DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 217 



which is neither serous nor mucous, hut partly the one and part ly the other ; and as blood- 

 vessels in considerahle numbers, as my preparations 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. D rosier*, of Cambric! B, for example) have endea- 

 voured to prove that they are adjuncts of the hums, and therefore assist in aerating the 



blood. This opinion was advocated by John Hunter as early 177 If, and is probably 

 correct, since the temperature of birds is higher than that of any other class of animals, 

 and because they are obliged to make great muscular exertions both in swimming and 

 flying. Others have viewed the air-sacs in connexion with the hollow bones frequent ly, 

 though not always, found in birdsj, and have come to look upon the heated air which 

 they contain as being more or less essential to flight §. That they have al .solutely nothing 

 to do with this function is proved by the fact that some excellent fliers (take the Bats e. //.) 

 are destitute of them, while birds such as the Ostrich and Apteryx, which are incapable 



work on the subject :— « The air-sacs are neither serous nor mucous, but they are more allied to the former. They 

 are very slightly vascular, and the vessels are long and slender, and do not arise from the pulmonary artery and 

 veins, as those who regard the air-sacs as expansions of the mucous membrane of the lungs or adjuncts of respiration 

 believe. On the contrary the arteries arise from the aortic system, and the veins run directly or indirectly into the 



venae cavse. 



Sometimes the arteries and veins run side by side, and sometimes separately. The air-sacs are in 

 some instances provided with a fibrous envelope, which invests them more or km completely, the envelope possessing 

 remarkable elastic properties which manifest themselves more specially when the sacs cannot be acted upon by the 

 true muscular system. The bird is furnished with a double diaphragm ; and the sacs are empUed and filled alter- 

 nately, so that there is a double respiratory process going ou." -Researches on the Respiratory Apparatus by Ph 

 C. Sappey : Paris 1847. Sappey found no lymphatics, although he looked carefully for them both in the living and 



the dead bird. 



* « 



On the Functions of the Air-cells and the Mechanism of Respiration in Birds, by W. II. Drosier, M.D., Caius 



College," Proc. Camb. Phil. Soc. Feb. 12, 18GG. 



f « An account of certain receptacles of air in birds, which communicate with the lungs, and are lodged among the 



fleshv parts and in the hollow bones of these animals," Phil. Trans., Lond. 1774. 



the Swallow, Martin, Snipe, and many birds of passage have no air in their bonei 



The same author, in a second communication (pp. 215 and 216), add- 



_ _ • i 



X 



(P 



that the Glossy Starling, Spotted Flycatcher, Whin-Chat, Wood-Wren, Willow-^ ren Black-headed Bunting, ami 

 Canary, five of which are birds of passage, have likewise no air in their bones. The followmg is Dr. Cr,p s sum 



Out of 92 birds examined he found :-« air in many of the bones, 5 (Falcomd*) ; air m the humeri and not 



mary : 



in the inferior extremities, 39 ; no air in the extremities and probably none in the other bones 48 



* - The power Which birds enjoy of raising and supporting themselves in the air . atded by the hgh.nea, of the body. 



The large and nsually air-filled cavities in the bones diminish their weight withont takmg away from the.r ,,r,„ gt h, 

 a hollow cylinder being stronger than a solid one of the same weight and length. But the spec.nc lev, , pnnapally 

 depends on the great ai,ee,ls which occupy almost every part of the body. The ,. wfc.hb.nU msp.re dtstends these 

 cells, and is rarefied by the heat of the body.' 



>/ Vertebrates, by Prof. 



p. 114. 



Dr. Bennett states « that the whole surface of the body of the Albatros is covered by numerous air-cells capable of 

 a voluntary inflation or diminution by means of a beautiful muscular apparatus. . . . . By this power he birds 



can raise or depress themselres at will." For refutation of this view see paper by Captain F. W Hutton, On s me 



of the Bir 



(p. 294-.")). This gentleman calcu- 



late, that, to raise an Albatros of 17 lbs. weight by means of heated air, the sacs wouU rnpnre to contam 1 820 cub.c 



feet, which would occupy a sphere of more than .5 feet in diameter. » If," he adds, ■ the heated a,r d,d succeed ■„ 



raising the Albatros, the object in view would be defeated, as .he bird, bemg hghter than the a,r, would dr,f. help- 

 lessly to leeward/' 



