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 least 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- 



