54 ORGANS OF BREATHING. 



There is another bird even more copiously supplied with 

 air than the above, called the Chavana Fidele, in which the 

 skin is entirely separated from the flesh, and filled with an 

 infinity of small air-cells, the legs and even toes partaking 

 of the same singularity, so that it appears much larger than 

 it really is, 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 be- 

 come 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 connexion 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, 



