ult, and others admitted as essential fo it, with which it is in no respect connected; that analogies and resemblances are sometimes conceived to exist between events, which are in truth extremely dissimilar; and that the wildest flights of fancy are sometimes permitted to occupy the place of those rational and Lainate hypotheses, which, if they are not the immediate ancipations of truth, are at least highly instrumental towards its dicovry.

Obvious as these remarks undoubtedly are, we fear that the el of inquirers who are chiefly interested in the work before m, have but too seldom been fully aware of their importance, Le science of Physiology-regarding it, in its widest extent, as that which treats of the functions or properties of animals and vegetable-has always attracted a considerable share of attenton; and yet there is none which has at all times abounded in w much extravagant theory. Even at the present day, we beLove that there is no branch of knowledge more imperfect; nor ang which, mnist a great, though slowly accumulated mass of curious and important truths, sill retains so large a proportion si what is vagite, fanciful, and erroneous.

It would not perhaps have been uninteresting, to have endracord to point out at length the causes which seem to have sulgected this science in particular to such an imputaBron: but, for the present, we must content ourselves with observing, that we believe they may all be reduced nearly to the following:-That the various departments of the science have hitherto been considered in a manner too unconnected and irregular, and have been too little cultivated by persons capable of devoting an undivided attention to their investigation, and of studying all the functions of life in their actual connexion with each other. It is unfortunate, too, that Physiology has been regarded as the peculiar province of persons connected with the profession of medicine: for the most able and intelligent individuals of this class do not always cherish a partiality for physiological inquiries; or, if they do possess any taste for such pursuits, they are usually prevented from prosecuring them with success, by the labour or multiplicity of their practical duties. The truth indeed is, that, in the vast variety of phenomena exhibited by organized beings, anatomists, physicians, metaphysicians, chemists, opticians, and mechanical philosophers, have all found ample field for occasional investigation. Each have selected, for separate speculation or inquiry, those subjects which were most conformable with their habitual studies or occupations. To their talents and industry Physiology is indebted for a large share of the established truth of which It has to boast; but, at the same time, we are obliged to im

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 con ducted 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 recte 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 sufficiently known, that every 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 suffers, from the action of animals and vegetables upon it; and in what manner those changes are effected. The original Inquiry' was published in 1807; but the author has, since that time, not only been led, in obviating the very few objections which have been ered 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 endeavour to lay before our readers in as few and as plain words as possible.

In the human body, from the first to the last moments of it 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 produced. As soon as this dilatation commences, it is obvious that a sort of vacuum must be formed between the sides of the thorax and the lungs. A current of air, therefore, immediately flows through the windpipe into the air-cells of the lungs, and gradually distends these organs, in proportion as the cavity containing them is increased.

This

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 lungs 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 enlargement, the cavity of the thorax may be diminished by the pressure of the abdoniinal 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 commences, 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 affixed to the ribs; consequently, the quantity of air expelled 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 upwards; and, as soon as these muscles 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,

membered, 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, even 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 bulk of air inhaled and exhaled, it will follow, that a full grown person respires 48,000 cubic inches in an hour, or 1,152,000 cubic inches in the course of a day; a quantity equal to about 79.hogsheads.

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 of oxygen, and of nitrogen, with a quantity scarcely percep tible of the acid gas. What, then, is the precise nature of the changes which this atmospherical air undergoes, when it is received into the lungs ?

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That air which has been breathed is loaded with moisture, seems at all times to have been generally known. Upwards 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, that a given quantity of atmospheric air, in passing once through the lungs, lost about tli part of its buik of nitrogen, about th of oxygen, and gained nearly 4th of carbonic acid; 100 cubic inches, for example, losing 1,47 cubic inches of nitrogen, and

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9.117 ce indios of expzen; while they acrmured 7.647 cubic inches of carotic ach, ty a single respiratio. Tris conclusion was dounced clay mum exaniments pub med by Mr Davy; in which he found, that wasn be applied his mouth to a tube connected with a mercurial air-bolt containing atmospheric air, and made a single insiration and expiration from and into this vessel, as muta in the manner of ordinary breathing as possible, the contents of the druder were diminished in buis, and antatted less nitrogen and oxygen, and more carbonic acid,—nearly in the proportions jak stated. To the deductions which had been made from such experiments, in as far as they related to the disappearance of nitrogen, Mr Enis had objected, that they were such as the resets cbtained did not warrant; since there was no proof, that the chest was reduced exactly to the same capacity after as before the experiments; and we could not therefore later, that the nitrogen ges which had disappeared from the airholder was not to be found in the lungs. We wish he had extended this obvious and substantial objection, to the inferences which had been drawn from the same experiments regarding the proportions of oxygen and carbonic acid; for it appears in all respects equally applicable to them. If a small quantity of the nitrogen of the inspired air remained in the lungs, merely because the thorax was of larger dimensions after than before the experiment, for the same reason a portion of oxygen, or carbonic acid, which otherwise would have been found in the airhoider, might have been retained in these organs.

Two memoirs on this subject, the joint production of Seguin and Lavoisier, were read to the Academy of Sciences of Paris, in 1789 and 1790; and Laplace has preserved the results of those experiments, in prosecution of the same inquiry, with which the philosopher last named was engaged when he was dragged to the guillotine;-experiments which he himself would have communicated to the world in detail, had not the short respite of a few days, which he requested for that purpose alone, been with such unprecedented barbarity denied. It would have been agreeable to us, if we could have attached any value to these investigations of two chemists so celebrated. But that caution which ought to be inseparable from every philosophical pursuit, precludes our placing the least 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 me moirs to which we have alluded, will hereafter be read, chiefly because they are among the last labeurs of one of the greatest philosophers