THE RESPIRATORY CHANGES IN THE TISSUES. 367 



Thus, on the one hand, the muscle seems to have the property of taking 

 up and fixing in some way or other the oxygen to which it is exposed, of 

 storing it up in its own substance in such a condition that it cannot be re- 

 moved by simple diminished pressure (so that the pressure of oxygen in the 

 muscular substance may be considered as always nil} and yet has not entered 

 into any distinct combination which we can speak of as an oxidation, but is 

 still available for such a purpose. On the other hand, the muscular substance 

 is always undergoing a decomposition of such a kind that carbonic acid is 

 set free, sometimes, as when the muscle is at rest, in small, sometimes, as 

 during a contraction, in large quantities. The oxygen present in this car- 

 bonic acid, as an oxidation product, comes from the previously existing store 

 of which we have just spoken. The oxygen taken in by the muscle, what- 

 ever be its exact condition immediately upon its entrance into the muscular 

 substance, in the phase which has been called " intra-molecular," sooner or 

 later enters into a combination, or perhaps we should rather say, enters into 

 a series of combinations. We have previously urged ( 30) that all living 

 substance may be 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 process, and 

 that through those processes it is made part of complex decomposable sub- 

 stances whose decomposition ultimately gives rise to the carbonic acid ; but, 

 as 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. 



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 

 gas freshly generated. 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 oxy- 

 gen 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. 



302. Our knowledge of the respiratory changes in muscle is more com- 

 plete 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 show 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 considerable quantity of carbonic acid. 

 And we may probably assert with safety with regard to all the tissues, that 



