xi EESPIRATOKY EXCHANGES 395 



also admits that " the oxygen combines in some way with the 

 tissues so as to constitute a provision for use when the animal is 

 unable to procure it from without." He based this assumption 

 particularly on the fact that muscle absorbs more oxygen during 

 rest, and spends more during activity, as though it accumulated 

 reserves to expend lavishly when need arises. Nothing definite is 

 known, however, as to the nature of the probable combinations 

 formed by oxygen with the different materials for building up 

 muscles and other tissues, or of the intermediate anabolic and 

 katabolic forms, through which it passes in combining with carbon 

 into carbonic acid. " The whole mystery of life," says Foster, 

 " lies hidden in the story of that progress, and for the present we 

 must be content with simply knowing the beginning and the 

 end." 



We know that carbonic acid is one of the ultimate products of 

 the katabolic processes, and that the variations in the amount 

 formed and eliminated by the tissues are, as Fano says, an 

 expression "of corresponding changes in the course of the 

 destructive processes. The assimilated oxygen on the contrary 

 enters, at least in part, into the molecular structure of our tissues, 

 is included in the series of synthetic processes, and may partially 

 be considered as an element which contributes to the anabolic or 

 constructive processes." We shall return to this argument in 

 treating of the metabolism or material exchanges of the body as a 

 whole. 



In regard to this subject of the respiratory gas exchanges 

 between the blood and the tissues, the facts observed by Pfliiger 

 and Strassburg, to the effect that the lymph, serous fluids, and 

 certain secretions (bile, urine, saliva, milk) formed within the 

 living tissues contain merely a trace of oxygen, and a comparatively 

 large amount of carbonic acid, are very remarkable: These authors 

 conclude that O 2 tension is low or practically nil in the tissues, 

 while the C0 2 tension on the contrary is high. 



The high tension of carbonic acid that prevails, according to . 

 recent researches, in living tissues is of especial significance, . 

 because it facilitates the dissociation of oxygen from oxyhaemo- 

 globin, and thus places at the disposal of the tissues the maximum 

 possible amount of the oxygen received from the blood. Bohr, 

 Hasselbach, and Krogh (1904) found that on bringing dog's blood, in 

 vitro, at 38 C. into the presence, simultaneously, of 2 at low tension 

 (5 mm. Hg), and of C0 2 at various tensions, the blood absorbs a 

 less degree of 2 than when it is in presence of 2 alone, and that 

 the decrease in absorption is proportional to the amount of C0 2 

 simultaneously present. This influence is much less felt if the O 2 

 tension is progressively raised, as is clearly shown on the diagram 

 (Fig. 172). 



The physiological value of this fact will be readily appreciated. 



