234 [FACULTIES OF BIRDS. 



body being always put in motion than in quadru- 

 peds*. 



'Besides these air-cells, which fill the whole cavity 

 of the body from the neck downwards, and serve the 

 double purpose of assisting in the assimilation of nu- 

 triment, by the supply of oxygen and the removal of 

 carbon, and of diminishing the weight of the body, 

 there are others situated in the bones themselves, 

 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) destitute of marrow t ; 

 and, as Camper has shown* are furnished with openings 

 for the purpose of communicating with the lungs J, 

 Experienced observers can tell from inspecting the 

 bone of a full-grown bird, whether or not it contains 

 air-cells, 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 air-bones 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 air-cells, 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 

 air-cells may be clearly seen from without by the 



* J. R. f Fred. II. De Arte Venandi, p. 39. 



J Kleine Schriften, i. 1, tab. 1 & 4. 



