Circulation in Fishes. 21 



commencement is swollen into an aortic bulb, from 

 which come off at least three pairs of branchial or 

 gill arteries ; these pass in the form of arches, right 

 and left, to the gills, and there break up into fine 

 branches in the soft, fringe-like folds. Here the 

 blood, being exposed to the air dissolved in the water, 

 absorbs oxygen and gives out carbonic acid, and is 

 thus purified. 



The purified blood, returning from the gills by the 

 branchial veins , enters the dorsal vessel or aorta, 

 which sends it to the different organs of the body to 

 supply the viscera. 



The blood of fishes is generally of the same 

 temperature as the medium wherein they live, or only 

 slightly warmer, and hence it usually feels cold to 

 the touch. It contains corpuscles, or little micro- 

 scopic bodies, of an oval shape and with a central 

 nucleus. 



Though the respiration of fishes is accomplished 

 by means of the air which is dissolved in water, yet 

 it is supplemented in some of them by direct exposure 

 of the gills to the atmosphere, and some fishes are 

 killed if prevented from rising to the surface. 



In most fishes there is a large sac filled with air, 

 placed beneath the vertebral column at the anterior 

 part of the body cavity, and communicating by a duct 

 with the digestive organs. This is called the swimming- 

 bladder, or the air-bladder, and, by expanding or 

 compressing it, the fish can rise or sink in the water. 

 This sac commences its existence in the embryo as 

 an outgrowth from the neck end of the alimentary 

 canal. 



