RESPIRATION IN FISHES. 217 



surfaces are still more minutely examined, they are found to 

 be covered with innumerable minute processes, crowded to- 

 gether like the pile of velvet; and on these are distributed 

 myriads of blood vessels, spread like a delicate net-work, 

 over every part of the surface. The whole extent of this 

 surface exposed to the action of the aerated water, by these 

 thickly set filaments, must be exceedingly great.* 



A large flap termed the Qperculum, extends over the 

 whole organ, defending it from injury, and leaving below a 

 wide fissure for the escape of the water, which has per- 

 formed its office in respiration. For this purpose the water 

 is taken in by the mouth, and forced by the muscles of the 

 throat through the apertures which lead to the branchial ca- 

 vities: in this action the branchial arteries are brought for- 

 wards and separated to a certain distance from each other; 

 and the rush of water through them unfolds and separates 

 each of the thousand minute filaments of the branchiae, so 

 that they all receive the full action of that fluid as it passes 

 by them. Such appears to be the principal mechanical ob- 

 ject of the act of respiration in this class of animals; and it 

 is an object that requires the co-operation of a liquid such 

 as water, capable of acting by its impulsive momentum in 

 expanding every part of the apparatus on which the blood 

 vessels are distributed. When a fish is taken out of the wa- 

 ter, this effect can no longer be produced; in vain the ani- 

 mal reiterates its utmost efforts to raise the branchiae, and 

 relieve the sense of suffocation it experiences in consequence 

 of the general collapse of the filaments of those organs, which 

 adhere together in a mass, and can no longer receive the vi- 

 vifying influence of oxygen.t Death is, in like manner, the 

 consequence of a ligature passed round the fish, and prevent- 

 ing the expansion of the branchiae and the motion of the 

 opercula. 



Dr. Monro computed that in the skate, the surface of the gills is, at the 

 least, equal to the whole surface of the human body. 



f It has been generally stated by physiologists, even of the highest au- 

 thority, such as Cuvier, that the principal reason why fishes cannot maintain 

 VOL. II. 28 



