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tity of air several times greater than what is ordinarily expelled. Perhaps the researches of Sir H. Davy afford us as accurate a view of the relative state of the lungs at different periods as any which have been yet made. He has estimated the quantity of air remaining in his lungs after a natural expiration at 118 cubic inches, and after a forced one at 41, making a difference of 77 cubic inches. The state of the lungs therefore after natural inspiration is to their state after natural expiration as 158 to 118, and the state of natural to that of fercible 'expiration as 118 to 41. He found, too, that after filling his lungs by a forced inspiration, he could then by a forcible expiration expel 190 cubic inches, so that the state of greatest dilatation is to that of greatest exhaustion as 231 to 41, These numbers must be understood, however, to indicate proportions only, which probably vary in some degree with every individual, since they must depend not only on the capacity of the chest, but also on the power of the muscles, and the degree of voluntary effort by which their action may be excited,

From these facts it appears that only about of the whole air contained in the lungs is renewed by each complete respiration, and as the whole will be changed in four such respirations or in about a quarter of a minute, it becomes easy to calculate the total consumption of oxygen for any given period by a single individual. But we must first stop to notice the diminution of bulk whichi the air is observed to sustain, We have observed already that the air which has been passed through the lungs with a view to the experimental determination of the changes which it undergoes, is generally found to have suffered a diminution of its volume. Sir H. Davy observed that when he respired atmospheric air in a natural manner, the diminution was about #s; when he made one single respiration of 100 cubic inches, the loss of bulk was too; when, after a complete exhaustion of his lungs, he respired 141 cubic inches, once only for a quarter of a minute the loss was about to; and when 161 cubic inches were breathed, for about a minute they were reduced is. The same fact has been observed by most other experimentalists, though the diminution does not seem to have been from a comparison of the experiments in any remarkable degree steady or uniform, Įt appears however from the experiments of Professor Pfaff, to bear' some proportion to the frequency with which the same air is respired. He breathed 144 cubic inches of air once only in ten or twelve seconds, and it suffered a diminution of four cubic inches or ; the same volume of air respired twice during twenty seconds, lost eight cubic inches or ; and when the same quantity of air was respired thrice, during thirty seconds, the diminution was about 12 cubic inches or of the original

volumé. A similar diminution of volume was found to take place by Sir H. Davy, in his experiments on the nitrous oxide. After exhausting his lungs by a forced expiration, he inspired 108 cubic inches of this gas, and this quantity when expired had lost te of its original bulk. When the same quantity of the gas was respired twice, the diminution was about : and when 102 cubic inches of the oxide mixed with of common air, was respired for half a minute, the volume of air after the seventh expiration was reduced to 62, or had suffered a diminution of 2.55. These facts establish the conclusion, that the diminution of volume is proportionate to the number of respirations which shall have been performed with the same quantity of air, and consequently that its amount is the least in natural respiration, or in those experiments which most closely imitate the natural process, and in which the air is respired only once.

This conclusion is very strongly marked in the recent experiments of Messrs. Allen and Pepys, which appear to have been conducted in the most unexceptionable manner. They caused a person to inspire 3460 cubic inches of atmospheric air from a gasometer, which were expired into another vessel of the same description. The time occupied by the experiment was eleven minutes and the number of respirations about fifty-eight, and the total diminution of volume was. 23. cubic inches or about so of the whole; and in a subsequent experiment in which 9890 cubic inches were breathed, the loss' was only 18 eubic inches or about såg of the whole. When, however, in other experiments they caused the same air to be respired eight or ten times successively, its bulk was considerably diminished, and the proportion of its constituent parts were also very different from that which had been respired once only.

Now to what cause is this diminution of bulk to be attri. buted ?-for until it shall be accounted for in some satisfactory manner, it is evident that all our conclusions must be liable to extreme uncertainty. The general opinion has been, that it is owing to the absorption of the air by the blood, in its circulation through the lungs; and Mr. E. qaotes the following observations froin the "Researches" of Sir H. Davy on the nitrous oxide, as an example of the views of chemical philosophers on this subject. After attributing the diminution of volume which he observed in his experiments on the respiration of nitrous oxide, to a rapid absorption of this elastic fuid by venal blood through the moist coats of the pulmonary veins, Sir Humphrey proceeds to observe, that it is also reasonable to suppose, that the whole compound atmospheric ait, passing through the moist coats of the vessels, is first dissolved by the serum of the venous blood, and, in its condensed state,

decomposed by the affinity of the red particles for its oxygen the greater part of the nitrogene being liberated unaltered, but a minute portion of it probably remaining condensed to the serum and coagulable lymph, and passing with them into the left chamber of the heart. Similar views may be found in the works of other authors who have treated on this subject. Mr. Ellis however presumes to think otherwise. He has examined the principles on which this hypothesis rests with philosophical precision, and with most perfect candour ; and to us he appears to have proved it to be destitute of any shadow of support. He objects to it both on anatomical and chemical principles. The anatomical structure of the lungs, he observes, is extremely unfavourable to the transmission of air into the blood vessels, since it must pass not only through the dense membrane forming the cells, but also through the coats of the veins. Nor does it appear at all possible that it can obtain admission into the blood by means of the absorbent vessels ; since their minuteness and the mucous secretion with which their inner surface is covered, must totally unfit them for such an office. Neither can it be supposed to be effected by chemical affinity, inasmuch as this does not act without actual contact, and the air, and blood are separated from each other, not only by the membranous sides of the cells and the coats of the vessels, but also by the cellular substance which intervenes between them. To arguments of this nature Mr. Elis adds the positive evidence of experiments; by which it has been proved, first, that no evidence of the existence of air in blood can be obtained by the use of those means which are usually found to disengage it from its combination with liquids : for neither when confined in its proper vessels, nor when exposed in an open vessel does it manifest the existence of an atom of air, under the exhausted receiver of an air pump. Secondly, the introduction even of a small portion of air into the blood vessels, so as that it may be allowed to pass into the circulation, and mix with the blood, is very speedily followed by the death of the animal. Mr. Ellis's own explanation is at once ingenious and satisfactory. The very small diminution observed in those experiments which may be considered as perfectly analogous to natural respiration, he thinks is to be attributed to the obvious difficulty of always bringing the respiratory organs into the same state of relaxation after a given inspiration, and this source of error will probably be increased in proportion as the attention of the individual shall be excited, as that must influence the voluntary power by which the action of the muscles is determined. When, however, the same quantity of air is repeatedly respired (under which circumstances the most remarkable diminution of volume in the expired air is

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always observed) then Mr. Ellis thinks the source of the discrepancy, is to be found in the natural and constant effects which are produced on the voluntary powers by the respiration of impure air. This is in all cases à progressive diminution, terminating in a total failure of voluntary power. The diminution consequently will be proportioned to the inability of the expiratory powers to expel the air ; and hence in experiments of this nature, the loss of bulk is found to be in proportion to the frequency with which the air is respired, and the consequent distress and oppression suffered by the respiratory organs from the vitiated state of the air. These observations apply with equal force to the experiments made by Sir H. Davy on the respiration of nitrous oxide, which produced a similar diminution of voluntary power, and finally a total inability to continue the experiment.

There is however another circumstance to which Mr. Ellis has adverted, and the influence of which is probably of considerable importance in modifying the results of experiments of this description, and which supplies a rational explanation of some facts, tending to prove that, in particular instances, a larger quantity of air has been contained in the lungs, than our knowledge of their capacity would lead us to think at all probable. This is the mechanical attraction which is known to exist between air and the surfaces of bodies, especially moist surfaces. Now as the congeries of cells of which the lungs are formed, present a surface which is perpetually moistened by its appropriate mucus, and of an extent which has been estimated at 21,907 cubic inches, or more than ten times that of the whole external surface of the body; it appears probable that this prodigious extent of surface must have a constant tendency to diminish the elasticity of the air in the lungs, and perhaps to occasion a larger quantity than usual to be retained under particular circumstances, without any remarkable injury or inconvenience to the animal. Mr. Ellis has supported his views on this subject, by adducing the analogous instance of the absorption of air by charcoal, which has been proved, by repeated experiments, to possess the power of absorbing and retaining different kinds of elastic fluids in its pores or cells, in a quantity exceeding by many times that of its own bulk, and which is very readily given out unchanged on the application of a low degree of heat.

On the whole, there appears therefore to be pretty strong reason to conclude that, in natural respiration, the volume of air suffers no diminution, and that the bulk of the expired air is exactly equal to what is inspired. This conclusion receives additional support, when the chemical changes which the air suffers are brought into the calculation. In the earlier periods

of his inquiry the conversion of oxygene into carbonic acid was supposed, by Mr. Ellis, to be accompanid by a small degree of condensation. This opinion however is now abandoned in favour of the evidence of more recent and correct experiments which have proved that the combination of carbon with oxygene does not produce any perceptible alteration in the volume of that gas. It is fair to presume, therefore, that the want of exact correspondence in the proportions of oxygene gas and carbonic acid, which has been so frequently noticed in the experiments on human respiration, must have been owing to some imperfection in the means of conducting them, or to some trifling error which esc&ped notice. In the most recent experiments, indeed, this want of correspondence has not been observed, except when the frequent respiration of the same air was attempted. In the experiments of Messrs. Allen and Pepys already noticed, in which the air was respired once only, the correspondence was perfect. The air before the experiment contained 21 parts of oxygene and 79 of nitrogene in 100 parts, and when examined afterwards it was found to have gained precisely as much carbonic acid as it had lost of oxygene. In the first experiment in which 3460 cubic inches of atmospheric cir were employed, 100 parts were found on analysis to contain 8.5 carbonic acid, 12.5 of oxygene, and 79 nitrogene; and in that in which 9890 cubic inches of air were respired, the proportions after the experiments were in 100 parts, 8 of carbonic acid, 13 of oxygene, and 79 parts of nitrogene; so that although there was in both a trifling loss of bulk, yet there was no alteration of proportion in the constituent parts of the air. This analysis was frequently repeated in the course of the experiments, and in every instance the oxygene which disappeared was replaced by the carbonic acid, and both together formed 21 per cent. of the whole, which was the original amount of the oxygene. When however the respiration was not natural, and the same air was repeatedly respired, there was a remarkable alteration in the proportions of the expired air at the conclusion of the process.

In one instance in which the same air was respired three minutes, and the respiration had become so extremely laborious that the operator was compelled to desist, the proportions were 9.5 of carbonic acid, only 5.5 of oxygene and 85 of nitrogene, and in a subsequent trial, which occupied about the same space of time, but in which the operator became insensible, the proportions in 100 parts of the expired air were 4 oxygene, io carbonic acid, and 86 nitrogene; so that in these instances about seven per cent. of oxygene had-actually disappeared, which was not replaced by a corresponding quantity of carbonic acid, but by an augmentation in the proportion of nitrogene. Our knowledge of respiration,

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