38 ANATOMY AND PHYSIOLOGY 



is collected by numerous vessels, which ultimately 

 unite in one single artery, coursing its way down 

 the body, under the protection of the vertebral 

 column. This, unlike the vascular apparatus of 

 any species of warm-blooded creatures, takes up- 

 on itself the action of a heart, — propelling the 

 blood, by successive pulsations, to the remotest 

 parts of the body. 



It is almost unnecessary to remark, that animals 

 breathing air, have a double heart; indeed man, 

 and not only man, but all animals that breathe at- 

 mospheric air, have two hearts, but for the sake 

 of economising the room — for the purpose of 

 packing the parts to the best advantage, the two 

 are united ; — hence they occupy less space than 

 would otherwise be the case, were they placed at 

 different parts of the body. One heart throws all 

 the blood, which has once been the round of circula- 

 tion, into the lungs; — here its office ceases. 

 The blood is collected from the lungs, where the 

 first heart left it, and gradually poured into the 

 other, or left heart, which forces the blood through 

 every artery in the body. Both hearts are forcing 

 pumps, and both have valves. The much ad- 

 mired invention of the ship-pump, with three 

 valves, is only an imitation, and a poor one too, of 

 the semilunar valves of the pulmonary artery. 

 Reptiles and fishes, having only one heart, the 



