

 cisely with the general bulk of the body. The size of the optic 

 lobes, e.g. has a more constant and direct relation to that of the 

 eyes, which soon acquire their full developement. 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, under the con- 

 ditions of its progressive motion, provision for occupying the 

 increasing capacity of the cranium is made by a concomitant 

 developement of the light cellular arachnoid, which has the 

 further advantage of regulating the specific gravity of the head. 



As the branchial respiration is a peculiarly active and im- 

 portant function in Fishes, and has an extraordinary apparatus of 

 bony or gristly arches with their muscles, we may associate there- 

 with the peculiar developement and complexity of the medulla 

 oblongata, as the centre of the vagal or respiratory nerves. The 

 Carp and other Cyprinoid Fishes, which have not the mechanical 



