CHANGES OF BLOOD IN THE L UNGS. 257 



Stoves in a room unless constantly supplied with fresh 

 air from without dry its air to an unwholesome extent. If 

 no appliance for providing this supply exists in a room it 

 can usually be got, without a draught, by fixing a board 

 about four inches wide under the lower sash and shutting 

 the window down on it. Fresh air then comes in by the 

 opening between the two sashes and in a current directed 

 upwards, which gradually diffuses itself over the room 

 without being felt as a draught at any one point. In the 

 method of heating by direct radiation the apparatus .em- 

 ployed provides of itself no means of drawing fresh air into 

 a room, as the draught up the chimney of an open fire- 

 place or of a stove does; and therefore special inlet and 

 outlet openings are very necessary. Since, fortunately, 

 few doors and windows fit quite tight, fresh air gets into 

 closed rooms in tolerable abundance for one or two in- 

 habitants. 



Changes undergone by the Blood in the Lungs. These 

 are the exact reverse of those exhibited by the breathed air 

 what the air gains the blood loses, and vice versa. The 

 blood loses heat, and water, and carbon dioxide, in the pul- 

 monary capillaries, and gains oxygen. These gains and 

 losses are accompanied by a change of color from the dark 

 purple which the blood exhibits in the pulmonary artery, 

 to the bright scarlet it possesses in the pulmonary veins. 



Why the Blood changes its Color as it flows through 



What are stoves apt to do? Point out a good way of supplying 

 fresh air to a room warmed by a stove? What are especially impor- 

 tant in a room heated by "direct radiation"? Why is it fortunate 

 that doors and windows do not fit air-tight? 



What is the relationship between the gains and losses of blood in 

 the pulmonary circulation, and the losses and gains of the breathed 

 air? What does the blood lose as it flows through the lungs? What 

 does it gain? What change in the color of the blood accompanies 

 these sains and losses? 



