PKOGRESSION IN OE THROUGH THE AIR. 



117 



show, ramify in their substance, and they are in many cases 

 covered with muscular fibres which confer on them a rhythmic 

 movement, some recent observers (Mr. Drosier ^ of Cambridge, 

 for example) have endeavoured to prove that they are ad- 

 juncts of the lungs, and therefore assist in aerating the blood. 

 This opinion was advocated by John Hunter as early as 

 1774,^ and is probably correct, since the temperature of birds 

 is higher than that of any other class of animals, and because 

 they are obliged occasionally to make great muscular exer- 

 tions both in swimming and flying. Others have viewed the 

 air-sacs in connexion with the hollow bones frequently, though 

 not always, found in birds,^ and have come to look upon the 

 heated air which they contain as being more or less essential 

 to flight. That the air-cells have absolutely nothing to do with 

 flight is proved by the fact that some excellent fliers (take the 

 bats, e.g.) are destitute of them, while birds such as the 

 ostrich and apteryx, which are incapable of flying, are pro- 

 vided with them. Analogous air-sacs, moreover, are met 

 with in animals never intended to fly; and of these I may 

 instance the great air-sac occupying the cervical and axil- 

 lary regions of the orang-outang, the float or swimming- 

 bladder in fishes, and the pouch communicating with the 

 trachea of the emu.^ 



^ " On the Functions of the Air-ceUs and the Mechanism of Respiration in 

 Birds," by W. H. Drosier, M.D., Caius College.— Proc. Camb. Phil. Soc, 

 Feb. 12, 1866. . 



2 "An Account of certain Receptacles of Air in Birds which communicate 

 with the Lungs, and are lodged among the Fleshy Parts and in the Hollow 

 Bones of these Animals." — Phil. Trans., Lond. 1774. 



3 According to Dr. Crisp the swallow, martin, snipe, and many birds of 

 passage have no air in their bones (Proc. Zool. Soc, Lond. part xxv. 1857, p. 

 13). The same author, in a second communication (pp. 215 and 216), adds 

 that the glossy starling, spotted flycatcher, whin- chat, wood- wren, willow-wren, 

 black-headed bunting, and canary, five of which are birds of passage, have 

 likewise no air in their bones. The following is Dr. Crisp's summary :— Out 

 of ninety-two birds examined he found " air in many of the bones, five 

 (Falconidce) ; air in the humeri and not in the inferior extremities, thirty- 

 nine ; no air in the extremities and probably none in the other bones, forty- 

 eight." 



4 Nearly allied to this is the great gular pouch of the bustard. Specimens 

 of the air-sac in the orang, emu, and bustard, and likewise of the air-sacs of 



