415 



NERVOUS SYSTEM AND SENSE ORGANS 



The cerebral hemispheres of the brain (Fig. 329) are large, 

 smooth, and rounded. The roofs of the lateral ventricles are 

 relatively thin, though nervous, but the corpora striata are large ; 

 with this development is connected the elaborate but stereo- 

 typed behaviour of birds. This is in large part based on a number 

 of fixed patterns of response, or instincts, but that considerable 

 learning is possible is shown by the rapid spread of such new and 

 peculiar habits as the opening of milk bottles by tits. The olfactory 



^S. 



Tn.o. 



Fig. 329. — The brain of a pigeon. — From Thomson. 



(i) Dorsal, (2) ventral, and (3) side view, c. Cerebral hemispheres ; ch. cerebellum ; m.o., medulla oblongata ; 



O.I., optic lobes ; 0//., olfactory lobes ; s.c, spinal cord. 



lobes are very small. The cerebellum and cerebrum meet over the 

 thalamencephalon, thrusting the round, hollow optic lobes to the 

 sides. The cerebellum is ridged transversely. There are twelve 

 cranial nerves, corresponding to those of the rabbit (p. 463). The 

 sense of smell is poorly developed in most birds, though good in 

 the kiwi [Apteryx) and some others. Hearing is acute, the labyrinth 

 possessing the organ known as the cochlea which w^as quite 

 rudimentary in the frog. Sight is very keen, and the eye is remark- 

 able for the presence of a vascular pigmented organ, known as 

 the pecten, which protrudes into the vitreous humour from the 

 * blind spot ' where the optic nerve enters. This is possibly con- 

 cerned with the appreciation of movement. There is a high 

 density of cones, and two special concentrations of them, or 



