FLIGHT 271 
val of carbon, and of diminishing the weight of the 
body, there are others situated in the bones them- 
selves, particularly the larger bones, both those 
which are cylindrical and those which are broad 
and angular. It is not a little remarkable that all 
these bones in birds are (at Ieast in the middle) des- 
titute of marrow ; and, as Camper has shown, are 
furnished with openings for the purpose of com- 
municating with the lungs. Experienced observ- 
ers can tell, from inspecting the bone of a full- 
grown bird, whether or not it contains aircells, 
without even seeing any of the openings through 
which the air enters, for such bones are, for the 
most part, of a purer white, and sometimes the 
bone is so thin that the cells may be seen through 
it; yet such appearances are not always to be 
trusted. The openings ought, therefore, in these 
investigations, to be sought for, though they are 
sometimes not a little difficult to discover. In the 
long bones, we may mention, they are generally 
situated close to one of the extremities, while in 
bones in pairs there is, for the most part, only one ° 
common opening. 
‘‘ The airbones in young birds” are described to 
be “ filled with marrow, which becomes gradually 
absorbed to make room for the admission of air. 
This gradual expansion of the aircells and absorp- 
tion of the marrow can nowhere be observed so 
well as in the young tame geese, when killed in 
different periods of the autumn and winter. The 
limits to the aircells may be clearly seen from 
without by the transparency of the bony walls. 
From week to week the aircells increase in size, 
till, towards the close of the season, the airbones 
become transparent. In all these bones the mar- 
row first disappears from the vicinity of the open- 
ing which admits the air, and continues longest at 
the points farther removed from this opening. To.- 
wards the close of the summer and beginning of au 
