2i6 THE ANIMALS AND MAN 



extinct forms, known through fossils, had large teeth set in 

 sockets on both jaws. The place of teeth is taken, as far 

 as may be, by the bill or beak formed of the two jaws, pro- 

 jecting 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. 



Fig. 112. PufEns. (Drawn from a photograph from life.) 



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, a condition helpful of course 

 in flight. The breast-bone 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 rudimentary 

 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, 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- 



