332 SYSTEMATIC ZOOLOGY 



of accommodating themselves to objects at very varied distances. 

 Farther back on the head are the external auditory openings, 

 often surrounded by a special row of stiff feathers. This open- 

 ing leads into a short tube called the external auditorv meatus, 

 at the deeper end of which is the tympanic membrane ; within 

 this is the tympanic cavity, which connects with the mouth by 

 means of the Eustachian tube. 



There are over twelve thousand known species of living birds 

 distributed over the whole world, and many of great economic 

 importance to man, not only as destroyers of insects and other 

 undesirable animals, but also as an article of food ; so we may 

 give a brief account of the structure of a typical bird, beginning 

 with the skeletal system (Fig. 333;. In most birds the bones are 

 extremely light, since they contain air chambers. The skull is 

 remarkably so, and there is great fusion of the various bones 

 which compose it. The vertebrae of the neck are very movable, 

 and vary in number from nine to twenty-three, the maximum 

 number being found in the swan, which has more cervical or 

 neck vertebrae than any other animal. The neck is succeeded 

 by the thoracic portion of the vertebral column, which is rigid 

 and bears the ribs, which join the sternum or breastbone on 

 the ventral side of the body ; the sternum bears a projection 

 or keel on its median line in all birds which use the wings for 

 flight. Posterior to the thoracic vertebrae is the pelvis, a la: 

 bony mass consisting of many fused vertebrae, and this is suc- 

 ceeded by a few caudal or tail vertebrae ; some extinct birds 

 were provided with a very long tail, but it is short in all living 

 species. The bones of the wing are attached to the pectoral 

 girdle, of which the most familiar bone is the wishbone, or 

 merrythought as it is called in England ; it is known technically 

 as the furcula, and corresponds to the collar bone in mammals. 

 In the wing we distinguish the same parts as in the human arm, 

 the upper arm, forearm, and hand. The skeleton of the poste- 

 rior appendage is attached to the pelvis, and in it again we dis- 

 tinguish the same parts as in man, thigh, leg, and foot. The 

 shape of the foot varies with the use to which it is put, such as 

 swimming, scratching, running, or grasping. 



Concerning the internal organs (Fig. 334), we note the brain 

 is more highly developed than in any of the vertebrates that we 



