188 



INTEODUOTOEY CHAPTEE. 



from which is distilled the oily substance with which they lubri- 

 cate their plumage. These receptacles are known as the uropygial 

 glands. On the lower surface is a layer of cellular tissue con- 

 taining a similar fluid, which seems to be connected with the 

 growth of the feathers. 



The feet of birds are as varied in different species as are their 

 wings. In birds of prey the claws are powerful and hooked. In 

 some the foot is flat and the claw straight and adapted for walking 

 (Figs. 56, 57, 58). The great toe is generally the strongest, 



Fig. 56. 



Fig. 67. 



Fig. 68. 



but this is not an absolute law : a projection which is found on 

 the leg of some birds, and is designated a spur, is a formidable 

 weapon in some species (Figs. 59, 60). Some birds walk by 



Fig. 69. 



'Fig. 60. 



bringing their feet forward alternately ; others by a simultaneous 

 motion, or a succession of leaps. Some run with great velocity, 

 while others walk with great difficulty, and that only on a flat 

 surface. Many have their toes joined by thin membranes, which 

 act as paddles by which they propel themselves through the 

 water. 



The beak, or bill, of birds is composed of two bony pieces, called 

 mandibles, surrounded by a horny substance, differing both in 



