282 ANATOMY OF VERTEBRATES. 



regard to the f cerebrum ' of the Cod. a median tract or con- 



c5 



volution is marked off by a longitudinal fissure, Avhich extends 

 along the back of each prosencephalon, defining a posterior and 

 inferior convolution ; the median convolution is vertically fissured 

 on its inner side. In the Amblyopsis, fig. 175, P, it is cleft 

 anteriorly ; and here, as in most fishes, the median longitudinal 

 tract is the most constant subdivision of the prosencephalic 

 superficies. 



The large elongated prosencephala are smooth in Chimrcra, 



fig. 179, P, Polypterus, and Lepidosiren, fig. 186, P, and in the 



186 still more developed confluent mass 



in the Sharks, fig. 187, P ; the 

 prosencephala are, also, smooth in 

 the Myxines, where they are rela- 

 tively smallest. The comparative 



Brain of Lepidosiren . , , p .-. , 



anatomists, who have failed to 



recognise the true homology of the prosencephalon in Osseous 

 Fishes, appear to have been misled chiefly by its small proportional 

 size, which is commonly that exhibited in the brain of the Cod, 

 the Carp, and the Globe-fish ; in some species the prosencephalon 

 is even smaller, as in the Gar-fish, the Herring, or the Lump-fish. 

 The prosencephalon equals the cerebellum in size, but is less than 

 the optic lobes in the Perch and Bream ; it equals the optic lobes, 

 but is less than the cerebellum, in the Eel ; in the Stickleback 

 and Gurnard the prosencephalon exceeds the cerebellum, still 

 more so in the Lepidosteus, but is less than the optic lobes ; in the 

 Lucioperca, the Amblyopsis, the Chimaera, and the Skate, neither 

 the cerebellum nor the optic lobes are so large as the prosence- 

 phalon ; in the large Sharks their united size scarcely equals that 

 of the prosencephalon ; and in the Salamandroid Polypterus and 

 the Lepidosiren the prosencephalic lobes surpass all the rest of 

 the brain, and vindicate their true cerebral character and impor- 

 tance. In the Amblyopsis the relative magnitude of the prosen- 

 cephalon is due to the diminution of the optic lobes in that blind 

 fish ; in the Plagiostomes it is due to absolute developement ; as it 

 is, also, in the Polypterus and Lepidosiren, where the prosence- 

 phalon presents the closest similarity in form and structure to that 

 division of the brain in the Batrachian Reptiles : each lobe, for 



(No. 1309), which Hunter truly, though briefly, describes as follows : "The cerebrum 

 fissured; the cerebellum a long projecting body, also fissured in a less degree; the 

 nates two projecting bodies: the optic nerves decussate one another." This is the 

 earliest recognition of the homology of the optic lobes with the anterior of the bigcini- 

 nal bodies of the human brain. 



