612 RESPIRATION OF THE TISSUES. [BOOK n. 



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 goes on very much 

 as before. 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 here call attention to the evidence in favour of the 

 view on which we are dwelling furnished by the behaviour of 

 certain easily oxidized substances when absorbed into the blood 

 from the alimentary canal or even when injected directly into 

 the blood ; they soon pass out by the urine either wholly or for the 

 most part unoxidized. In some of these instances, such as that of 

 pyrogallic acid, it may be that the substance is really oxidized but 

 subsequently undergoes, in the urine, an equivalent reduction. 

 But this does not apply to organic acids, such as citric, which even 

 when given in combination with alkaline bases, are only partially 

 oxidized, and when given as acids, not as salts, are hardly oxidized 

 at all. Did any large amount of oxidation take place in the 

 blood stream itself, we should expect that such substances as the 

 above would be oxidized during their transit through that blood 



