304 THE VITAL FUNCTIONS. 



and leaving below a wide fissure for the escape 

 of the water, which has performed its office in res- 

 piration. 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 cavities : in this action the branchial 

 arches are brought forwards 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 

 branchia3, so that they all receive the full action 

 of that fluid as it passes by them. Such appears 

 to be the principal mechanical object 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. AVhen a fish is taken out of the 

 water, this eff'ect can no longer be produced ; in 

 vain the animal 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 vivifying influence of oxygen*. 



* It has been generally stated by physiologists, even of the 

 highest authority, such as Cuvier, that the principal reason why 

 fishes cannot maintain life, when surrounded by air instead of 

 water, is that the branchiae become dry, and lose the power of 



