516 THE RESPIRATORY EXCHANGE 



oxygen, not to an increase of carbon dioxide, not to any toxin 

 discharged from the human body, but to the absence of free ven- 

 tilation, to the warm and humid atmosphere, which reduces the 

 respiratory exchange and metabolism of the inmates and renders 

 them less resistant to the attacks of micro-organisms. The danger 

 to health appears to lie in the massing of numerous dwellings upon 

 a small area, which, owing to the exigencies of water supply, is 

 often situated in a valley ; there are doubtless many other factors, 

 but temperature, moisture, and wind are the ones which have a 

 marked effect upon respiratory exchange. 



The power of adaptation must not be overlooked. It is known 

 that men and animals become " seasoned " after living for some 

 time in a climate very different from that in which they were bred. 

 This has also been proved by experiments upon the respiratory 

 exchange ; the first marked change of weather is the most felt, 

 and produces the greatest effect upon the respiratory exchange. 

 The body by practice improves its power of regulating its tempera- 

 ture on exposure to heat and cold, and thus the physiological 

 resistance to changes of temperature can be maintained by train- 

 ing. The idea that the danger of " colds " can be warded off by 

 the maintenance of an equable temperature in living-rooms and 

 bedrooms is widely prevalent, but is erroneous, and in practice 

 leads to deterioration and defeats its own aim. Weaklings and 

 debilitated subjects need artificial conditions ; but rules of health 

 should be directed to the survival of the fit, not of the unfit. 



Exchange of Gases between the Blood and Air. The gaseous 

 exchange which is in constant progress between the blood and 

 the air may be studied in two ways : the changes in the composi- 

 tion of either the blood or the air during its passage through the 

 lungs may be determined. The former method involves a com- 

 parison of the gases in the venous blood of the right side of the 

 heart with the gases of the arterial blood, the latter method com- 

 prises qualitative and quantitative analyses of the inspired and 

 expired air. The data so obtained give the gross results, but do 

 not yield information upon the important questions how and why 

 the gaseous interchange occurs. The air which is in contact with 

 the epithelium covering the pulmonary capillaries has not the 

 composition of the inspired air, for owing to the exchange of gases 

 it is poorer in oxygen, richer in carbon dioxide, and saturated with 

 aqueous vapour at the temperature of the body ; nor has it the 



