280 INTEODUCTION TO ZOOLOGY. 



ture h\ which fn'eat lightness is combined with strcnffth, and 

 the hollows of the bones in the adult birds are filled net with 

 marrow, but with air. This remark is inappUcable to aquatic 

 birds like the Penguin, which are unable to fly, but refers to 

 those which, like the Eagle or the Swift, have the power of 

 llight in its full development. In them, the bones, even to the 

 extremities of the body, can, at the pleasure of the bird, be 

 filled with air, the buoyancy of which is increased by the high 

 temperature of the interior of the body. Thus we observe 

 the opposite qualities of great strength and great lightness so 

 admirably combined, that the greatest architects or engineers 

 would here find their utmost skill surpassed, and learn how^ 

 imperfect is human mechanism, compared with that evinced in 

 the structure of every individual of those countless myriads by 

 which the aii' is traversed. 



Temperature. — The circulation of the blood in birds need 

 not here be dwelt upon ; its leading features are shown in the 

 accompanying figure (^Fig. 241); but it is worthy of remark, 

 that the temperature of their bodies is, in some instances, 

 several degrees higher than that of man. The blood heat of 

 the human body is 9S. and a tliermometer held in the hand 

 will not reach to within two or three degrees of that tempera- 

 ture ; but, placed under the wings of different birds, it will rise 

 to upwards of 100, and sometimes even to 110. This great 

 amount of internal warmth gives to birds a power of enduring 

 cold which, to our ideas seems incompatible with their habits. 

 As an instance of this, we may mention that, on the bleak 

 shores of Terra del Fuego, Hummiusf-birds were seen during 

 a snow-shower, hovering over the expanded blossoms of a 

 Fuchsia.* Wliat a strange sight ! The Hummingr-birds and 

 the snow — the representatives of the Tropic and the Arctic 

 regions — united in the same picture. 



Respiration. — The lungs of birds (^Fig. 242) do not fill the 

 cavity cf the chest ; they adhere to the ribs and have man}' 

 openings through which tubes pass, conveying the air to the 

 numerous air-cells distributed throughout the body. By means 

 of this apparatus every part of the body can be inflated, the 

 bones themselves rendered buoyant, and air propelled even into 



* I owe the knowledge of this fact to the kindness of my valued friend, 

 Captain Thomas Graves, R.N., H.5I.S. Volaya, who at the time was 

 one of the officers in the expedition mider conmiand of Captain King, in 

 whose " Voyages" it is also recorded. 



