« PrécédentContinuer »
philosophers of the eighteenth century. And, indeed, it is impossible to observe the tone of enthusiasm which these essays have in some parts receivedl, from the prevailing spirit of the times in which they were written, without melancholy reflections without feeling again awakened in us those emotions of den regret with which the recollection of that period of disappointment must ever be contemplated.
As we never can be assured that the capacity of the lungs.is the same afier as before the experiment, we cannot decidłe, wità certainty, how far the relative proportions of the gases in the air expired, may not have been influenced by this cause. But it is obvious, that if the diminution which may have been com served in the bulk of air by one inspiration, depended on any tatural and constant process in the animal economy, by whica air is continually abstracted from the cells of the lungs, the sea duction in volume which would take place, in breating a large quantity of air, would be directly proportional to the number of respirations necessary to transmit the whole of this air throach the lungs. Whereas, were it dependent on any such accidents circumstances as we have now alluded to, no such increase ought to be observed: the dimination might even be least, when ihs quan:ity of air inspired was largest; and, at all events, ve should not expect to find it, in any instance, exceeding 20 cu. bic inches, or half the bulk of an ordinary inspiration. This point has been determine, in the most satisfactory manner, he the experiments of Messrs Allen and Pepy's. About three year's am, they constructed an apparatus, by which from 3000 do near ly 10,000 cubic inches of atmospheric air could be transmittel once through the lungs, hy easy respirations, beginning ami ending with a forced or extreme expiration : And, of thirteen experiments of this kind which they performed, the greatest de hciency in the expired air appeared in one where 8360 la inches had been inspired, in which it arounted to 62 curie incho; in another, where 3620 cubic inches had been breathe ed, the dininution was only 4 cubic irrhes; and, in a third, 912) cubic inches lost cniv 18. But, althouch it is thus estaban Lisbied, that there is one canse to which the diminution is ni* orin, we do not feel ourselves entitled to assert, positively, al it, in all these instances, it ought to be ascribed solely to the Herence between the extent of the expiration immediately ecoing, and of that closing the experiments. It may, kereziera be shown to be dependent on circumstances entirely difference In the mean time, it may be remarked, that even the greatest deficiency in these experiments was not equal to half the diet enez between an ordinary and an extreme expiratien,
Wien, too, a large volume of air is breathed only once, and in the manner practised by Messrs Allen and Pepus, any slight variety in the proportions of the gases composing the small quantity of air which may be retained in the chest, can very little aftect the composition of the much greater bulk which has been cxhaled. The analysis of this expired air may therefore be regarded as affording a very satisfactorvillustration of the changes which air in general undergoes, in its passage through the lungs during natural respiratio. Accordingly, Messrs Allen and Pepys hare demonstratexi, by experiments which seem to be unexceptionable, that, by its transmission through the lungs in ordinary breathing, the air loses about 8 per cent. of oxygen, and acquires an e.cactly equal bulk of carbonic acid, while its nitrogen remains unalterel. They conclude, that 39,5:54 cubie inches of carbonic acid are emited daily from the lungs of a full grown person. But we perfectly agree with Mr Ellis in his objections to this calculation, and are inclined to estimate the a- verage quantity at about 92,160 cubic inches.
Not being acquainted with any attempt, since the imperfect trial made by Sir Abernethy, to ascertain the eomposition of the fluid which is brought oil' by the expired air in the state of vapour, we lately subjected a small quantity of this fluid to accurate analysis. For this purpose, we made å full grown per-" son, in perfect health, expire through a thin glass tube, about three feet in length and a quarter of an inch in diameter, kept at a low temperature, by the evaporation from a slip of muslin moistened in spirits of wine, which was wrapped roumd it. In four hours, an ounce measure of a perfectly transparent, colourless, and insipid fluid, of the consistence of water, was collected in drops from the extremity of the tube. This fluid did not produce the least alteration on the colours of litmus or turmeric paper. It suffered no change on the auldition of corrosive niuriate of mercury, tannin, or nitrate of silver. And when a small glass, containing half an ounce of it, was connected, by filaments of moistened cotton, with other two glasses containing each about two drams of pure water, and these were then attached to the opposite extremities of a galvanic battery, consisting of 24 four-inch double plates of copper and zinc, charged with diluted muriatic acid, at the end of four hours, we could not detect the slightest indication of the presence of albumen in the negative, or any saline substance in the positive glass. Hence we have been led to conclude, that the Auid dissolved in the exhaled air is pure water.
All those animals which suckle their young, constituting the class Mammalia,--for example, the ape, the horse, the dog, the
mouse, the seal, and the whale ; Reptiles, such as the turtle, the lizard, the frog, and the snake, and the whole class of Birds, are provided with organs resembling the lungs of man, ito which they are constantly, during life, receiving fresh air. Ana- . logy alone, therefore, would lead us to suppose, that these animals produce similar changes on it by their respiration. But this inference is in a manner confirmed, by the appeal which Mr Ellis has made to the experiments of various physiologists on rabbits, guinea pigs, rats, mice, sparrows, vipers, tortoises and lizards; and by some very neat experiments of his own on toads and frogs. In all these it appeared, that the nitrogen of the air respired suffered no change, but that a quantity of oxygen was removed,--and a volume, nearly or exactly equal, of carbonic acid substituted in its place.
The opinion which seems to have prevailed almost universally of late years, is, that, during respiration, a portion of the oxygen or nitrogen of the atmosphere, or a quantity of the compound atniospheric air itself, actually passes out of the lungs into the bloodvessels of these organs, and was combined with, or absorbed by the blood; and respiration has been familiarly spoken of as a process analogous, if not absolutely identical, with that of combustion. This combination being assumed as a fact, a variety of other phenomena, more or less at variance with actual experience, have been suppose ed to succeed or accompany it. According to one hypothesis, the carbonic acid exhaled might have existed ready formed in the blood of the pulmonary arteries; and the blood might have a stronger attraction for oxygen gas than for carbonic acid, and, combining consequently with the oxygen of the air, might part with the carbonic acid, whieh would as easily pass from the vessels into the cells, as the oxygen from the cells into the vessels. Another doctrine supposed, that a portion of oxygen being attracted by the blood in the lungs, was combined, during the circulation of that fluid through the other parts of the body, with a portion of carbon, so as to form an oxide of carbon ; which, on being brought back to the pulmonary vessels, was there united with an additional quantity of oxygen, assumed the state of carbonic acid, and was discharged." In like manner, it was conceived by some, that the water emitted in the state of vapour might be formed, by the union of a portion of absorbed oxygen with hydrogen existing in the blood, so as to constitute an oxide of hydrogen, which, on passing into the pulmonary arteries, combined with another portion of oxygen, and was then exhaled. And, finally, with respect to the nitrogen, some were of opipion, that while the blood actually attracted a large volume of VOL. XIX. NO. 37,
this gas out of the cells of the lungs, it combined only with a small portion of it,--the remainder passing back into the cells again; while others thought it more probable, that no was absorbed by the blood from the cells, than this fluid permaactly retained.
This rapidl and easy transmission of gases through the siles of the cells and vessels of the lugs, which is the common foundation of all these theories, Mr Ellis maintains, is not only totally devoid of proof, but, if sound philosophy only allows us to conjeciure, respecting phenomena unscuri, from what we have experienced of similar events actually perceived, that it is not legitimate to entertain it, even as an hypothesis. Dr Lower had indeed observed, that when dark-coloured blocsi was brought into contact with atmospheric air, it assumed a forid colour; and various other physiologists after Priestley, had proved that this, or any other air containing oxygen, so exposed, lost part of its oxygen, and gained carbonic acid. Lower had also demonstrated, by experiments on quadrupeds, that the change of colour from modena to a scarlet red which tlic blood underwent in the lungs, depended entirely on the presence of fresh air in their coils : And Priestley found, that when a quantity of dark-coloured bloodl was tied up closely in a moistened bladder, and hung in the air, the whole lower surface of the blood acquired a couting of a florid red colour, as thick as it no bladder had intervened. From all this, it had been inferred, that, during respiration, cither some part of the air passed through the sides of the cells and vessels of the lungs into the blood, or that something was given out by the same course, from the blood to the air, so as to alter the colour of the one, and the coniposition of the other. Bui, without denying that, in these instances, the change of colour in the blood depended on the presence of atmospheric air, or of air containing oxygen, Mr Ellis has shown, by the most satisfactory experiments, that, in the case where the bladder intervened, neither did the air afiord any portion of its gases to the blood, nor did the blood communicate any matter to the air. Thus, when he put a quantity of black blood into a small bladder, and suspended it in a glass jar containing 13.1 cubic inches of atmospheric air inverted over mercury, he found that the blood soon reddened ; that, at the . end of two days, the whole of the oxygen of the included air had disappeared--but that an equal quantity of carbonic acid had been formed. Hence it is obvious, that as all the oxygen which had disappeared was converted into carbonic acid, none could have penetrated the bladder, or combined with the blood. On
the other hand, when Mr Ellis suspended, in the same manner, bladders filled with water, or bladders empty, but moistened, in jars of atmospheric air, the oxygen was equally found to be converted into carbonic acid. Since, therefore, it thus appears that a moistened bladder is of itself capable of affording carbon to form carbonic acid with the oxygen of the air, there is no reason for supposing, that the carbon is derived from any other source, where the bladder is filled with blood; and the conclusion seems irresistible, that when dark-coloured blood is reddened by the air, through the sides of a moistened bladder, the air yields no oxygen to the blood, nor acquires from it any carbon ; but the carbon of the bladder, by its combination with the oxygen of the air, passes into the state of carbonic acid
The doctrine, then, of the entrance of gases into the blood from the air cells of the lungs, can no longer be regarded as receiving the best support from Priestley's experiment. But although the result had been otherwise, and the direct passage of something through the bladder had been unequin vocally proved, we should still have been disposed to maintain with our author, that it would not necessarily follow, that any similar transmission of air took place through the sides of the cells and vessels of the lungs. On the contrary, we regard it to be a fact, as well established as any in Physiology, that no part of the body, provided with vessels, however delicate it may be, has ever been observed to permit the smallest quantity
any kind of fluid to permeate through it, as long as the circulation continues in that part; though, as soon as death has taken place, transudation goes on in all textures with the utmost facility. We should not, therefore, be entitled to infer, merely because a dead bladder may seem to allow of the transmission of air, that the cells and vessels of the living lungs are equally permeable to that fluid.
Mr Davy had concluded from experiment, that 71 or 93 cubic inches of nitrous oxide might, in tlie short period of half a minute, be absorbed by the venous blood, through the moist coats of the pulmonary veins. Our author's observations, alone, would have lot very little doubt in our minds, that, in these experiments, though a portion of gas had disappeared from the airholder, none had passed into the vessels of the lungs. But we have, oursoves, found, by repeated trials with nitrous oxide and atmospheric air, that, when a given quantity of either of these is frequently breathed, the desire, or sympathetic stimulus to inspire, becomes gradually so strong, and the expirations proportionally so short and restrained, that, at last, when the ex