CHARACTERS OF TITEMATOTHERMA. 5 



"No approach to torpidity with loss of animal temperature has 

 been determined to take place in any bird. The insectivorous 

 kinds migrate — Smfts and Swallows, e. g., to and fro between 

 England and Africa; and migration is performed by numerous 

 other birds in relation to localities furnishing the food most 

 appropriate for the nourishment of their newly-hatched young. 

 Experiments have failed to induce torpidity in birds through 

 artificial cold. 



§ 123. Characters and Orders of Birds. — The two H^mato- 

 thermal classes Aves and Mammalia, are defined in vol. i. p. 6 ; 

 and I here proceed to a fuller exposition of the avian charac- 

 teristics, and of the modifications on which the class has been 

 divided into orders or other primary groups. 



Birds constitute a class of oviparous vertebrate animals, with 

 warm blood, a double circulation, and a covering of feathers. 

 They are organised for flight, and as this, the fleetest and most 

 vigorous kind of locomotion, demands the greatest energy in the 

 contractility of the muscular fibre, so the respiratory function 



finds its highest developement in the present class. Not only the 

 ramifications of the pulmonary artery, but many of the capillaries 

 of the systemic circulation, from the singular extension of the 

 air-cells through the body, are submitted to the influence of 

 the atmosphere, and hence Birds may be said to enjoy a double 

 respiration. 



Although the heart resembles in some particulars that of the 

 Reptilia, the four cavities are as distinct as in the Mammalia, but 

 they are relatively stronger, their valvular mechanism is more 

 perfect, and the contractions of this organ are more forcible and 

 frequent in Birds in accordance Avith their more extended resi)i- 

 ration and their more energetic muscular actions. 



As Birds exceed Mammals in the activity of those functions on 

 which the waste and renovation of the general system more 

 immediately depend, so they possess, as has been shown, a higlicr 

 standard of animal heat. 



