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This constitutes what is denominated Inspiration. The quantity of air which is inhaled, in any single inspiration, is of course determined entirely by the extent to which the chest is dilated. In individuals who are healthy and at rest, inspiration consists merely of a gentle enlargement, produced by a partial contraction of the diaphragm; and such may be termed an Ordinary Inspiration. The quantity of air, which rushes into the lung's during an inspiration of this kind, is very different in different individuals, according to the size of their chests, or the extent to which the diaphragm contracts, in the inspirations of each. It has been variously estimated, in adults of a middle stature, at 13, 17, 20, 35, and 40 cubic inches; affording 25 cubic inches as a mean. But all these calculations have not been founded on equally satisfactory data. Dr Menzies's experiments alone, which estimate the average bulk of an ordinary inspiration at about 40 cubic inches, seem to have been performed in an unexceptionable manner; and we place the more confidence in his calculation, that we have found it to correspond with some late experiments of our own. In larger inspirations, the thorax is increased in all directions; and the average bulk of air, at temperature 60° Fahrenheit, which is inhaled by the utmost possible inspiring effort, or by what may be called an extreme inspiration, is probably about 130 cubic inches.
After previous enlargeinent, the cavity of the thorax may be diminished by the pressure of the abdon inal viscera, the elasticity of the parts with which the ribs are connected, and the muscles which pull these bones downwards, exactly to its natutural capacity, or even considerably below it. When the diminution comniences, the lungs are compressed; and the air, being thus forced out of their cells, escapes by the trachea and mouth. This constitutes Expiration. In health, and during rest, it consists of a reduction of the thorax to its natural state only ; and this seems produced merely by the compression of the relaxed diaphragm, and the elasticity of the cartilages and softer parts afiixed to the ribs: consequently, the quantity of air espelled is exactly equal to the quantity previously inhaled. Such may be called an Ordinary Expiration. In all larger expirations, where the chest is compressed below its natural state, the compression is produced and sustained entirely by the action of powerful muscles, drawing down the ribs, and forcing the diaphragm upwarris ; and, as soon as thiese niuscles cease to act, the thorax returns to its natural state again. We are inclined to think, from experiment, that the quantity of air which, on an average, is expelled by an extreme expiration, after a previous extreme inspiration, is about 260 cubic inches. It is to be re,
membered, pute to the partial views of these very men, the greater proportion of the error with which it abounds.
If any thing, however, can contribute to render an imperfect science speedily perfect, it is the publication of inquiries conducted on the plan of those which form the subject of the present article. We scarcely know any work in physiology, where an author has displayed a more extensive knowledge of every fact contributing, in the most reincte manner, to elucidate the object of his investigation ; in which, he has sought the opinions of others with more diligence, or stated them with more uniform candour; or where he has himself interrogated Nature, by experiments more judicious or more successful.
It is a fact, which has been long suficiently known, that erery thing which lives, whether animal or vegetable, requires, for the continuance of its life, a constant supply of fresh air. The great purpose of Mr Ellis's Inquiry, is to discover why it is that air is necessary to the vital existence of organized bodies. In the present volumes, he has particularly in view, to show the precise nature of the changes which the air suffer, from the aciion of animals and vegetables upon it; and in what manner those changes are eficcted. The original • Inquiry' was puhlished in 1807; but the author has, since that time, not only been led, in obviating ihe very few objections which have been rred to his doctrines, to the discovery of some new and inte resting facts, but has corrected his original views by various additional experiments. The result of the whole we shall endcavour to lay before our readers in as few and as plain words as possible.
In the human bods, from the first to the last moments of its existence, we remark, that a certain quantity of air is alternately rushing into and out of the mouth and nostrils. The chest, or thorax, is so constructed, that, merely from the elasticity of its sides, and the pressure of the surrounding parts upon them, it has a tendency to assume a certain permanent capacity or dilatation. Accordingly, after death, when there no longer exists any counteracting cause, this is the capacity which it assumes and retains. We may call it the natural state of the thorax. In the living body, however, it is found that, by the action of the surrounding muscles, a further enlargement of the chest, beyond its natural state, may be produceri. As soon as this dilatation commences, it is obvious that a sort of racuum must be formed between the sides of the thorax and the lungs. A curreut of air, therefore, immediately fiows through the windpipe into the air-cells of the lungs, and gradually distends these orans, in proportion as the cavity containing them is increased.
Inembered, however, that we cannot, by any muscular effort whatever, reduce the dimensions of the chest so far, as to empty the lungs entirely of their contents. After the most violent expiration, a considerable quantity of air still remains within their cells ;-nay, it is found extremely difficult to expel this residual air altogether, eren by subjecting the lungs to very great compression, after they have been removed from the body. That, after an extreme expiration, they still retain, on an average, about 40 cubic inches, seems probable, from considering both the structure of the lungs, and the extent to which the thorax scems capable of being diminished by muscular action, as well as the result of an experiment of Mr Davy.
These two processes, of inspiration and expiration, generally alternate with each other, while the body is at rest, about 20 times in a minute. If, therefore, we adopt 40 cubic inches, as the average buik of air inhaled and exhaled, it will follow, that a full grown person respires 19,000 cubic inches in an hour, or 1,152,000 cubic inches in the course of a day; a quantity equal to about 79 hogaleads.
It has been long ascertained, however, that the air which is emitted by expiration, does not possess the same properties as that which has been inspired. Now, the only gaseous substances which chemists have hitherto found existing, permanently and uniformly, in the atmosphere, are oxygen, nitrogen, and carbonic acid ; the relative properties of which may be judged of from the analysis of one cubic inch, which gives nearly yo's of oxygen, and no of uitrogen, with a quantity scarcely perceptible of the acid gns. What, then, is the precise nature of the changes which this atmospherical air undergoes, when it is received into the lungs?
That air which has been breathed is loaded with moisture, seems at all times to have been generally known. Cpwards of fifty years ago, the celebrated Dr Black demonstrated, that it was also combined with much more carbonic acid ; and Dr Priestley proved, in 1776, that it contained much less oxygen than the air inhaled. Yet, at the publication of Mr Ellis's inquiry, physiologists had not established, either the proportion of these gases existing in it, or the composition and quantity of the vapour with which it is united; nor had they ascertained what relation its nitrogen bore to that of the surrounding atmosphere. It appears, indeed, to have been the prevalent opinion, ihat a given quantity of atmospheric air, in passing once through the lungs, lost about s'ili part of its buik of nitrogen, about to the of oxygen, and gained nearly 'th of carbonic acid; 100 cubic inches, for example, losing 1.17 cubic inches of nitrogen, and 9.17 € nets arran; stare?.647 cubic incess of cartes 23,12zeres TO's conclusoa pas ceuccedci Trans med bs dir D:55; in such he hate his mouth to a tu muda mraz 003: tra atmosphere ar, and care au exirstea from and in this res, 28 Ischia ierada o cuinary breathing as pomaza, tie estedisaiteer sene dizinished in bus, candauerdin sa 2, ad rrure carbonic acid,-akt proportions ici. To the deducis ca toa ride on suchen eines in as far 2 trid: he crearance of Ln, Vir Eiis had ohjecie, that ihr were as be reis kanel did not warrant; sicee tere was au poi, that the chasi mos raluced exactly to the same capaciir ater as before the experiments; and we could not therciore luier, that the nitrogen ass which had disar.carca from the arhiver was not to be found ia the lungs. We wish he had extended this cbricus and se'stantial objection, to the interences which had been drawn from the same experisenis reguding the proportions of orren and carbonic achi; for it appears in ail respects equaat spicable to them. If a small quantity of the nitrogen of ihe inspired air remained in the lungs, merely because the thorax was of larger dimensions after than before the exzeriment, for the same reason a portion of oxygen, or carbonic acki, wach otherwise would have been found in the airhoider, mit have been retained in tese org.is.
Two memoirs on this subject, the joint production of Seguin and Lavoisier, were read to the Academr of Sciences of Paris, in 1789 ard 1790; and Laplace has preserved the results of those experiments, in prosecution of the same inquiry, with which the philosopher last samel wis engaged when he was dragged to the guillotine ;-experiments which he himself would have communicated to the word in detail, had not the short respite of a few dars, which he requested for that purpose alone, been with such unprecedented barbarity denied. It would have been agreeable to us, it' we could have attached any value to these investigrions of two chemists so celebrated. But that caution which ought to be inseparable tron erery philosophical pursuit, præcludes our placing the last reliance on results of experiments, when they are not detailed with the most circumstantial minuteness. We honour the memory of Lavoisier, and respect the talents of his surviving coadjutor. But their momoirs to which we have alludert, will hereafter be read, chiefly secause they are among the lasi labeurs of cae of the greatesë