840 SPECIAL PHYSIOLOGY. 



phenomenon. The production of carbonic acid is one of the ultimate 

 results. It is supposed by some, that here a repetition of the false or 

 moist diffusion process may take place, oxygen passing from the blood 

 in the capillaries to the substance of the tissues and glands, whilst 

 carbonic acid passes from them back into the blood. This interchange 

 of the two gases at the systemic capillary circulation, constitutes the 

 parenchymatous respiration, which, so far as the blood is concerned, 

 is exactly the reverse of the pulmonary respiration; for in the former, 

 the blood loses oxygen and gains carbonic acid, whilst in the latter, it 

 loses carbonic acid and gains oxygen. This consumption of oxygen 

 especially by the nervous and muscular tissues, and its combination 

 with their substance, are said to explain the so-called stimulating effect 

 of oxygen upon those tissues, when they are actively engaged in their 

 special offices in the living economy. The quantity of oxygen consumed, 

 and of carbonic acid evolved, in any given case, are found to be pro- 

 portioned to the degree of activity of the nervous and muscular struc- 

 tures. The muscles especially, require a large supply of oxygen for 

 their nutrition, but more for their effective action; the blood returning 

 from a muscle at rest, as we have seen, contains 7.5 vols. per cent, of 

 oxygen, but when in exercise, only 1.25 vols.; whereas arterial blood 

 contains 17 vols. The prepared muscles of a frog, of course deprived 

 of circulating blood, continue to absorb oxygen, and to give off carbonic 

 acid, so long as their contractility is manifested. (G. Liebig.) Whe- 

 ther during life, this denutritive oxidation is entirely completed in the 

 tissues, and the resulting carbonic acid is then transmitted from them, 

 to the returning systemic blood, or whether some intermediate products 

 of disintegration enter the blood, and are oxidated therein, or, lastly, 

 whether both varieties of this combustive process takes place during 

 the ordinary nutritive changes in the tissues, is not well known. That 

 substances properly belonging to the blood are also oxidated within 

 the systemic capillaries, cannot be denied ; they are probably fatty 

 matters and carbhydrates, or their derivatives, introduced into the 

 blood from the food, constituting the so-called respiratory food ; but it 

 has hitherto been supposed that these are quite as actively oxidized 

 in the lungs, and in the arterial blood-current generally, as in the sys- 

 temic capillaries. But many have believed, and recent researches ap- 

 pear to show, that although the nutritive processes of the muscular 

 tissue demand oxygen for the removal of disintegrated albuminoid and 

 other materials, yet that in the active contraction of muscle, or in the 

 development of animal motion, it is not, as was supposed, the muscular 

 substance which wastes, by being more actively oxidized, but, rather, 

 that some combustible substances in the blood, of the nature of respi- 

 ratory food, either fatty matters or carbhydrates, then undergo oxida- 

 tion, that this chemical action yields the carbonic acid formed in the blood 

 of muscles, and that it is at once the source of the motive power exer- 

 cised by the muscles and of the heat evolved in the system. (See the 

 Section on Animal Dynamics.) 



In conclusion, then, it appears that the oxygen taken in during res- 

 piration, combines, first, with the tissues during their action and nutri- 

 tion, especially with the nervous and muscular tissues; secondly, with 



