40 The Bird 
is provided with a thick, callous pad, which, by constant 
use, is thus kept bare of feathers. In addition, the under 
sides of the degenerate wings are also free of plumage, 
owing no doubt to the continual close application of 
these organs to the sides of the body. The other bare 
areas are almost obliterated, but the legs are bare, thus 
allowing perfect freedom in action. 
Some birds, such as vultures and cassowaries, have lost 
all feathers on the head and neck, or other portions of 
the body, from various causes, as for cleanliness, or, in 
some cases, probably for ornament. This will be spoken 
of more in detail in a later chapter. 
Moult 
The waste of internal tissues and organs in animals 
is repaired by means of the blood which brings them 
fresh material and carries away worn-out cells, as it 
traverses arteries and veins. Entire parts, as the tails of 
tadpoles, may even be absorbed; but, in general, skin 
structures when old and worn out are cast off and renewed 
from the lower, or derm, layer. This takes place in various 
ways. The skin, even to the covering of the eyeballs, 
may come off entire, as is the case among snakes, or por- 
tions peel off and tear away, as in lizards. Warm-blooded 
animals also shed, or cast, their outside covering; mam- 
mals shedding their coats of hair, and birds their feathers. 
In the latter class this process is called moulting. 
The nestling down and the feather which replaces it 
can hardly be considered as separate structures, as the 
