426 



ANIMAL RESPIRATION 



out into slender thin-walled air-sacs, which penetrate between the 

 other organs of the body. How far these organs are of use in 

 breathing is not known. 



LUNGS OF BIRDS (Aves) 



Although richly supplied with blood, the lungs of Birds are 

 comparatively small, and instead of being mobile are closely fixed 

 to the ribs and backbone. From the main air-passages which 

 traverse them very complex branches are given off, the linings 

 of which are raised up into folds by which the breathing surface 

 is greatly increased. A number of large thin- walled aii'-sacs are 

 connected with the lungs (fig. 551), and these 

 not only fill up most of the space between 

 the other internal orsfans, but are also, as a 

 rule, continuous with air-spaces in the bones. 

 How far purification of the blood takes place 

 in the neighbourhood of these air-sacs is 

 doubtful, but in any case it would appear 

 certain that their presence conduces to the 

 rapid and frequent tlow of air through the 

 main channels of the lungs, and hence pro- 

 motes rapid breathing. It has been explained 

 elsewhere that only Birds and Mammals 

 among backboned animals are hot-blooded, 

 maintaining a constant temperature whatever 

 may be that of their surroundings (see pp. 

 208, 244). In these two groups the problem 

 cut through and rods pushed into gf adapting to lung-breathing requirements a 

 heart and blood-vessels inherited from eill- 

 breathing ancestors has been for the first time solved, and 

 the impure blood poured into the heart from the general body 

 is kept separate from the pure blood which that organ receives 

 from the lungs. Hence increased activity, associated with more 

 efficient breathing, one manifestation of which is a higher body- 

 temperature. Birds are more active than Mammals, and it is not 

 therefore surprising that they possess hotter blood (103° to 104° F. 

 as against about 98° F.). 



The relation of air-sacs to flight will be considered in another 

 section. As to the means by which air is caused to enter and 



Fig. 551- — Lungs and Air-sacs 

 of a Bird, seen from under side. 

 The air-sacs of only one side are 

 shown, and two of them have heen 



