CIRCULATION IN WARM-BLOODED ANIMALS. 131 



the 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 car- 

 rying it directly to the heart, as in man, unite in a single 

 large trunk a, which passes down the back, and per- 

 forms 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. 



388. In fishes, the heart is exceedingly small when 

 compared 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. 



389. 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 con- 

 tained in the air of the water, nfust be exceedingly minute. 

 Hence it is, that their flesh is white, presenting a remark- 

 able contrast to the red color of that of animals belonging 

 to the higher orders, as quadrupeds and man. 



CIRCULATION IN WARM-BLOODED ANIMALS. 



390. In proportion as animals rise in the scale of organi- 

 zation and capacity, so does the complexity of the appara- 

 tus for carrying on the circulation increase. 



391. Amphibious animals and fish, as just shown, are 

 provided 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 sys- 

 tematic on the left. 



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

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

 fish, and that of a quadruped ? 



