164 AIR AND LIFE. 



that Brown-Sequard and d'Arsonval did not take pains enough to pre- 

 vent the contamination of the liquid, either by solid, and probably 

 living, particles of nasal or buccal origin, or by impurities belonging 

 to the apparatus and receiver in which condensation was effected. We 

 can not, therefore, accept their original statement although there is a 

 probability in favor of its truth. Further experiments are required 

 to settle the matter. 



III. — Biological Bole of the Chemical Constituents of the 



Atmosphere. 



Having now considered the constituents of the atmosphere, their 

 relative proportions in the aerial mixture, their mode of production and 

 distribution — that is, their mode of equilibration — and taking it as an 

 established fact that the composition of air varies but slightly, remain- 

 ing constant within the limits previously mentioned ; having also briefly 

 reviewed the part played by animate life m maintaining the composi- 

 tion of the atmosphere, we may now proceed to consider the chemical 

 and physical influence of the atmosphere on the life of organisms. 



For the sake of convenience and clearness, we shall begin with the 

 chemical influence, and review in turn the influence of each separate 

 constituent. 



The life-maintaining gas of atmosphere, par excellence, is, to all 

 appearances, oxygen — and we shall deal first with this element. 



That its presence in air is indispensable for the proper execution of 

 the respiratory functions is a fact familiar to all. Physiology has most 

 clearly demonstrated, for a century past, the great importance and use- 

 fulness of this gas. It is essential to respiration. Man consumes large 

 quantities of it. 1 



Inspired air, containing on the average 20 or 21 per cent of oxygen 

 by volume — expired air containing only 16 per cent — 4 per cent have, in 

 consequence, been absorbed by the organism, and in twenty-four hours 



1 It should be noticed that neither men nor animals ever breathe pure air, nor 

 can they do so uuder normal and natural circumstances. The reason is obvious. 

 The lungs are never totally emptied. Even after the deepest expiration, there 

 remains in the lungs and air passages a residue of air that can not be expelled 

 (owing to the anatomical impossibility of total pulmonary contraction), and 

 such air is vitiated and unfit for respiratory purposes. The next inspiration brings 

 a certain amount of pure air, but, as a matter of course, it mixes with the impure 

 residual air, and therefore becomes vitiated to some extent. The only parts 

 which receive strictly pure air are the superior air passages. At the end of expira- 

 tion they are full of impure air; but the very first result of inspiration is to return 

 all this impure air to the lungs, and to fill the air passages with pure air. A part of 

 this goes to the lungs, and all that remains in the nose, trachea, etc., is pure. All 

 mucous membranes have some respiratory functions, so that a proportion of this 

 pure air is used; but the most important of the respiratory organs is bathed in 

 a vitiated atmosphere, and one may truly say that neither men nor animals ever 

 breathe really pure atmospheric air. A very simple and ingenious experiment has 



