CIRCULATION IN WARM BLOODED ANIMALS. 135 



ths blood to the gills, g, h. There it is aerated, or ex- 

 posed to the air, which the water contains. It is then 

 collected by the branchial veins i, which, instead of 

 carrying it directly to the heart, as in man, unite into a 

 single large trunk , which passes down the back, and 

 performs the office of the aorta, by distributing it to the 

 different parts of the body. The circulating fluid is 

 then conveyed to the auricle d, by the large vein c, which 

 answers to the vena cava. The blood then passes into 

 the ventricle and begins its circulation as before. 



In Fishes the heart is exceedingly small when com- 

 pared to that of other animals of the same bulk, its 

 weight being only to that of the body, as 1 to 351, or 

 even 1 to 768 in the different species ; while in man 

 the weight of the heart is to that of the body, as 1 to 

 about 160. 



The proportion of blood in this class is also very 

 small, and the vessels few in number. The quantity of 

 oxygen likewise, which Fishes obtain, being only that 

 contained in the air of the water, must be exceedingly 

 minute. Hence it is, that their flesh is white, presenting 

 a remarkable contrast to the red color of that of ani- 

 mals belonging to the higher orders, as quadrupeds and 

 man. 



CIRCULATION IN WARM BLOODED ANIMALS. 



In proportion as animals rise in the scale of organiza- 

 tion and capacity, so does the complexity of the appa- 

 ratus for carrying on the circulation increase. 



Amphibious animals and fish, as just shown, are pro- 

 vided with a single auricle, and a single ventricle only. 

 But in all warm blooded animals there are two auricles 

 and two ventricles, and two systems of circulation. In 

 the first, the heart is single ; in the last, it is double ; 

 one being for the circulation through the lungs, called 

 the pulmonic ; the other for the general circulation, 

 called the systematic. The pulmonic is on the right 

 side ; the systematic on the left. 



What office does the heart perform in the fish 7 What is said of the 

 size of the heart in fish 1 What is the difference between the heart of a 

 fish and that of a quadruped 1 



