CHAP. IL] RESPIRATION. 581 



bile, urine, and milk contain a mere trace of free or loosely 

 combined oxygen, but a very considerable quantity of carbonic 

 acid. And we may probably assert with safety with regard to all 

 the tissues that in the tissues themselves, in the lymph which 

 bathes their lymph-spaces, and in the secretions which some of 

 them pour forth free oxygen is either wholly absent or so scanty 

 that their oxygen-pressure may be regarded as nil, while carbonic 

 acid is so abundant that the pressure of carbonic acid in them 

 may be regarded as exceeding that of venous blood. An excep- 

 tion seems to be presented by the case of the lymph flowing along 

 the larger lymphatic vessels, for in this the amount of carbonic 

 acid, while usually higher than that of arterial blood, is lower 

 than that of the general venous blood ; but this probably is due 

 to the fact that the lymph in its passage onwards is largely 

 exposed to arterial blood in the connective tissues and in the 

 lymphatic glands, where the production of carbonic acid is slight 

 as compared to that going on in muscles. All the facts point to 

 the conclusion, that it is the tissues, and not the blood, which 

 become primarily loaded with carbonic acid, the latter simply 

 receiving the gas from the former by diffusion, except the (pro- 

 bably) small quantity which results from the metabolism of the 

 blood-corpuscles; and that the oxygen which passes from the 

 blood into the tissues is at once taken up and placed under such 

 conditions that it is no longer removable by diminished pressure. 



In further support of this view may be urged the fact that if, in 

 a frog, the whole blood of the body be replaced by normal saline 

 solution, the total metabolism of the body is, for some time, 

 unchanged. The saline medium is able owing to the low rate of 

 metabolism, and large (cutaneous) respiratory surface of the 

 animal, to supply the tissues with all the oxygen they need, and to 

 remove all the carbonic acid they produce. It is difficult to 

 believe that, in such an experiment, the oxidation took place in 

 the saline solution itself while circulating in the blood vessels and 

 tissue-spaces of the animal. 



We may add, that the oxidative power which the blood itself 

 removed from the body is able to exert on substances which are 

 undoubtedly oxidized in the body is so small that it may be 

 neglected in the present considerations. If grape-sugar be added 

 to blood, or to a solution of haemoglobin, the mixture may be kept 

 for a long time at the temperature of the body, without undergoing 

 oxidation. Even within the body a slight excess of sugar in the 

 blood over a certain percentage wholly escapes oxidation, and is 

 discharged unchanged. Many easily oxidized substances, such as 

 pyrogallic acid, pass largely through the blood of a living body 

 and are discharged in the urine without being oxidized ; though 

 perhaps in some of these cases, what appears to be an absence of 

 oxidation is really an oxidation followed by a subsequent equiva- 

 lent reduction taking place in the urine or elsewhere. The organic 



