TUB ENGLISH SPABBOIV 



389 



The nervous system consists of the brain, spinal cord, and 

 the sympathetic system. The brain shows a great advance 

 over reptiles, particularly in the development of the hind 

 brain, or cerebellum — the centre of coordination (Fig. 356). 

 Since flight demands extreme coordination, we can under- 

 stand the large cerebellum of birds. 



/7Cf 



Fig. 357. — Median section through the eye of the pigeon : en, cornea ; 

 ir, iris; I, lens; cLjir, lens muscles; pet, pecten ; opt.nr, optic nerve; 

 rt, retina ; eh, choroid coat ; scl, sclerotic coat. After Vogt and Yung, 

 from Parker and Haswell's "Zoology." 



Sense-organs. — As befits such active animals, pigeons, like 

 birds in general, have well-developed sense-organs. Organs of 

 smell, lying at the base of the beak, and those of hearing, having 

 much the same structure as in man, are present. But the 

 most perfect organ is the eye which has relatively a large size. 

 It differs from that of man in its lenticular (compressed) form, 

 well adapted to the vision of distant objects. A peculiar organ 



