RESPIRATION. 



145 



Fig. 89. 



tial to life, than in the lower orders. A Fish soon ex- 

 pires if taken out of the water, but an oyster will live 

 for weeks, with only that which it retains in the shell. 



Respiration in Fishes. One side of the gills of a 

 Fish is distinctly represented by Fig. 89, which also 

 shows the heart and artery by which 

 the blood is sent to these parts. Of 

 the heart, d, is the auricle, e, the ven- 

 tricle, b, the enlargement of the arte- 

 ry, called the bulbous arteriosus, 

 which is shown distinctly in Fig. 85, 

 ^/, the brancial artery, g, g, the gills. 

 We here have an opportunity of ob- 

 ' serving how near the heart of the Fish 

 is to the lungs, and consequently of 

 inferring the importance of well sup- 

 plying these parts with blood. This 

 arises from the circumstance that the 

 heart throws the blood only to the 

 gills, and not to the other parts of the 

 body as in the other animals, hence a 

 large proportion of the blood of the 

 whole system, is constantly in the gills, 

 to be well purified by the air before it circulates 

 through the other parts, and this is the reason why these 

 parts are highly colored with blood, while the other 

 parts of the Fish are white. 



The gills consist of filaments arranged somewhat 

 like the leathers of a quill. When these filaments are 

 closely examined, they are found to be covered with 

 minute processes crowded close together, and on which 

 may be observed millions of capillary blood vessels, 

 spread like a net-work over the whole surface. It is 

 through the thin coats of these vessels that the air acts 

 upon the blood they contain. 



In the osseous or bony fishes, there is a large flap 

 called the operculum, which covers the gills from injury, 

 and below which there is an opening for the escape of 

 the water, after it has performed its office. The pro- 



How is this function carried on in the fish ? 

 13 



