hi THE FEATIIEUED TKIBIOS. 



bulk, j'ct, provided whh a huge laj'or of oily substance called blubber, iuimediately 

 under the skin, they proceed hither and thither at pleasure. In other instances, the 

 organ called " the swinnning bladder" answers as cfiectually the same purpose. 



The air, which in iishes is contained in one cavity, is distriliuted into numerous cells iu 

 various parts of the bodies of birds. These cells are divided by partitions, piovidcd, iu 

 the case of large birds, with muscular fibres, supposed to be employed iu sending the air 

 back to the lungs, as is done by the diaphragm in other animals, but which is wanting iu 

 birds. 



In addition to these air-cells, which till the whole cavity of the body from the neck 

 downwards, and serve the double purpose of assisting in the assunilation of nutriment, 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. This is particularly the case 

 with the larger bones. Those accustomed to such observations 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. Such bones ai'c generally of a purer 

 white, and sometimes they are extremely thin. The long bones are usually situated close 

 to one of the extremities, while in bones in pairs, there is, for the most part, only one 

 common opening. 



As Blumenbach observes : " 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 absorption of the marrow, can nowhere be 

 observed so well as in the yomig tame geese, when killed iu different periods of the 

 autumn and winter. The limits to the air-cells may be clearly seen from without by the 

 transparency of the bony walls. From week to week the air-cells increase in size, till, 

 towards the close of the season, the air-bones become transparent. 



" In all these bones the marrow first disappears from the vicinity of the opening which 

 admits the air, and continues longest at the poiuts furthest removed from this oijeniug. 

 Towards the close of the summer, and beginning of autunm, although in external appear- 

 ance the young goose resembles the parent, no trace of air-cells can be discovered in its 

 bones, the interior of the bones being then filled with marrow. About the fifth or sixth 

 month the marrow begins to disappear. This circumstance, which applies also to other 

 birds, shows with what caution one should form an opinion, from yomag birds only, on the 

 size of the air-cells. In many kinds of biid.s the air-cells of some bones are nearly fully 

 developed, although they have the openings of tha bones which lead to the air-cells." 



The quills of the feathers accord, in this instance, with the structure of the bones. 

 While growing, they are filled with an organised pulp, but as soon as they arrive at their 

 full giowth, the absorption of this pidp renders them liglit. Tlie lightness is increased 

 by air being introduced into their cavity, through a small opening at the termination of 

 the furrow where the quill end.s and the plumelets of the feathers begin. Air is also 

 admitted, in a similar manner, into tlie plumelets themselves. 



Colonel Jlontague made many experiments to observe the air-cells in the structure of 

 the gannct. lie first introduced into the trachea a pipe, and when air was {jropellcd 

 thiough it, the whole internal cavity of the body was iutlated, but no air passed into tlie 

 external cells between the slcin and the body. An incision was then made in the lower 

 part of the abdimien into the body, \\hen it readily I'ound its way through the larjnix, 

 uroducins a noise siinilnr to the sound emitted bv tlie livint'- bird. 



A .small opening was then made in the skin on (lie lel't side, about midway between the 

 wing and the thigh, and a jiipe was introduced, fhuse (h'rectly communicating ^vith the 

 internal ])aits having first been stopjied. It was now obvious tliat when air was forced 

 through this orifice, the skin on that side, as far as tlie niiildle line of tlie body, was 

 greatly inflated, extending into the lower pint of the neck, along the larger jninls of llie 



