xi EESPIEATOEY EXCHANGES 397 



The gas exchanges of respiration are, however, more complicated, 

 as is readily seen on examining the changes induced in external 

 air by animal respiration. 



Expired air, in comparison with inspired air, presents the 

 following differences : 



(a) It contains 5 vols. per cent less of oxygen (according 

 to Vierordt 1 6*033 vols. per cent instead of 20*95 vols. per cent). 



(5) It contains a considerable amount of carbonic acid (from 

 3*3-5'5 vols. per cent, according to Vierordt). 



(c) According to Eegnault and Eeiset, and Seegen and Nowak, 

 expired air contains a slight excess of nitrogen, but whether this 

 is a waste product from the tissues owing to decomposition of 

 protein is doubtful, as already stated ; more probably it comes 

 from absorption in the blood of the excess nitrogen contained in 

 the intestinal gases. 



(d) It contains traces of free hydrogen, of marsh gas, of 

 ammonia, and of other gases of hitherto unknown composition. 

 It is certain that these substances arise partly from the absorption 

 of intestinal gases, partly from exhalations of putrefactive excreta 

 that clog the skin and integuments (Hermann, 1883), partly from 

 pulmonary exhalations which when collected and condensed in 

 sterilised water, and inoculated subcutaneously, have a toxic 

 action (Brown-Sequard and D'Arsonval). Formanek has recently 

 (1900) occupied himself exclusively with the question of the 

 toxicity of expired air. He found that air passed by the method 

 of Brown-Sequard and D'Arsonval through cages of animals 

 contained a really toxic substance, which was identified with 

 ammonia. This ammonia was not, however, derived from the 

 expired air, but from the decomposition of urine and faeces within 

 the cages. When this source of impurity was eliminated, expired 

 air was obtained which had no toxic action. Formaiiek concluded 

 that no toxic substances were developed in the lungs of man and 

 other healthy animals. The sense of malaise which sensitive 

 persons experience in crowded rooms must arise reflexly, from 

 disturbance of thermal regulation, or the respiration of foul- 

 smelling substances. 



(e) The expired air is saturated or nearly so with the aqueous 

 vapour exhaled along the respiratory passages. 



(/) It is partly deprived of the dust, and the germs or 

 sporules that are never absent from inspired air, and which are 

 arrested all along the respiratory tract by the moisture of its walls 

 these being clothed with vibratile epithelia, whose function is 

 to expel them, along with the mucus secreted by the muciparous 

 cells. 



(g) Its temperature is approximately that of the body 

 (35-36 C.), consequently its volume when it issues from the 

 respiratory passages exceeds that of the inspired air, to which the 



