Chap. XIII] RESPIRATORY SYSTEM 247 



In addition the expired air contains a certain amount of organic 

 matter which comes principally from the mouth and particles of 

 food left in the mouth. 



Ventilation. — Since at every breath the external air loses ox;y'gen, 

 gains carbon dioxide, and a certain amount of organic matter, it 

 was formerly taught that the general discomfort, headache, and 

 languor that result from staying in a badly ventilated room were 

 due to the increase in carbon dioxide, and the poisonous effects of 

 the organic matter. The results of many experiments seem to 

 prove that people can become so accustomed to a low percentage 

 of oxygen and a high percentage of carbon dioxide that they 

 suffer little discomfort, and the organic matter is not poisonous, 

 though when present in any amount it causes disagreeable odors 

 and makes the air stuffy. It is now thought that the injurious 

 effects of remaining in a badly ventilated room are due to inter- 

 ference with the heat-regulating mechanism of the body. The air 

 is heated to a high temperature and becomes saturated with watery 

 vapor. Both of these conditions prevent loss of heat from the 

 body and produce a fever temperature. Even when these condi- 

 tions exist it has been found that moving the air, i.e. keeping it 

 in circulation even without renewing it, has a stimulating effect 

 and lessens discomfort. Because of these facts we are now taught 

 that the maintenance of proper air conditions must be based (1) on 

 the normal composition of the air as regards ox;^^gen and carbon 

 dioxide, (2) on the temperature, and (3) on the degree of humid- 

 ity. Any system of ventilation that is based solely on the first 

 condition, and neglects to take into account the second and third 

 is sure to prove unsatisfactory. 



ABNORMAL TYPES OF RESPIRATION 



Dyspnoea. — The word dyspnoea means difficult breathing. 

 It is caused by (1) an increase in the percentage of carbon dioxide 

 in the blood, (2) a decrease in the ox;\^gen, and (3) any condition 

 that stimulates the sensory nerves and causes pain in the lungs. 



Hyperpnoea. — The word hyperpnoea means excessive breathing 

 and is applied to the initial stages of dyspnoea, when the respira- 

 tions are simply increased. 



Apnoea. — The word apnoea means a lack of breathing. 



