88 SCIENCE PRIMERS, ^ [§ vi. 



blood within the capillaries to the flesh outside; 

 from the flesh outside the carbonic acid has passed to 

 the blood within the capillaries. And this goes on all 

 over the body. Everywhere the flesh is breathing 

 blood, is breathing gas dissolved in the blood, just as 

 a fish breathes water, i,e. breathes the air dissolved in 

 the water. ; , 



Goes on everywhere with one great exception. 

 There is one great artery, >yith its branches, in which 

 blood is not bright, scarlet, arterial, but dark, purple, 

 venous. There are certain great veins in which the 

 blood is not dark, purple, venous, but bright, scarlet, 

 arterial. You know which they are. The pulmonary 

 artery and the pulmonary veins. The blood in the 

 pulmonary veins contains more oxygen and less 

 carbonic acid than the blood in the pulmonary artery. ' 

 It has lost carbonic acid and gained oxygen, 

 as it passed through the capillaries of the 

 lungs. 



38. What are the lungs? As you saw them in the 

 rabbit, or as you may see them in the sheep, they are 

 shrunk and collapsed. We shall presently iearn why. 

 But if you blow into them through the windpipe, 

 which divides into branches, one for either lung, you 

 can blow them out ever so much bigger. They are 

 in reality bladders which can be filled with air, but 

 which, left to themselves, at once empty themselves 

 again. , ' '^ 'j 



They are bladders of a peculiar constmction. 

 Imagine a thick short bush or tree crowded with 

 leaves; imagine the trunk and the branches, small 

 and great, down to the veriest twigs, all hollow ; 

 imagine further that the leaves themselves were little 



