BLOOD SYSTEM. 



375 



Again, fishes are all throat breathers. They must be, 

 for they must force the water outward through their 

 gills. Now, amphibians also have gills, at least in early 

 life, and therefore must be throat breathers, at least in 

 early life. When they become lung breathers they 

 retain the throat method, and now force air down to the 

 lungs, instead of water out through the gills. Reptiles 

 never have this mode of breathing. 



Amphibians were once regarded as an order of 

 reptiles, but now they are recognized as not only a 

 different class, but as more nearly related to fishes than 

 to reptiles. 



SECTION III. 

 Blood Cir delation — Vertebrates. 



Having given the general morphology of respira- 

 tory organs among vertebrates, we are now prepared 

 to give the course of circulation, and show how it is 

 controlled by the necessity of aeration. We take first 

 mammals, and, as usual, man as the type. 



Circulation in Man. — Circulation means a going 

 round in a circle and a coming back to the starting- 

 point. A machine for circulation implies (i) a pumping 

 organ and a system of pipes. In the animal body the 

 pumping organ is the heart and the pipes are the 

 blood vessels. It implies (2) two kinds of pipes, 

 one carrying away, efferent; the other bringing back, 

 afferent. In the animal body these are called arteries 

 and veins. There is, however, a third kind, connecting 

 these with one another, called the capillaries. The 

 arteries carry to the tissues, the capillaries among the 

 tissues, and the veins back again from the tissues. The 

 arteries carry blood to the work, the veins bring it 

 back after the work, but the work itself is done in the 

 capillaries. (3) In a good machine the pump must have 



