118 ANATOMY AND PHYSIOLOGY. 



The mechanism of respiration is considerably modified in other 

 classes. Whales (which breathe air) have parts that are thought 

 to serve as reservoirs, both of venous and arterial blood ; and this 

 is conjectured to be the reason why they are able usually to remain 

 under water twenty minutes, and sometimes upwards of an hour, 

 without breathing. The lungs of birds, instead of being free in the 

 chest, are fixed to its sides, and also have openings in them which 

 allow the air to pass into air-cells that pervade almost every part of 

 their bodies. As a proof of this, if the windpipe of an eagle be tied, 

 and the largest bone of its wing (humerus) be broken, it can breathe 

 through the broken bone instead of its windpipe. It is this ar- 

 rangement that causes the respiration of birds to be called double, 

 for the air acts on the blood, 1st, in passing through the lungs to 

 the air-cells ; 2dly, in passing out of these, and probably also while 

 it remains in the air-cells. Hence, they consume more air than any 

 other class of animals. 



Reptiles can act but imperfectly on the air, from the cells of their 

 lungs being very large, and from this cause of course diminishing 

 the surface upon which the blood-vessels have to be distributed. 

 The frog has no ribs, nor has it any diaphragm, the abdomen and 

 chest forming but one cavity. As a substitute for these, the air is 

 forced into the lungs by a species of deglutition. A frog perishes 

 if its mouth is kept open, because, before this deglutition can be 

 accomplished, the mouth must be closed. 



The surface occupied by the gills of fishes is often very consider- 

 able. Those of one kind are said to have a surface nearly equal to 

 that of the human body. The reason why air cannot usually be 

 directly breathed by gills, is believed to be principally because they 

 become collapsed and dry. The eel, the crab, and some other 

 species, that breathe by gills, can, however, breathe in air for a 

 considerable time. 



The only other modification of the respiratory apparatus we 

 shall refer to, is that of insects. The veined appearance of the 

 wings of the butterfly is produced by what are called tracheae, that 

 have openings on the surface (stigmata) for admitting the air, and 

 extensive ramifications over the body. There are similar openings 

 on the side of the bee-worm and in other species. If these are 

 closed, the animal immediately dies. In all the lowest classes of 

 animals, and even as high as the class of reptiles, the skin is also 

 an active respiratory organ. 



What has been said may render intelligible the mechanism by 





