1 52 ZOOLOGY. 



The thalamencephalon is similar to the rabbit's ; its sides 

 forming the optic tracts, its floor depressed into an inf undi- 

 bulum with the pituitary at its end and the roof giving rise 

 to a pineal stalk. But the pineal itself is disconnected with 

 the brain, and lies close under the skin of the dorsal side of 

 the head, having been nipped off by the growth of the bones 

 of the skull. In the most primitive of the Amphibia (the 

 class in which the frog is placed), long since extinct and 

 only known by their fossil bones, the skull had a large 

 round opening at this point, and there can be little doubt 

 that in them the pineal was a functional organ of vision, 

 and its stalk an optic nerve emerging by this foramen. 



The optic lobes are large, and tbey are only one pair 

 (hence sometimes called corpora bigemina, in contrast with 

 the rabbit's quadrigemina), and each contains a large 

 cavity. The floor and sides of the mid-brain do not show 

 any marked differentiation from those of the pons and bulb, 

 nor these again from one another or from the spinal cord. 

 On the roof of the hind brain comes the cerebellum, which is 

 a very small upgrowth, quite without the complexity of 

 structure of the rabbit's. The thin roof of the rest of the 

 hind-brain is fully exposed, and gradually narrows back into 

 the dorsal fissure of the cord. 



The small size of the cerebellum is easily understood when 

 we consider how much broader and less high a frog's body 

 is than a rabbit's, and hence that a frog's normal position 

 is one of stable equilibrium and needs no elaborate co-ordi- 

 nation of muscular action to keep it balanced. We see a 

 result of this feeble cerebellar development in the way a 

 frog alights from a leap : its leaps are mere leaps in the 

 dark. Instead of coming down on its feet as a dog does it 

 really falls down anyhow, and is as ready to leap over a 

 precipice as on flat ground. This is also partly due to the 

 imperfect development of its cerebral cortex, the structure 

 of which is far simpler than in the rabbit, and quite in- 

 adequate to the forming of accurate judgments of distance 

 or to easy education by experience. 



14. Cranial Nerves, Of these, i., n., in., iv., vi., 

 and VITI. are so exactly like the same in the rabbit tjiat 



