STRUCTURE OF BIRDS. 



523 



no bony union of the two pubic bones, nor do the ischia 



unite with the sacrum or each other, except in Rhea. In the 



ostrich, the pubic bones are solidly united. The hind limbs 



(Fig. 456) are two, three, or four toed, the ostrich having 



but two digits ; in most four-toed birds, one toe (the hallux) 



is directed backwards, while in the parrots and trogons, 



etc., there are two toes in front and two toes -behind, and 



in the swifts and certain other forms all 



four toes are turned forwards. The bones of 



the skeleton are dense and hard ; both the 



long bones and the bones of the skull are 



commonly hollow, containing air; the air-sacs, 



in connection with the lungs, communicating 



with the hollows of the bone. In some birds 



which fly well, only the skull-bones have air- 



cells, while in the ostrich which is unable to 



fly, the bones have even a greater number of 



cavities than the gull. The body during 



flight is thus greatly lightened, and the bird 



can sustain itself in the air for many hours in 



succession. 



With all these characters, the most re- 

 markable and diagnostic external feature is 

 the presence of feathers; no reptile on the 

 one hand, or mammal on the other, is clothed 

 with feathers, though the scales on the legs 

 and feet of birds are like those of reptiles, 

 and it should be borne in mind that feathers 

 are fundamentally modified scales or hairs. 

 The ordinary feathers are called pennae or 



_* * , 



contour feathers ; as they determine by their seen from in front; 



, ,! ,-,. , r <ld', d"d"', the four 



arrangement the outline ot the body. They toes. After oegen- 

 are, like hairs, developed in sacs in the skin ; l 

 the quill is hollow, partly imbedded in the derm ; this merges 

 into the shaft, leaving the outgrowths on each side called barbs, 

 which send off secondary processes called barbules. These 

 tertiary processes (called barbules and hooklets) are com- 

 monly serrated, and end in little hooks by which the bar- 

 bules interlock. Down is formed of feathers with soft, 



Fi g 456 __ Hind 



theaame 



piece isolated, and 



