Internal Workings I45 



BREATHING 



The purpose of breathing is the same in all animals: to 

 take the prime mover, oxygen, into the body, and to get rid 

 of the waste-product, carbon dioxide. This is accomplished 

 in the higher animals by exposing the blood, in vessels so 

 thin that gases can get through their walls, to a source of 

 oxygen. If there is less oxygen in the blood than on the 

 outside, oxygen will pass into the blood. If there is more 

 carbon dioxide in the blood than on the outside, carbon 

 dioxide will pass out of the blood. 



To man the source of oxygen is air, and because of its 

 drying action the thin-walled blood-vessels into which the 

 oxygen is to be absorbed must be placed in lungs where they 

 can be kept alive and moist and permeable. To the fish the 

 source of oxygen is water, and since water has no drying 

 effect, the blood-vessels can be exposed on gills in the 

 mouth. The gills are the red objects which can be seen by 

 raising the gill covers and looking through the gill opening 

 into the gill chamber underneath. They consist of a multi- 

 tude of filaments full of tiny blood-vessels, arranged on 

 bony arches somewhat as the flanges are arranged on the 

 tubes of an automobile radiator. There are four of these gill 

 arches in most fishes. Between them are openings, or clefts, 

 through which the water passes. Since there is no opening 

 from the fish's nostrils into the mouth cavity, as there is in 

 humans, breathing has to take place through the mouth. The 

 gill covers are closed, and water is drawn into the open 

 mouth by expanding what might be called the cheeks. The 

 mouth is then closed, either by the shutting of the jaws or by 

 the automatic action of flaps of skin which act as check valves 

 and thus block the orifice even though the jaws remain apart. 

 The gill covers are opened, and the water is forced out over 

 the gill filaments by contracting the "cheeks." 



