AVES. 401 



Is there any recorded evidence that the character of the gizzard 

 in a given individual may vary somewhat in accordance with 

 the food used? 



428. Respiration. The trachea corresponds in length to 

 the length of the neck. Its rings are rigid (ossified). It 

 divides into a right and left bronchus which pass to the re- 

 respective lungs. The lungs are closely applied, and even 

 attached, to the dorsal wall of the thorax and are small in pro- 

 portion to the size of the animal. Some of the bronchial tubes 

 connect with air spaces (nine in the pigeon) among the viscera 

 and extending even into the hollow bones. They are probably 

 chiefly respiratory in function. 



Bird notes are produced not at the upper end of the trachea 

 as in other vertebrates but near its lower end, where it joins 

 the bronchi. The organ is called the syrinx. Its mode of 

 action is somewhat similar to that of the vocal cords in the 

 larynx of mammals. 



429. The Nervous System and Organs of Special Sense. 

 -The cerebral hemispheres are relatively larger than in any 



of the groups yet studied. Their surface is smooth. The 

 cerebellum is also large and concentrated chiefly in a central 

 or median lobe. By the growth of these two portions the 

 well-developed optic lobes are crowded into a lateral position. 

 The olfactory lobes are small and the sense of smell is not so 

 acute as in many other vertebrates. The optic lobes and the 

 eyes are well developed and the sense of sight is correspond- 

 ingly acute. The eye protrudes as a somewhat rounded cone 

 in front. This is supported by a ring of sclerotic (bony) 

 plates. The power of accommodation, that is, of focusing the 

 eye upon objects at different distances, is very great in birds. 

 In addition to the upper and lower lids a transparent fold of 

 the conjunctiva (nictitating membrane) may be drawn over 

 the eye from the inner corner. Hearing is acute, and the con- 

 dition of the ear is interesting chiefly in the facts of the ab- 

 sence of the concha of the external ear, and in the presence 

 of a well-developed but uncoiled cochlea in the internal ear. 

 27 



