MECHANICS OF RESPIRATION 



the space is closed and connected with the mouth only. 

 Often, when a dead fish is being examined to see if 

 it is fresh, the gill-cover is raised to see if the redness 

 of the gills proves that it is. The gills are then de- 

 pressed, one on top of another, and it is not possible 

 to perceive their exact connection. In order to do so 

 the mouth must be examined, and then the structure 

 may be seen in its details. It is possible to observe 

 how it works by considering this remarkable structural 

 arrangement, the floor of the mouth, like a window 

 with a grill, the bars of the grill bearing the gills, and 

 behind them, the shutters of the gill-covers opening 

 or shutting this window to the outer world, everything 

 being constructed to act in the water. 



The fish, thus equipped, is obliged to behave 

 accordingly and use its equipment. Consequently, 

 its respiratory mechanism necessarily differs from that 

 of the vertebrates nearly related to us, and from our 

 own. On land, breathing usually necessitates the play 

 of the pectoral and abdominal muscles and of the 

 diaphragm, and the elasticity of lungs capable of 

 contraction. The head itself and the nostrils do not 

 move much, if at all. This is by no means the case 

 in fishes, whose gills hang motionless upon the arcaded 

 bars which support them. So far as they are con- 

 cerned, the process consists of bathing these gills with 

 constantly renewed water which enters by the mouth 

 and comes out by the gill-slits. So, to carry out its 

 work, it employs other means, and has recourse to the 

 muscles situated around the mouth and gill cavities. 

 Respiration, in the fish, produces a characteristic ex- 

 pression. Its muscles as they contract, its details as 

 they move, alter the contours and appearance of the 

 head, and give them a definite expression, connected 

 with the function they are fulfilling. One might 

 almost say that the fish, far from being the cold in- 

 different creature it is often supposed to be, shows in 

 its face the vital activity by which it is animated. 

 216 



