October 20, 1892] 



NATURE 



599 



than that the parallel between the burning candle and the living 

 organism should be thought to represent truly the real con- 

 'itions? That as the candle consumes the oxygen, burns, and 

 ives out carbon dioxide, so the living thing breathes in oxygen, 

 .iiid gives out in place of that consumed carbon dioxide. And 

 as in each case heat is produced, what would be more natural 

 than to look upon respiration as a simple combustion? This 

 was the generalization of Lavoisier (1780-89). As he saw it, 

 the oxygen entered the lungs, reached the blood, and burned the 

 carbonaceous waste there found, and was immediately given 

 out in connection with the carbon with which it had united, 

 and as the gas given off in a burning candle makes clear lime- 

 water turbid, so the breath produces a like turbidity. 



But here, as in many of the processes of nature, the end pro- 

 ducts or acts were alone apparent, and while the fundamental 

 idea is probably true that respiration is, in its essential process, 

 a kind of combustion or oxidation, yet the seat of this action is 

 not the lungs or blood. If the myriads of microscopic forms 

 are considered, these have no lungs, no blood, and many of 

 them even no organs ; they are, as has been well said, organiess 

 organisms, and yet every investigation since the time of Vinci 

 and Von Helmont, Boyle and Mayow, has rendered it more 

 and more certain that every living thing must in some way be 

 supplied with the vital air or oxygen, and that this is in some 

 way deteriorated by use ; and the nearer investigation approaches 

 to the real life-stuff or protoplasm, it alotie is found to be the 

 true breather, the true respirer, as was shown long ago by Spal- 

 lanzani (1803-7). If one of the higher animals, as a frog, 

 is decapitated and some of its muscle or other tissue exposed in 

 a moist place, it will continue to take up oxygen and give out 

 carbon dioxide, thus apparently showing that the tissues of the 

 highly organized frog, may, under favourable conditions, absorb 

 oxygen directly from the surrounding medium, and return to it 

 directly the waste carbon dioxide. This shows conclusively 

 that it is the living substance that breathes, and the elaborate 

 machinery of lungs, heart, and blood-vessels, are only to make 

 sure that the living matter, far removed from the external air, 

 shall not be suffocated. Still more strange, it has been found 

 that if some of the living tissue is placed in an atmosphere of 

 hydrogen or nitrogen entirely devoid of oxygen, it will perform 

 its vital functions for a while, and although no oxygen can be 

 obtained, it will give off carbon dioxide as in the ordinary air. 

 If it is asked, " how can these things be ? " the answer is appar- 

 ently plain and direct. Not as the oxygen unites directly with 

 the carbon in the burning candle does it act in the living sub- 

 stance. The oxidations are not direct in living matter, as in 

 the candle ; but the living matter first takes the oxygen and 

 makes it an integral part of itself, as it dues the carbon and 

 nitrogen and other elements ; and, finally, when energy is to be 

 liberated, the oxidation occurs, and the carbon dioxide appears 

 as a waste product. 



The oxygen that is breathed to-day, like the carbon or the 

 nitrogen that is eaten, may be stored away and represent only 

 so much potential energy to be used at some future time in 

 mental or physical action. 



So far only living animal substance has been discussed. If 

 plants are considered, what can be said of their relations to the air ? 

 The answer was given in part by Priestley (1771), who found that 

 air which had been vitiated by animal respiration became pure 

 and respirable again by the action of green plants. He thus 

 discovered the harmonizing and mutual action of animals and 

 plants upon the atmosphere ; and there is no more beautiful 

 harmony in nature. Animals use the oxygen of the air and give 

 to it carbon dioxide, which soon renders it unfit for respiration ; 

 but the green plants take the carbon dioxide, retain the carbon 

 as food and return the oxygen to the air as a waste product. 

 This is as thoroughly established as any fact in plant physio- 

 logy ; and yet, in his experiments, Priestley had some of what he 

 called " bad experiments " ; for instead of the plants giving out 

 oxygen and thus purifying the air, they sometimes gave off car- 

 bon dioxide, and thus rendered it more impure, after the manner 

 of an animal. What investigator cannot sympathize with 

 Priestley when he calls these '* bad experiments " ; they appeared 

 so rudely to put discord into his discovered harmony of Nature. 

 But Nature is infinitely greater than man dreams. The "bad 

 experiments " were among the most fruitful in the history of 

 scientific discovery. Ingenhausz (1787) followed them up, care- 

 fully observing all the conditions, and found that it was only in 

 daylight that green plants gave out oxygen ; in darkness or in- 

 sufficient light they conducted themselves like animals, taking up 



oxygen and giving out carbon dioxide. Finally it was proved 

 by Saussure (1804) and others that for green plants, and those 

 without green, like the mushroom, oxygen is as necessary for 

 life as for animals. It thus became evident that this use of oxy- 

 gen and excretion of carbon dioxide was a property of living 

 matter, and that the very energy that set free the oxygen of the 

 carbon dioxide was derived from oxidations in the green plant 

 comparable with those giving rise to energy in animals. Further 

 that the purification of the air by green plants in light is a 

 separate function — a chlorophyll function, as it has been happily 

 termed by Bernard — and resembles somewhat digestion in 

 animals, the oxygen being discarded as a waste product. In- 

 deed so powerful is the effort made to obtain oxygen for the life 

 processes by some of the lowest plants— the so-called organized 

 ferments — that some of the most useful and some of the most 

 deleterious products are due to their respiratory activity. In 

 alcoholic fermentation, as clearly pointed out by Pasteur and 

 Bernard, the living ferment is removed from all sources of free 

 oxygen, and in the effort for respiration the molecules of the 

 sugar are decomposed or rearranged and a certain amount of 

 oxygen set free. 



It has been found that the motile power of some bacteria like 

 Bacterium terino depends on the presence of free oxygen in the 

 liquid containing them. When this is absent, they become 

 quiescent. This fact has been utilized by Engelmann and 

 others in the study of the evolution of oxygen by green and other 

 coloured water plants. The bacteria serving as the most delicate 

 imaginable oxygen test, so that when the minutest green plant is 

 iliuoiinated by sufficient daylight, the previously quiescent bac- 

 teria move with great vigour and surround it in swarms. Out 

 of the range of the plant, the bacteria are still, or move very 

 slowly, as if to conserve the minute energy-developing substance 

 they have in store until it can be used to the best advantage. 



May we not now approach the problem directly, and answer 

 for the whole organic living world the question, " What is re- 

 spiration?" by saying it is the taking up of oxygen and the giving 

 out of carbon dioxide by living matter? This is the universal 

 and essential fact with all living things, whether they are animals 

 or plants, whether they live in the water or on land. But the 

 ways by which this fundamental life process is made possible, 

 the mechanisms employed to bring the oxygen in contact with 

 the living matter, and to remove the carbon dioxide from it, are 

 almost as varied as the groups of animals, each group seeming to 

 have worked out the problem in accordance with its special needs. 

 It is possible, however, in tracing out these complex and varied 

 methods and mechanisms, to discover two great methods — the 

 Direct and the Indirect. 



In the first, there is the direct assumption of oxygen from the 

 surrounding medium, and the excretion of carbon dioxide 

 directly into it. The best examples of this are presented 

 by unicellular forms like the amoeba, where the living substance 

 is small in amount, and everywhere laved by the respiratory 

 medium. But as higher and higher forms are destined to appear, 

 evidently the minute, organiess amoeba could not in itself 

 realize the great aim toward which Nature was moving. There 

 must be an aggregation of amoebas, some of them serving for 

 one purpose and some for another. Like human society, as 

 civilization advances, each individual does fewer things, becomes 

 in some ways less independent, but in a narrow sphere acquires 

 a marvellous proficiency. Or, to use the technical language of 

 science, in order to advance there must be aggregation of mass, 

 differentiation of structure, and specialization of function. 

 Evidently, however, if there is an aggregation of mass, some of 

 the mass is liable to be so far removed from the supply of 

 oxygen, and the space into which carbon dioxide can be elimi- 

 nated, that it is liable to be starved for the one and poisoned by 

 the other. Nature adopted two simple ways to obviate this — 

 first to form its aggregated masses in the form of a network or 

 sponge, with intervening channels through which a constant 

 stream of fresh water may be made to circulate, so that each 

 individual cell of the mass could take its oxygen and eliminate 

 its carbon dioxide with the same directness as its simple proto- 

 type, the amoiba. 



But in the course of evolution forms appeared with aerial 

 respiration, and the insects, among these, solved the mechanical 

 difficulty of respiration by a most marvellous system of air-tubes, 

 or tracheae, extending from the free surface, and therefore from 

 the surrounding air, to every organ and tissue. By means of 

 this intricate network, air is carried and supplied almost directly 

 to every particle of living matter. The respiration Is not quite 



NO. II 99. VOL. 46] 



