350 CHANGES IN THE TISSUES. [BOOK n. 



versely, the carbonic acid produced by the decomposition of the 

 muscular substance will tend to raise the carbonic acid tension of 

 the muscle until it exceeds that of the blood ; whereupon it will 

 pass from the muscle into the blood, its place in the muscular 

 substance being supplied by freshly generated carbonic acid. 

 There will always in fact be a stream of oxygen from the blood to 

 the muscle and of carbonic acid from the muscle to the blood. 

 The respiration of the muscle then does not consist in throwing 

 into the blood oxidizable substances there to be oxidized into car- 

 bonic acid and other matters; but it does consist in the assumption 

 of oxygen (as intra-molecular oxygen), in the building up by help 

 of that oxygen of explosive decomposable substances, and in the 

 occurrence of decompositions whereby carbonic acid and other 

 matters are discharged first into the substance of the muscle and 

 subsequently into the blood. We cannot as yet trace out the steps 

 taken by the oxygen from the moment it slips into its intra- 

 molecular position to the moment when it issues united with carbon 

 as carbonic acid. The whole mystery of life lies hidden in the 

 story of that progress, and for the present we must be content 

 with simply knowing the beginning and the end. 



Our knowledge of the respiratory changes in muscle is more 

 complete than in the case of any other tissue; but we have no 

 reason to suppose the phenomena of muscle are exceptional. On 

 the contrary, all the available evidence goes to shew that in all 

 tissues the oxidation takes place in the tissue, and not in the 

 adjoining blood. It is a remarkable fact, that lymph, serous fluids, 

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

 combined oxygen, and saliva or pancreatic juice a very small quan- 

 tity only, while the tension of carbonic acid in peritoneal fluid and 

 probably in the tissues of the intestinal walls is higher than that 

 of venous blood, and in bile and urine is still greater. The tension 

 of carbonic acid in lymph, while 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 these 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 in some combination, so 

 that it is no longer removable by diminished tension. 



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 solu- 

 tion, the total metabolism of the body is, for some time, unchanged. 

 The saline medium is able owing to the low rate of metabolism, 



