BACKBONED ANIMALS WHICH BREATHE IN AIR 



427 



leave the breathing organs, it need only be said that the arrange- 

 ment is much the same as in Reptiles (see p. 424) when a bird 

 is standing or walking, but 

 during flight it would appear 

 that the breastbone is rela- 

 tively fixed, while the hinder 

 part of the backbone is 

 moved up and down. The 

 same end is effected in either 

 case. 



Owing to the length of 

 a bird's neck the windpipe is 

 correspondingly elongated, 

 and in some cases (Cranes) 

 it is thrown into a number 

 of loops situated in the 

 breastbone (fig. 552). No 

 plausible explanation has 

 been offered as to the use of 

 this peculiar arrangement, 

 but it is difficult to believe 

 that it has not some special 

 meaning. 



Fig- 55 2 - Windpipe of a Crane 



a a a, Windpipe; bb, breastbone; cdef, hyoid bone (which 

 supports tongue); gg, shoulder-blades; h, right coracoid bone; 

 /, merry-thought. 



LUNGS OF MAMMALS (MAMMALIA) (see vol. i, p. 45-47) 



The lungs of a Mammal ^are very complicated spongy organs 

 which fill most of the thorax (fig. 553). The windpipe divides 

 into two branches, as a general rule, one for each lung. If one 

 of these is traced it will be found to branch in a tree-like way, 

 the smallest, very thin -walled branches being known as bron- 

 chial tubes. Each of these ends blindly, swelling up into a little 

 bunch of air-cells, the walls of which are invested by a net-work 

 of capillary blood-vessels. It is in these air-cells that the blood 

 is purified, and they present collectively an enormous breathing 

 surface. In short, the spongy lung of a Mammal is a very perfect 

 contrivance for packing into a comparatively small space an ex- 

 tended area over which exchange of gases between the impure 

 blood and the air can readily take place. Amphibians, Reptiles, 



