278 VERTEBRATE ANIMALS. 
the barbules, the ends of which are hooked. Towards the base 
of the shaft, however, the barbs are usually more or less sepa- 
rate and placed at a distance from one another, constituting 
what is known as the “down.” In the Ostriches and the birds 
allied to them, all the barbs are disunited and placed at a dis- 
tance, and they are often not at all unlike hairs in appearance. 
The feathers of birds not only greatly conduce to the high tem- 
perature of the body, but also serve to keep out moisture, to 
which end there is a peculiar oil-gland at the base of the tail, 
with the secretion of which the bird anoints its plumage. 
The skeleton of birds exhibits many points of peculiar interest, 
mostly in adaptation to an aerial mode of life; but only some 
of the more important of these can be noticed here. The entire 
skeleton is at the same time peculiarly compact and singularly 
light, the compactness being due to the presence of an unusual 
quantity of phosphate of lime, and the lightness to the fact that 
many of the bones are filled with air in place of marrow. The 
cervical region (neck) of the vertebral column is unusually long 
and flexible, since the fore-limbs are useless as organs of pre- 
hension, and all these functions have to be performed by the 
beak. In all birds the neck is, at any rate, sufficiently long to 
allow of the application of the beak to the tail, so as to permit 
of the cleaning and oiling of the whole plumage. The vertebrz 
which form the back or dorsal region of the spine are generally 
more or less immovably connected together, so as to give a 
base of resistance to the wings. In the Ostrich, however, and 
in other birds, in which the power of flight is either very limited 
or is absent, the dorsal vertebrae are more or less movable one 
upon the other. The vertebra which follow the dorsal region 
of the spine are all amalgamated together to form a single bony 
mass, which is termed the “sacrum,” and this, in turn, is united 
on both sides with the bones which form the pelvic arch, which 
carries the hind-limbs. The vertebrz of the tail are more or 
less movable upon one another ; and in almost all living birds, 
when fully grown, the last joint of the tail (fig. 152, B, s) is a 
long, slender, ploughshare-shaped bone, which is really com- 
posed of several vertebrze united together. It is usually set on 
at an angle nearly perpendicular to the axis of the body, and it 
serves to support the great tail-feathers, which act as a rudder 
during flight. It also serves to support the oil-gland, which 
supplies the secretion with which the feathers are lubricated. 
