and we may presume that, at this period, nearly all the air originally contained in the lungs will be expelled, and its place occupied by the aqueous vapour. Now, according to the method in which Mr. Coleman performed his experiments, a great portion of this vapour would be condensed, the moment that the lungs, by being removed from the thorax, were subjected to the pressure of the atmosphere, and any part of it that remained, would be destroyed in passing through the water of the inverted jar And besides the complete destruction of the aqueous vapour, a considerable proportion of the permanently elastic gas contained in the lungs would be carbonic acid, a part of which would also be absorbed in its passage through the water. It may be farther observed, as in the case of Goodwyn's experiments, that from the form and texture of the vesicles, and still more of the bronchia, it is not possible, by mere pressure, to expel all the air which they contain. It appears therefore evident, that Mr. Coleman has very much under-rated the capacity of the lungs in their state of complete expiration, although it is obviously impossible to ascertain the amount of the error."

7 The same cause may probably operate, to a certain extent, in death, produced by any cause, except by hanging; that the violent effort to expire will expel a portion of air, the place of which will be partly occupied by aqueous vapour, and thus make the residual contents of the lungs appear too small. Messrs. Allen and Pepys estimate the quantity at 108 cubic inches; Phil. Trans. for 1809, p. 412; this, I conceive, from various considerations above stated, to be below the average. In VOL. II.

D

From the above data, which, although confessedly imperfect, are the best which we possess, we may form some approximation to the knowledge of the quantity of air contained in the lungs in thoir dif ferent states of distention. Assuming 170 cubic inches as the quantity which may be forcibly expelled, and that 120 will be still left in them, we shall have 290 cubic inches as the measure of the lungs in their natural or quiescent state; to this quantity 40 cubic inches are added by each ordinary inspiration, giving us 330 cubic inches as the measure of the lungs in their distended state. Hence it will appear that about of the whole contents of the lungs is changed by each respiration, and that rather more than can be expelled by a forcible expiration. Supposing that each act of respiration occupies 3 seconds, or that we respire 20 times in a minute, a quantity of air rather more than 24 times the whole contents of the lungs will be expelled in a minute, or about 4000 times their bulk in 24 hours. The quantity of air respired during this period will be 1,152,000 cubic inches, or about 666 cubic feet.

There are two curious subjects of inquiry, con nected with the mechanism 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 alternations of inspiration and expiration during the

another part, indeed, of their papers they state 141 cubic inches as the residual quantity; Phil. Trans. for 1808, p. 270. 8 See Sprengel, Instit. Med. t. i. p. 470.

66

remainder of life. The first of these queries was proposed by Harvey as a problem for the consideration of his contemporaries. Quomodo nempe embryo," he asks, "post septimum mensem in utero matris perseveret? cum tamen eo tempore exclusus statim respiret; imo vero sine respiratione ne horulam quidem superesse possit; in utero autem manens ultra nonum mensem, absque respirationis adminiculo, vivus et sanus degat.' The question may be stated more generally, why is the animal which has once respired, under the necessity of continuing the respiration without intermission, when, if the air had never been received into the lungs, the same animal might have remained for some time without exercising this function?1

"9

Many solutions were proposed of this problem, depending upon principles which are obviously erroneous, and are now totally discarded; but the hypothesis of Whytt deserves attention, on account of the reputation which it long maintained, in some of the most distinguished schools of physiology. He observes, that before birth the blood of the fœtus is properly elaborated by the mother, but that when

9 Exer. de Gener. p. 361.

There is scarcely any opinion in physiology, however absurd it may appear at first view, which has not found some supporters, and accordingly attempts have been made to prove that the foetus breathes whilst still in the uterus. See Boyle's Works, v. i. p. 110; Haller, El. Phys. xxix. 4. 54; also Whytt on Vital Motions, sect. 9. p. 111..114, where the subject is very fully and satisfactorily discussed.

the communication is cut off, it becomes necessary for the young animal to produce the requisite change in its fluids by means of its own respiration. In furtherance of this end he supposes that, immediately after birth, an uneasy sensation is experienced in the chest from the want of fresh air, which may be regarded as the appetite for breathing, in the same manner as hunger and thirst are the appetites for food and drink. To supply this appetite, the sentient principle, with which the body is endowed, causes the expansion of the chest, in order to prevent the fatal effects which would ensue, were not the lungs to be immediately brought into action. This appetite for air is supposed to commence at birth, because, in consequence of the struggles of the fœtus at this period, the circulation will be quickened, and an additional quantity of blood will now pass through the lungs, which stimulates them into action, and seems to be the immediate cause of this appetite. He considers the exercise of the function of respiration "as owing to a peculiar sensation of the body, which determines the mind or sentient principle to put certain muscles or organs into motion." With respect to Harvey's problem, he regards it" to be of so very easy solution, that it is not a little surprising, that many physiological writers should have attempted it in vain." He explains it upon the principle of the change which takes place in the direction of the blood, the whole of which now passes through the vessels of the lungs, and which would

stagnate in them, were it not propelled through them by the alternate motions of the chest.2

Haller refers the cause of the first inspiration to the habit which the foetus had acquired, while in the uterus, of taking into the mouth a portion of the fluid in which it is immersed, and supposes that it still continues to open its mouth, after it leaves the mother, in search of its accustomed food; the air will therefore rush into the lungs, expand them, and thus reduce them to the state of a breathing animal, in consequence of which change of which change they will require a regular supply of fresh air, to prevent the blood from stagnating in its passage from the right to the left side of the heart.3

A somewhat similar view of the subject is taken by Darwin. He coincides with Haller so far as to conceive, that the fœtus acquires the power of deglutition before it leaves the uterus; but he remarks, that the acts of swallowing and of breathing are essentially different. When the fœtus is separated from the mother, an uneasy sensation is experienced from the want of air; to remove this uneasiness all the muscles of the body are called into action, and among others those of the thorax, and the uneasiness being by this means relieved, to use his own expres

2 On Vital Motions, sect. 9. p. 109.. 122. The author remarks that many physiologists have ascribed the first inspiration to instinct, but as he dislikes "the use of words whose meaning may be obscure or indefinite," p. 114, he prefers the explanation which is given in the text.

3 El. Phys. viii. 5. 2.