BRANCH CHORD AT /I ; CLASS AYES: THE BIRDS 337 



the fine bristles or vibrissae about the eyes and nostrils 

 called thread-feathers. The fore limbs are modified to 

 serve as wings, which are well developed in almost all 

 birds. However, the strange Kiwi or Apteryx of New 

 Zealand with hair-like feathers is almost wingless, and the 

 penguins have the wings so reduced as to be incapable of 

 flight, but serving as flippers to aid in swimming under- 

 neath the water. The 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 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, 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, 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. 



