BREATHING A VEIN. 79 



the body ; in other words, the left auricle and ven- 

 tricle are entirely wanting the branchial veins 

 changing into arteries without any muscular en- 

 velope. The muscular apparatus of their circula- 

 tion is composed of the auricle, the ventricle, and 

 the bulb of the pulmonary artery, and the auricle 

 itself is preceded by a large sinus, in which all the 

 veins of the body terminate, a structure which 

 gives rise to four cavities separated by restrictions, 

 into which the blood must flow in its progress from 

 the body to the gills or branchiae. 



SECTION XIV. 



The Respiration of Fishes. 



IT is by an almost infinite subdivision of the 

 vessels over the surface of these branchiae, that the 

 blood of fishes becomes subjected to the influence 

 of an ambient fluid. This fluid is of course water, 

 which is made to flow incessantly between the 

 leaves of the branchiae, by the movement of the 

 jaws, and of the opercular and hyoidean apparatus. 

 This mode of respiration is equally necessary to 

 fishes as the direct respiration of air is to other 

 animals ; but the action of water on the blood is 

 much more feeble than that of air. If the portion 

 of air which is held in solution, or mingled with 

 the water, be expelled by ebullition, fishes will not 

 live, and many species are obliged to rise frequently 





