BREATHING.] , . . PHYSIOLOGY. 87 



in the arteries arterial blood, and the dark purple 

 blood which is found in the veins venous blood. 



What is the difference between the two? If you 

 were to pump away at some arterial blood, as you did 

 at the water in which you put your fish, you would be 

 able to obtain from it some air, or, more correctly, some 

 gas ; a great deal more gas, in fact, than you did from 

 the water. A pint of blood would 5deld you half a pint 

 of gas. This gas you would find on examination not 

 to be air, i.e. not made up of a great deal of nitrogen 

 and the rest oxygen. (Chemical Primer, Art 9.) There 

 would be very little nitrogen, but a good deal of 

 oxygen, and still more carbonic acid. 



If you were to pump away at some venous blood 

 you would get about as much gas, but it would be 

 very different in composition. The little nitrogen 

 would remain about the same, but the oxygen would 

 be about half gone, while the carbonic acid would be 

 much increased. 



This, then, is one great difference (for there are 

 others) between venous and arterial blood, that while 

 both contain, dissolved in them, oxygen, 

 nitrogen, and carbonic acid, venous blood 

 contains less oxygen and more carbonic acid 

 than arterial blood. ^ 



37. In passing through the capillaries on its 

 way to the vein, the blood in the artery has 

 lost oxygen and gained carbonic acid. Where 

 has the oxygen gone to ? Whence comes the carbonic 

 acid? To and from the islets of flesh between the 

 capillaries, to the bloodless muscular fibre or bit of 

 nerve or skin which the blood-holding capillaries 

 wrap round. The oxygen has passed from the 



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