210 EESPIKATIOX. 



capillaries, whence, being -with the rest of the blood arte- 

 rialized, it is carried to the pulmonary veins and left side 

 of the heart. 



Changes of the Air in Respiration. 



By their contact in the lungs the composition of both air 

 and blood is changed. The alterations of the former being 

 manifest, simpler than those of the latter, and in some 

 degree illustrative of them, may be considered first. 



The atmosphere we breathe has, in every situation in 

 which it has been examined in its natural state, a nearly 

 uniform composition. It is a mixture of oxygen, nitrogen, 

 carbonic acid, and watery vapour, with, commonly, traces 

 of other gases, as ammonia, sulphuretted hydrogen, etc. 

 Of every 100 volumes of pure atmospheric air, 79 volumes 

 (on an average) consist of nitrogen, the remaining 2 1 of 

 oxygen. The proportion of carbonic acid is extremely 

 small; IO,OOO volumes of atmospheric air contain only 

 about 4 or 5 of carbonic acid. 



The quantity of watery vapour varies greatly, according 

 to the temperature and other circumstances, but the at- 

 mosphere is never without some. In this country, the 

 average quantity of watery vapour in the atmosphere is 

 I '40 per cent. 



The changes produced by respiration on the atmospheric 

 air are, that, I, it is warmed; 2, its carbonic acid is in- 

 creased ; 3, its oxygen is diminished ; 4, its watery vapour 

 is increased ; 5, a minute amount of organic matter and of 

 free ammonia is added to it. 



I. The expired air, heated by its contact with the in- 

 terior of the lungs, is (at least in most climates) hotter 

 than the inspired air. Its temperature varies between 97 

 and 99J, the lower temperature being observed when 

 the air has remained but a short time in the lungs, rather 

 than when it is inhaled at a very low temperature ; for 

 whatever the temperature when inhaled may be, the air 



