MECHANICS OF RESPIRATION 



different. The creatures which live in water only 

 secure it after it has first been dissolved in the medium 

 that surrounds them: they can secure it in no other 

 way. Moreover, in the vertebrates — the fish among 

 them — there are other differences besides those due 

 to the environment. Land vertebrates, in their bodies, 

 have a special apparatus to take the air and its oxygen 

 to the lungs through which they breathe. They have 

 a continuous passage from the nostrils to the lungs for 

 this purpose. Fishes have nothing of the sort. Their 

 respiratory organs, the gills, situated behind the cheeks 

 on the sides of the head, communicate at once with 

 the outer world and the back of the mouth. It is 

 through the latter that they receive the water with its 

 dissolved oxygen. They are obliged to fill the mouth 

 with water in order to breathe, whereas birds and 

 mammals, surrounded by air, can breathe all the air 

 they need through their nostrils, with their mouths 

 closed all the time. 



The modest goldfish, in its little globe, will serve 

 as an example. As it moves about, we see it yawning 

 all the time, opening and shutting its mouth con- 

 tinually, as if it were obliged to keep on swallowing. 

 We can also see, behind its cheeks, the golden gill- 

 covers beating gently, rising and falling continually. 

 As we watch closely, it is not difficult to realize that 

 both movements correspond and are successive. There 

 is a beating of the gill-cover for each yawning of the 

 mouth. A little experiment will serve to complete the 

 demonstration. With a pipette, let free near the 

 mouth a small quantity of some harmless, impalpable 

 dust, chalk or something of the sort, or a few drops 

 of some coloured solution. Then you will see several 

 parts of these substances, taken in during the yawning, 

 enter the mouth and come out again beneath the gill- 

 covers. Thus we can determine their passage. The 

 fish takes in water all the time, fills its mouth with 

 it, then makes it pass over the gills and finally ejects 



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