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, pig 456,—-Hina 
but it should be borne in mind that feathers limb of a Hawk, 
Buteo vulgaris. a, 
Fatty . 07 ire. * femur; 2, tibia; 0, 
are distinct in origin and structure from hairs. unig arses 
The ordinary feathers are called penne or atarsus;c’, the eame 
piece isolated, and 
contour feathers ; as they determine by their seen from in front; 
arrangement the outline of the body. They tak aneroaren: 
are, like hairs, developed in sacs in the skin; °*"” 
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 bardules. 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. 
* Jeffries’ The Epidermal System of Birds. Proc. Bost. Soc. N. H. 
1883. 
