xi RESPIRATORY EXCHANGES 393 



IX. We know very little as yet about the physico-chemical 

 processes which complete the gas exchanges between the blood 

 circulating in the aortic capillaries, and the living cells of the 

 tissues, by the agency of the interstitial lymph (internal 

 respiration). 



It was formerly supposed that the transformation of arterial 

 into venous blood took place within the capillaries. But there are 

 well-ascertained facts which prove the blood, when extracted and 

 kept at body temperature, not to be the seat of any very energetic 

 oxidative phenomena. The oxygen it contains gradually dis- 

 appears, i.e. drawn arterial blood slowly becomes venous. On the 

 other hand, circulating arterial blood is known not to become 

 venous along the entire course of the aortic system, but only when 

 it is passing through the capillaries. This fact was explained on 

 the hypothesis that the intermediate products of tissue consumption, 

 which reach the arterial blood by the capillaries, consisted of 

 reducing substances, i.e. are avid of oxygen, which they rapidly 

 subtract from the oxyhaemoglobin. This supposition is no 

 longer admissible, since it has been demonstrated that only the 

 erythrocytes, not the blood plasma, nor the lymph of asphyxiated 

 animals (in which there must be an accumulation of many 

 reducing substances), are capable of chemically combining with 

 oxygen. Neither the blood plasma, then, nor the lymph, contain 

 reducing substances, since the latter do not pass into these fluids, 

 but arise in the living cells of the tissues which breathe in virtue 

 of their metabolism, i.e. they take up oxygen and give off carbonic 

 acid. It is therefore evident that the tissues are the seat of 

 internal respiration, and if the blood also breathes, however slowly, 

 this is in virtue of the mass of corpuscles which it contains, and 

 by which it functions as a tissue. 



Many direct proofs might be adduced as to thfr correctness of 

 this theory. When fragments of living tissue, particularly of 

 muscle, are dissolved in drawn arterial blood or in a solution of 

 oxyhaemoglobin, a rapid reduction follows, by which the oxyhae- 

 moglobin is promptly converted into haemoglobin in that portion 

 of the fluid which immediately surrounds the fragments (Hoppe- 

 Seyler). If benzylic alcohol or salicylic aldehyde are added to 

 the arterial blood drawn from the vessels, it does not oxidise to 

 any appreciable extent ; if, on the other hand, this blood, plus one 

 of the above substances, is circulated in the vessels of organs 

 recently extracted from a living animal (kidney or lung), consider- 

 able quantities of benzoic or salicylic acid are at once produced 

 by oxidation (Schmiedeberg). The oxidation performed by the 

 isolated surviving organ seems due to the action of special enzymes 

 contained within the cells of the tissues (Schmiedeberg, Jacquet, 

 Buchner). 



All tissues breathe (Paul Bert), but it is particularly in muscle 



