CHAP, ii.] RESPIRATION. 611 



regarded as incessantly undergoing changes of a double kind, 

 changes of building up and changes of breaking down. In the 

 end-products of the breaking down, in the carbonic acid given 

 out by muscle for instance, we can recognize an oxidation product ; 

 but we do not know exactly at what stage or exactly in what 

 way the oxygen is combined with the carbon. We may imagine 

 that the oxygen, as it comes from the blood, is caught up so to 

 speak by, and disappears in, the building up processes (forming, 

 possibly at the very beginning, with some constituent of the 

 muscular substance a combination like to but firmer and more 

 stable than its combination with haemoglobin) and that through 

 those processes it is made part of complex decomposable sub- 

 stances whose decomposition ultimately gives rise to the carbonic 

 acid ; but, so far as actual knowledge goes, we cannot as yet trace 

 out the steps taken by the oxygen from the moment it slips from 

 the blood into the muscular substance 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. 



But if the oxygen-pressure of the muscular tissue be thus 

 always nil, oxygen will be always passing over from the blood - 

 corpuscles, in which it is at a comparatively high pressure, through 

 the plasma, through the capillary walls, the lymph-spaces and the 

 sarcolemma, into the muscular substance, and as soon as it arrives 

 there will be in some manner or other hidden away, leaving the 

 oxygen-pressure of the muscular substance once more nil. Con- 

 versely, the carbonic acid produced by the decomposition of the 

 muscular substance will tend to raise the carbonic acid pressure of 

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

 acid will pass from the muscle into the blood, its place in the 

 muscular substance being supplied by freshly generated supplies. 

 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 

 carbonic acid and other matters ; but it does consist in the 

 assumption and storing up of oxygen somehow or other in its 

 substance, in the building up by help of that oxygen of explosive 

 decomposable substances, and in the carrying out of decompositions 

 whereby carbonic acid and other matters are discharged first into 

 the substance of the muscle and subsequently into the blood. 



359. 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 that 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, 



