302 INTRODUCTION TO ZOOLOGY. 



structure by which great lightness is combined with strength, 

 and the hollows of the bones in the adult birds are filled not 

 with marrow, but with air. This remark is inapplicable 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 flight 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 air 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 98, and a thermometer held in the hand 

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

 perature; 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, Humming-birds were 

 seen, during a snow-shower, hovering over the expanded 

 blossoms of a Fuchsia.* What a strange sight! The Humming- 

 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 of the chest; they adhere to the ribs, and have many 

 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 



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

 Captain Thomas Graves, R.N., H.M.S. Volage, and who at the time was 

 one of the officers in the expedition under command of Captain King, in 

 whose " Voyages " it is also recorded. 



