MECHANICS OF RESPIRATION 



hog-fish seem good-humoured creatures, with no 

 pretentions to Rightfulness: their weapons are not to 

 be taken particularly seriously. 



Although they stay on the bottom and do not move, 

 the fact that they are alive is clearly manifested by 

 movements of the mouth and gill-covers, and these 

 movements are made still more evident by the large 

 size of the organs which produce them. They per- 

 form a regular series of respiratory movements, the 

 various stages of which may easily be followed, since 

 they are confined to the head, the rest of the body 

 remaining still. The creature, as it takes in the water 

 which reaches its gills, plainly reveals an inspiration 

 followed by an expiration, as in the case of land verte- 

 brates. It begins by parting its lips and opening its 

 mouth; then it takes in the water and gradually fills 

 the buccal cavity. Little by little the cheeks expand, 

 the gill-covers swell, while their edges still remain in 

 contact with the body in such a way as to keep closed 

 the gill-chamber which they cover. The head grows 

 larger and broadens. When the cavities of the mouth 

 and of the two gill-chambers are absolutely full, the 

 inspiration ceases and expiration begins. The lips 

 come together again, and the mouth shuts; an inner 

 valvular fold, forming a sort of tongue, makes the 

 closing more complete. The gill-covers sink, their 

 edges rise; the cheeks retract; and the head becomes 

 smaller. The water, thus put under pressure, tries to 

 get out of the cavities in which it is contained, and 

 since it cannot do so by the mouth, which is closed, 

 and the only way left is through the gill-slits which 

 are open, it takes that way and is swept over the gills. 



Fish provide us with examples of the mechanical 

 phenomena of respiration, first an inspiration and then 

 an expiration, as do the vertebrates which live in air; 

 but there are a few differences. The fishes have to 

 deal with dense water, not with a light gas like air. 

 The courses followed in the two phenomena are quite 

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