RESPIRATION. 101 



needed. Doors and windows cannot be left open with im- 

 punity. During recess, or whenever a room is unoccupied 

 for a short time, they should be thrown open for a few 

 minutes, but, strictly speaking, the air should be warmed 

 before it is brought into the room, and the foul air drawn 

 off through openings in the walls. These openings should be 

 large, and placed low down, so as not to carry off the newly 

 admitted fresh air, which, being warm, rapidly ascends. 



Various systems of ventilation exist from which we may 

 choose, but in making a selection expert advice should be 

 followed. In this connection it may be stated that dwellings 

 should never be built over soil which is polluted by organic 

 matter, such as decaying vegetable matter. Decomposition 

 of animal and vegetable material is attended with the pro- 

 duction of poisonous gases. When a dwelling is placed over 

 such matter, foul gases will rise and penetrate the building. 

 In winter, especially, the heat of the house tends to draw 

 those gases through the lower floor, when they will ascend 

 into all parts of the house. 



10. How the Heat of the Body is kept up. Body- 

 heat is generated or produced by the oxidation or burning 

 of certain materials in the body. The heart and lungs are 

 not stoves for providing heat and distributing it to the body, 

 but by their combined influence the red corpuscles of the 

 blood are made capable of producing changes in the system 

 which result in heat. They are the oxygen carriers and 

 give it out to the tissues, where it unites with certain 

 elements in the form of combustion. The mysteries of life 

 have not so far been revealed sufficiently to show exactly 

 in what proportion certain materials form the fuel. Not 

 unlikely it is to some extent the worn-out material, but 

 chiefly the refuse of nutrition. The evidences of economy 

 shown in the human body, and in its various functions, 

 support the belief that it is the remnants of the nutritious 



