66 
ORGANS OF BREATHING. 
killed at different periods of the autumn and winter. 
From week to week the air-cells increase in size, till, 
as the season advances, the air-bones become trans- 
parent. Towards the close of the summer and 
beginning of autumn, although in external appear- 
ance the young Goose resembles the parent, no trace 
of air-cells can he discovered in its bones, — the in- 
terior being still filled up with marrow, which does 
not entirely disappear till about the end of the fifth 
or sixth month. 
In the Eagle, Hawk, Stork, Lark, and other birds 
in the habit of soaring, the air-cells are very large, 
particularly those in connexion with the wing. On 
the other hand, Ostriches, and those birds which 
either never, or seldom fly, those of the wing are 
comparatively small ; hut as a compensation, it has 
been remarked, that as great strength as well as 
lightness are desirable to enable them to run swiftly, 
their bones are almost all of them remarkably hol- 
low. Such are some of the advantages derived 
from this abundant supply of air. 
We have alluded to the additional warmth pos- 
sessed by birds, in comparison with other animals, to 
which this greater quantity of air must essentially 
conduce. We may here again refer to the Gannet, 
which, passing so much of its time in the depth of 
winter exposed to the severest weather, would, if 
not provided with additional means of keeping itself 
warm, often perish from cold ; but having, as we have 
observed, a power of filling up the space between its 
flesh and outward skin with air, it is thus furnished 
with a light, but at the same time admirable coat, 
which effectually prevents it from feeling the effects 
of cold, however severe. 
