With respect to the bulk of an ordinary inspiration, the first writer who gives us any very accurate information is Jurin. By breathing into a bladder, and making the ne cessary allowance for temperature and pressure, he esti mated the quantity at forty cubic inches. Goodwin em ployed a peculiar apparatus, in which he measured the bulk of an inspiration by the quantity of water raised up from one vessel to another. Sir H. Davy endeavoured to find the quantity, by examining the chemical changes which a certain portion of air had experienced by passing through the lungs; and other methods have been employ ed, which gave results that varied from a few inches to above fifty. But of all the experiments of this kind that have been performed, those of Menzies seem to be enti tled to the greatest confidence, both from the nature of the apparatus, and from the general uniformity of the re sult's. Ile performed two sets of experiments. In the first, lie employed a membranous bag, of such a size as to enable him to take the average of a number of inspira tions, and so fitted with valves as to keep the inspired and expired air distinct from each other. In the second set of experiments, a man was immersed up to the chin in a vessel of water, which was closed at the top, and furnish ed with a projecting tube, in which any alteration in the level of the water might be readily perceived, and thus the corresponding changes in the bulk of the body be ac curately estimated. By employing every necessary pre caution, he found the result of both sets of experiments to be nearly similar, and also to correspond with the sim ple process of Jurin, indicating the average bulk of a single inspiration to be forty cubic inches.
The quantity of air that can still be expelled from the lungs after an ordinary expiration, depends upon the size of the individual, and the peculiar formation of the chest. It appears, however, that a middle-sized man can expel between 150 and 200 cubic inches by a powerful voluntary effort. But it is obvious, from the formation of the lungs and chest, that no voluntary effort can expel all the air from them ; and here again, as on all the other points, the opinions of anatomists and physiologists have been very various. Perhaps the experiments of Goodwin are the most to be relied on, which make this quantity some-• what more than loo cubic inches. From these data, which, although confessedly imperfect, seem to be the best which we possess, it may be estimated, that, by each ordinary expiration, between one-sixth and one-seventh or the whole contents of the lungs is discharged ; and that, by the most violent effort of expiration, two-thirds, or even a greater proportion of the air, is evacuated. Supposing that each act of respiration occupies about three seconds, a bulk of air, nearly equal to three times the contents of the lungs, will be expelled in a minute, or about 4000 times their bulk in 24 hours. This quantity will amount to between 600 and 700 cubic feet.
There are two curious subjects of inquiry connected with the function of respiration, which have abundantly exercised the genius of physiologists—what is the cause of the first inspiration in the newly born infant ? and what is the cause of the regular successions of inspiration and expiration during the remainder of life ? With respect to the first of these points, Harvey asks the following ques tion, Why is the animal, when it has once breathed, un der the necessity of continuing to respire without inter mission, when, if the air had never been received into the lungs, the same animal might have remained some time without exercising this function ? \Vhytt gave an an swer to this question, which at the time was very general ly acquiesced in. He supposes that, immediately after
birth, an uneasy sensation is experienced in the chest from the want of air, which may be regarded as the appe tite for hreathing. To supply this appetite, the intelligent principle with which the body is endowed produces the expansion of the chest, being aware of the fatal conse quences that would result from the exclusion of fresh air. It is sufficient to remark concerning this hypothesis, that it labours under the defect of all the speculations of the metaphysical physiologists, that it confounds the final with the efficient cause, and supposes the agency of a principle, the existence of which is itself a point which •equires to be proved.
Perhaps it would be difficult to give any answer to Har vey's query, which should be altogether satisfactory; but there is one point which may tend to throw some light up on this obscure subject. We refer to the mechanical -change which the chest experiences when the young ani mal leaves the uterus. Before this time, in consequence of the position of the foetus, the lungs are squeezed up into as small a space as possible, and are nearly impervi ous to the blood ; but when the trunk is extended, and the parts are allowed to exercise their natural elasticity, the ribs rise up, and the diaphragm descends, so that the di mensions of the chest are extended in both directions. The thorax being thus brought into its state of average distention, and there being a free passage through the -mouth, the air necessarily rushes in to supply the vacancy thus produced.
The second subject of inquiry that was pointed out, re zards the regular alternation of inspiration and expira tion. The attempts that have been made to solve this _problem are very numerous; but we are disposed to re gard them all as inadequate to the end in view ; nor are we able to afford any explanation which is entirely satis factory. We are indeed disposed to consider this action as depending upon different principles, according to the manner in which it is carried on, whether in the ordinary process of respiration, or when the lungs, from any cause, are excited to an extraordinary effort. In the first case, it would appear that when the blood has passed through the systemic circulation, it undergoes a change which renders it no longer fit for performing the functions ne cessary for the continuance of life. In some way or other, which we shall not now attempt to explain, when the blood comes into this state, an uneasy sensation is expe rienced about the heart, which sensation is removed by taking a portion of fresh air into the lungs. ‘Vhether the sensation itself serves as a stimulus to the oiscles, or to the nerves connected with the organs of iespiration ; or whether the blood produces some more cotnp.ic.,,ted train of changes, which eventually ends in the Loni•action of the diaphragm. is a point which we are unable to de termine ; but the result is, that a necessi,ry connexion is established between these actions, in consequence of which the diaphragm contracts, wherever the blood is returned to the heart in the venalized state.