354: ©N THE RESPIRATION OF FISHES. 



the same, since in both cases air is equally introduced into 

 the lungs, there to produce and undergo a certain action. 

 The real dilFereuce is only in the mode of inspiration, and 

 the connexion of its continuance with the motion that 

 empties the lungs. In fact, the air, inspired by small quan- 

 tities in succession, is always introduced at the nostrils, and 

 issues from the mouth at a single discharge of longer or 

 shorter duration. During the stay of this .large quantity of 

 air inspired in the lungs, which seem to have been greatly 

 expanded for the purpose, it divests itself of the oxigen it 

 contains, so that the moment of expiration is to a certain 

 degree arbitrary. 

 Difference of Most of the circumstances that have been mentioned will 



structure be- -^^ exhibited at large in the following examination of the 



t ween these and ° v 



fishes respiratory apparatus of fishes. AH vertebrate animals, 



that breathe air alone, have only two apertures in the 

 lower part of the mouth. One, intended for the passage of 

 Cood, is the cesophagus : the other, admitting the passage of 

 the air into and out of the lungs, is the glottis. Fishes on 

 the contrary, without exception, have the throat, perfo- 

 rated with four, six, or seven large apertures on each side, 

 beside the passage leading to the stomach. These apertures 

 supply the place of the glottis; and are commonly furnished 

 on their edges with fringed appendages, which fit into each 

 olher, but are capable of being separated to a greater or 

 less distance, to strain the water that passes through them. 

 Kach of these apertures directs the liquid into a space bor- 

 dered by two bronchial laminae, which are covered by a 

 membrane, in the substance of which are commonly solid 

 radii capable of motion, and a thin plate called the oper- 

 culum, which can be moved so as to form a chink on each 

 side. This aperture alone furnishes an issue to the water, 

 which by staying on the bronchiae iias served for the respi- 

 ration of the fish. 

 Manner in When a fish respires, it begins by opening the mouth, to 



^hich fishes suck in the liquid in which it is immersed. The water inir 

 mediately comes in to fill the vacuum produced by the sepa- 

 ration and protraction of the labial bones. The bottom of 

 the throat is then raised, and the bronchial apertures fre- 

 quently closed. But presently the lips approach each 



other \ 



respire. 



