xi RESPIRATORY EXCHANGES 391 



salivary cells, in obedience to nervous impulses, secrete saliva in 

 defiance of a counter-pressure greater than that of the arterial blood 

 circulating in the gland (Ludwig). 



Haldane and Smith (1896) investigated the tension of oxygen 

 in the pulmonary blood by a method applicable to man. The 

 method is based on the fact already demonstrated by Haldane in 

 1895, that with simultaneous action of oxygen and carbonic oxide 

 on the blood, the amount of the carbonic oxide fixed by the blood 

 is proportional to the oxygen tension, so that it is possible from 

 the degree of absorption of carbonic oxide in the blood to calculate 

 the tension of the oxygen present. If a man or other animal is 

 made to breathe a gaseous mixture containing a small quantity 

 (exactly determined) of CO for a time long enough to make the 

 carbonic oxide content constant in the blood (measuring it at 

 intervals by small samples of blood taken from the subject), it is 

 possible from the degree, thus measured, of fixation of CO by the 

 blood, to calculate the O 2 tension that prevails in the pulmonary 

 blood. It results from these experiments that the 2 tension 

 in the blood of a man's lungs amounts to 26*2 hundred ths of an 

 atmosphere equal to 200 mm. Hg, a figure that is inexplicable on 

 the hypothesis of simple gas diffusion as the cause of the absorption 

 of oxygen in the lung. Identical results were obtained from 

 experiments on birds and mice (1897), by means of which these 

 authors were also able to show how want of oxygen acted as a 

 stimulus to the active absorption of this gas by the pulmonary 

 epithelium. 



With regard to the process of the elimination of C0 2 from the 

 lungs in the air of the pulmonary alveoli, Grandis (1900) called 

 attention to a new factor which had till then escaped the notice of 

 physiologists. 



It is known that in addition to C0 2 , a considerable amount of 

 water-vapour is eliminated by the expired air from the blood 

 plasma circulating through the lung (according to Loewy, alveolar 

 air contains about 6 per cent of aqueous vapour). The blood 

 accordingly undergoes a temporary increase of concentration 

 during .its passage through the lungs, which, by raising the C0 2 

 tension, must facilitate its expulsion into the alveolar air. 

 Grandis confirmed the importance of this fact by certain experi- 

 ments in vitro, in which he artificially increased the concentra- 

 tion of the blood, by adding strong solutions of sodium chloride 

 and sugar, with the effect of a prompt rise of tension in the gases 

 of the blood. 



On the ground of these experiments he thinks it probable that 

 in living animals also the greater concentration of the blood, on 

 evaporation of the water in the pulmonary alveoli, must facilitate 

 the expulsion of CO. 2 . The experiments of Grandis, however, show 

 that the greater concentration of the blood raises the tension, not 



