El VERTEBRATA : BRAIXS 131 



The brain of a fish is very small, compared with 

 the spinal cord into which it is continued, and 

 with the nerves which come off from it : of the 

 segments of which it is composed the olfactory 

 lobes, the cerebral hemispheres, and the succeed- 

 ing divisions no one predominates so much over 

 the rest as to obscure or cover them; and the so- 

 called optic lobes are, frequently, the largest 

 masses of all. In Reptiles, the mass of the brain, 

 relatively to the spinal cord, increases and the 

 cerebral hemispheres begin to predominate over 

 the other parts ; while in Birds this predominance 

 is still more marked. The brain of the lowest 

 Mammals, such as the duck-billed Platypus and 

 the Opossums and Kangaroos, exhibits a still 

 more definite advance in the same direction. The 

 cerebral hemispheres have now so much increased 

 in size as, more or less, to hide the representatives 

 of the optic lobes, which remain comparatively 

 small, so that the brain of a Marsupial is extremely 

 different from that of a Bird, Reptile, or Fish. A 

 step higher in the scale, among the placental 

 Mammals, the structure of the brain acquires a 

 vast modification not that it appears much 

 altered externally, in a Rat or in a Rabbit, from 

 what it is in a Marsupial nor that the proportions 

 of its parts are much changed, but an apparently 

 new structure is found between the cerebral 

 hemispheres, connecting them together, as what is 

 called the " great commissure " or " corpus 



