the most probable opinion seems to be one of an intermediate nature, that there are separate groups of cells, which are connected together, while these groups are themselves distinct.?

9 Haller's general description of the lungs is contained in El. Phys. viii. 2.9.. 18; his account of their minute structure, ibid. 26..30. He gives an ample detail of the controversy concerning the question, whether air that is impelled into the bronchial tubes, can pass into the intervals between the lobules, and the reverse; the authorities as to the fact appear to be nearly balanced, § 26; but it may be suspected, that when the transmission does take place, it is in consequence of the rupture of a portion of the delicate cellular membrane. Haller himself inclines to the opinion, that the cells do communicate, but that there is no communication between the lobules, § 30. With respect to the figure of the cells, Hales says that they appear in the microscope to be spherical; Stat. Ess. v. i. p. 241. Monro Secundus supposed that the lungs are composed of cells, in which the bronchia terminate, but that the cells communicate with each other, and that this is likewise the case with the lobules; Elements of Anat. v. ii. p. 89. et seq. See also Sprengel, Inst. Med. lib. 4. c. 4. sect. 1. p. 450, and Boyer, Anat. t. iv. p. 263, who appear to adopt the opinion of Haller. For what may be regarded as the current opinion among the French respecting the structure of the lungs, see the 4th vol. of Bichat, Anat. Descrip. p. 69, written by Buisson, who supplied the last portion of the work, which was left imperfect by the premature death of his precep tor also the article "Respiration," p. 13, by Chaussier and Adelon, in the Dict. Scienc. Med. published in 1820; also Dumas, Physiol. p. 3. § 2. c. 2. p. 45.. 50. Sommering, Corp. Hum. Fab. t. vi. § 14, supposes that the individual air cells are separate from each other, but that they all communicate by the bronchial tubes; Magendie, like Helvetius, conceives that the lungs are composed of a spongy substance, the cells of which freely communicate with each other; Physiol. t. ii. p. 262. et seq.; Journ, Physiol. t. i. p. 79; and Richerand, Physiol. p. 206,

But whatever may be our opinion respecting the precise form of the vesicles, or the mechanical connexion which there is between the air cells and the blood-vessels, we know that the parts are so arranged, as to enable the air and the blood to act upon each other, by the blood being divided into a great number of small portions, and thus expose as large a surface as possible, and by being separated from the air merely by the interposition of a very delicate membrane. The extent of the surface of the membrane lining the cavity of the air vesicles, must necessarily be very considerable; but with respect to the estimates which have been made of it by Keill,2

says, that most anatomists adopt the opinion of Helvetius, an assertion which appears to be scarcely warranted. Blumen. bach considers the cells as being unconnected with each other; Physiol. § 139; and this appears to be the opinion of Cuvier; Tabl. Elem. p. 41, 2. For a very complete investigation of the structure of the lungs we are indebted to Reissessen, who seems to have examined the parts with the greatest minuteness. He describes the vesicles as the closed terminations of the bronchial tubes, possessing a cylindrical and somewhat rounded figure; he states that they do not communicate with each other, or with the cellular substance in which they are enveloped. Edin. Med. Journ. v. xxi. p. 448. et seq. There is a peculiarity in the circulation of the lungs, which appears to have been first announced by Reissessen, and is confirmed by the present Prof. Monro, Elements of Anatomy, v. ii. p. 96, that there is a direct communication between the bronchial artery and the pulmonary vein, so that the greatest part of the blood which is conveyed to the lungs by the former vessel is retained by the latter; Edin. Med. Journ. v. xxi. p. 454. et seq.

20 Capacity of the Lungs in their different States.

3

Hales, and other physiologists of the last century, there is reason to suppose that they are, in a great measure, imaginary; nor do we appear to have any data from which we can form a more correct conclusion.*

Many attempts have been made by physiologists to ascertain the quantity of air taken into the lungs by a single inspiration. All, however, that we can obtain on this point is the average quantity; for, as was remarked above, the action of the chest is so far under the control of volition, that we are able to receive into it at pleasure very different quantities of air. There is also a considerable difference in different individuals with respect to the size and form of the chest, and it is also probable that peculiar states of the constitution, and perhaps even

3 Stat. Ess. v. i. p. 241.

4 An interesting account of the comparative anatomy and mechanism of the respiratory organs in the five classes of the mammalia, birds, amphibia, fishes, and insects, is given by J. Bell, Anat. v. ii. p. 133. . 168. It is written in that interesting and impressive manner, which is so characteristic of his works, although, in certain points, it is not technically correct. We have a very valuable article on the respiration of birds in Rees's Cyclop.; see also Hunter on the same subject in Phil. Trans. for 1774, p. 205, et seq.; he endeavours to prove that birds possess a proper diaphragm, but unless we use the word quite in a technical sense, it appears not proper to apply this term to any part of their thorax. Cuvier's "Leçons," on this, as well as on every other subject on which he treats, cannot be too carefully studied; t. iv. leç. 26. passim, and leç. 27. sect. 2; also Blumenbach's Comparative Anatomy, c. 14. with Mr. Lawrence's valuable notes.

Bulk of a single Inspiration; Goodwyn's Experiments. 21

particular habits, may have an effect upon the quantity of air received into the lungs. And besides the question respecting the average bulk of a single inspiration, there are three others connected with it, that are both curious and important. We may

inquire, first, what is the quantity of air left in the lungs after an ordinary inspiration, which may be considered as the natural or quiescent condition of the thorax; second, what farther quantity we are able to expel by the greatest voluntary exertion; and, lastly, what quantity is still left in the lungs after the most complete expiration.

With respect to the bulk of an ordinary inspiration, the first writer who attempted to ascertain this point by experiment appears to have been Borelli ; the method which he employed was afterwards improved upon by Jurin, who obtained results which would seem to be nearly correct. By breathing into a bladder, and making the necessary allowance for temperature and pressure, he estimated that he took into the lungs about 40 cubic inches. Since his time many attempts have been made to solve the problem, and results have been obtained which vary from a few inches to above 50. Goodwyn bestowed much attention upon the point, but his apparatus, although more complicated than Jurin's, does not appear to have been capable of furnishing

5 De Motu Anim. P. 2. prop. 81.

6 Phil. Trans. No. 355; v. xxx. p. 757, 8; La Motte's Ab. of Phil. Trans. v. i. p. 415.

equally accurate results, at the same time that his estimate involves some physiological positions, which are at least very questionable. His apparatus consisted of a closed vessel, provided with two tubes, through one of which he inspired, while the other terminated in a second vessel containing water; when he inspired from the closed vessel, an equal bulk of water was drawn into it from the second vessel, and by weighing the first vessel before and after the experiment, the weight of water raised, and consequently the bulk of air displaced, was ascertained. By taking the average of 30 inspirations, he concluded that the bulk of air received in the ordinary action of the lungs is no more than 14 cubic inches.R

But independently of other considerations, there are two obvious objections to this method of ascertaining the bulk of a single inspiration; first, that in breathing from the closed vessel, the water being raised from the open vessel contrary to its specific gravity, a greater effort would be necessary to receive the due quantity of air into the lungs; and although Goodwyn was aware of this circumstance, and attempted to obviate it," the nature of the apparatus seems to render this impossible. In the second place, as was remarked by Menzies,' when the mouth is removed from the tube, the external air will imme

7 Connexion of life with respiration, p. 28. 9 Essay, p. 32, 3.

8 Ibid. p.

36.

1 On Respiration, p. 18, 9.