136 PHYSIOLOGY AND HEALTH. 



returns back to the heart from the lungs, it is one homoge- 

 neous compound ; it has lost its carbonic acid and water, and 

 received a supply of oxygen, which now pervades the fluid. 



299. The difference of the color of the blood is seen in 

 the veins of the hand and arm, which^appear to be blue, 

 while the flushed cheek is of scarlet red, from the presence 

 of the blood in the arteries of the skin. When a person is 

 bled from the arm, the vein is opened ; the blood that flows 

 is venous, and of course purple. The inexperienced mistake 

 this natural color for the effect of disease, and often remark, 

 " that the blood is very black ; the patient needed bleeding 

 to be relieved of such dark impurities." 



300. This change of the blood is effected in the lungs , 

 of course, there must be a corresponding change in the air. 

 The oxygen which the blood receives is the oxygen of the 

 air ; and the carbonic acid and water which are thrown off 

 from the blood are mingled with the air. The air is therefore 

 changed by this process, and to this extent. During a state 

 of repose, the air at each respiration has a little more than 

 four per cent, of its volume of carbonic acid gas added to 

 it, and the oxygen is diminished in proportion necessary 

 to form this acid. If the same air be respired over and 

 over several times, all the oxygen is consumed, and the 

 air becomes loaded with carbonic acid gas. 



301. Sir Humphry Davy enclosed one hundred and sixty 

 cubic inches of air in an oiled silk bag, and breathed this 

 for the space of one minute. In this time, he made nineteen 

 respirations. On examination of the air, he found that 

 nearly one half '(if if ) of the oxygen was consumed, and its 

 place supplied by 15.2 inches of carbonic acid, which had 

 been generated in the blood-vessels, and given out from the 

 lungs in one minute.f 



302. The quantity of carbonic acid gas which is found 

 in the air that has been breathed, varies in different circum- 

 stances, and in different conditions of the human body. 



* Lehmann's Physiological Chemistry, Vol. II. 436. 

 f Mullcr's Physiology, p. 295. 



