Chap, ii.] KESPIKATIOK 469 



enters into a series of combinations. We have previously 

 urged (§ 30) that all living substance may be regarded as inces- 

 santly undergoing changes of a double kind, changes of build- 

 ing 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, and that 

 through those processes it is made part of complex decompos- 

 able substances whose decomposition ultimately gives rise to 

 the carbonic acid ; but, so far as actual knowledge goes, we 

 cannot as jet trace out the steps taken by the oxygen from the 

 moment it slips from the blood into 5 the muscular substance to 

 the moment when it issues united with carbon as carbonic 

 acid. 



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. Conversely, the carbonic acid produced bj^ 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 sup- 

 plied by freshly generated supplies. There will always in fact 

 be a stream of oxygen from the blood to the muscle and of car- 

 bonic 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 assumj^tion 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. 



§ 290. 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, bile, urine, and milk contain a mere 

 trace of free or loosely combined oxygen, but a very consider- 



