178 THE INFLUENCE OF INANIMATE SUKEOUNDINGS. 



CHAPTER VI. 



THE INFLUENCE OF A STILL ATMOSPHERE. 



The most important influence of stagnant air on the animals 

 living in it is strikingly exhibited by those organs which are 

 intended to respire air and convey it to the interior of the 

 body. The physiological action of these air-breathing organs is 

 exactly the same as that of the skin and giUs in water-breathing 

 animals. They bring the blood into the closest possible contact 

 with the oxygenated medium. But as regards structure no greater 

 contrast can be conceived of than that between the gills of a fish 

 and the lungs (fig. 51) of the higher Vertebrata, or the tracheae, 

 as they are termed, of Insects, Myriapoda, and Arachnoidse. 

 These last (fig. 52) are usually fine tubes, with elastic walls 

 with spiral thickening, which ramify in all directions, and 

 which allow of the alternate inspiration of fresh air and 

 expiration of foul air — charged, that is, with carbonic acid. 

 This is effected by the opening and closing of the stigmata, or 

 openings of the tubes, by the act of I'espiration, and by the 

 expanding and contracting of the tubes themselves. These 

 tracheae thus convey the air, in extraordinarily fine particles, to 

 all the organs,^^ so that their finest living portions certainly 

 and abundantly absorb the oxygen they require direct from 

 the air which is so brought to them. It is otherwise with 

 the Vertebrata. Here the air is taken into sacs of a spongy 

 structure (see fig. 51), of which the walls contain an exces- 

 sively intricate network of blood-vessels ; her?, exactly as 

 the skin or gills of fishes absorb oxygen from the water, the 

 oxygen from the air passes by endosmosis through the mucous 



