54 ORGANS OF BREATHING. 



and when pressed by the finger the skin sinks in, but resists 

 pressure like a foot-ball or other elastic body. The air, in 

 this case, is supposed to assist in producing a powerful 

 screaming voice, the bird being a wader, and not calculated 

 for lengthened flights. 



Generally speaking, the bones of birds, excepting when 

 young, are without marrow, the gradual absorption of which, 

 till the bones become a hollow tube, is most easily perceptible 

 in young tame Geese, when killed at different periods of the 

 autumn and winter. From week to week the air-cells in- 

 crease in size, till, as the season advances, the air-bones become 

 transparent. Towards the close of the summer and beginning 

 of autumn, although in external appearance the young Goose 

 resembles the parent, no trace of air-cells can be discovered in 

 its bones, the interior 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 connection with the wing. On the other hand, in 

 Ostriches, or those birds which either never or seldom fly, 

 those of the wing are comparatively small ; but as a compen- 

 sation, it has been remarked, that as great strength as well as 

 lightness is desirable to enable them to run swiftly, their 

 bones are almost all of them remarkably hollow. Such are 

 some of the advantages derived from this abundant supply of 

 air. 



We have alluded to the additional warmth possessed 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. 



