276 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



ostriches and cassowaries also have only rudimentary wings 

 and are not able to fly. Legs are present and functional in 

 all birds, varying in relative length, shape of feet, etc., to suit 

 the special perching, running, wading, or swimming habits of 

 the various kinds. Living birds are toothless, although certain 

 extinct forms, known through fossils, had on both jaws 

 large teeth set in sockets. The place of teeth is taken, as far as 

 may be, by the bill or beak formed of the two jaws, projecting 

 forward and tapering more or less abruptly to a point. In 

 most birds the jaws or mandibles are covered by a horny 

 sheath. In some water and shore forms the mandibular 

 covering is soft and leathery. The range in size of birds is 

 indicated by comparing a humming-bird with an ostrich. 



Many of the bones of birds are hollow and contain air. The 

 air-spaces in them connect with air-sacs in the body, which 

 connect, in turn, with the lungs. Thus a bird's body contains 

 a large amount of air. The breastbone is usually provided 

 with a marked ridge or keel for the attachment of the large 

 and powerful muscles that move the wings, but in those 

 birds like the ostriches, which do not fly and have only rudi- 

 mentary wings, this keel is greatly reduced or wholly wanting. 

 The fore limbs or wings are terminated by three "fingers" 

 only. The legs have usually four toes, although a few birds 

 have only three toes and the ostriches but two. 



As birds have no teeth with which to masticate their food, 

 a special region of the alimentary canal, the gizzard, is provided 

 with strong muscles and a hard and rough inner surface by 

 means of which the food is crushed. Seed-eating birds have 

 the gizzard especially well developed, and some birds take small 

 stones into the gizzard to assist in the grinding. The lungs 

 of birds are more complex than those of amphibians and rep- 

 tiles, being divided into small spaces by numerous membranous 

 partitions. They are not lobed, as in mammals, and do not 

 lie free in the body cavity, but are fixed to the inner dorsal 

 region of the body. Connected with the lungs are the air- 

 sacs already referred to, which are in turn connected with the 

 air-spaces in the hollow bones. By this arrangement the bird 

 can fill with air not only its lungs but all the special air-sacs 



