848 SPECIAL PHYSIOLOGY. 



time, and frogs as long as fourteen hours, without any injurious effects; 

 and nitrogen is equally harmless to frogs. (Collard de Martigny, Mul- 

 ler, Bergmann.) In the experiments on frogs, carbonic acid is exhaled, 

 for a time, in as great, or even greater quantity, than if the animals 

 had breathed atmospheric air; more is excreted in hydrogen than in 

 nitrogen. The total quantity of carbonic acid, so given off, is, how- 

 ever, limited, doubtless because no more oxygen can be absorbed ; the 

 lungs of the frog have been usually emptied of air, by compression, or 

 the use of the air-pump, and the only oxygen left in the animal was 

 that in the blood or the tissues. Some of the hydrogen and nitrogen 

 seems to be absorbed, but only in small quantity. The exhalation of 

 carbonic acid, in these cases, must be owing to the successive moist 

 and dry diffusion taking place into the hydrogen or nitrogen ; and the 

 diffusive force, in the former gas especially, would be much greater 

 than that into air. In, the case of Warm-blooded animals, only the 

 newly-born, or very young, can support such an experiment without 

 the rapid extinction of life; but they may live a short time and yield 

 carbonic acid to the artificial atmosphere of hydrogen or nitrogen. 

 Fully-grown Birds and Mammalia expire rapidly in pure hydrogen or 

 nitrogen ; the symptoms being instantaneous difficulty of respiration, 

 gasping, loss of muscular power, and, at the end of two or three min- 

 utes, cessation of the heart's action ; the lungs are found engorged 

 with venous blood. The animals, indeed, are asphyxiated from the 

 deprivation of oxygen, of which they require a larger and more con- 

 stant supply, in comparison with the -young of the same species, or 

 with Cold-blooded animals. That the nitrogen is not in itself inju- 

 rious, is obvious from the large proportion of it about four-fifths in 

 ordinary atmospheric air; and that the same is true of hydrogen, is 

 shown by the fact that, if this gas be mixed with oxygen in the same 

 proportions as nitrogen and oxygen exist in the air, animals live and 

 breathe in such a mixture without the least inconvenience. 



From all that has preceded it is evident, that of the two gases in the 

 atmosphere, the oxygen is the active ingredient in respiration ; for 

 nitrogen alone, as we have seen, causes suffocation. Hence oxygen 

 has been named vital air. Considered in reference to its office, it is a 

 supporter of life, or of the proper animal functions ; but, as regards 

 the body itself, it is a destructive, not a constructive, agent, operating 

 constantly in its disintegration and oxidation, in the various processes 

 of animal life. The proper medium for healthy respiration, is pure 

 atmospheric air, which contains, besides a minute trace of carbonic 

 acid, four-fifths of nitrogen, and only one-fifth of oxygen. But an ad- 

 dition to the normal quantity of oxygen is of more or less importance. 

 Twice or three times the usual quantity in air, at first, causes no ap- 

 parent inconvenience, and no special change in the products of respi- 

 ration; but it is probable that, after a time, certain injurious conse- 

 quences would ensue, though experiments are wanting to determine 

 the point. Pure oxygen, however, is highly injurious; the vital func- 

 tions are stimulated as if by a fever; the pulse and respiration are 

 increased in frequency ; after an hour, insensibility gradually comes 

 on, complete coma then ensues, and death occurs in from six to twelve 



