When two sides of two contiguous air passages or cells come into opposition, the resulting partition is not composed of two layers, but one. If the cells were formed by the protrusive force of the air in enter ing in the first act of inspiration, such par titions would, of mechanical necessity, consist of two layers : they are, however, formed by an act of organisation. This curious and distinctive fact in the history of the human and the mammal lung vvill be again referred to. As the partitions of the cells are organised b'ore birth, it follows that the geometrical outline of each cell must be determined before the first act of inspiration. The satne argument applies to the foramina between the cells. They are not accidental perforations ; they are definitively and de signedly organised orifices, and are sustained in a permanently patulous state by an arch like arrangement of elastic fibres, which will be afterwards described.
As the air-cells of the lungs of mammals generally bear no proportion in size to that of the body of the animal, so in the human subject there is no relation between the di mensions of these cells and the stature of the body ; and it is probable that no estimate can be formed of the vital capacity of the lungs from a calculation of the individual dimensions of the air-cells. It should be observed that the orifices, by which one cell communicates with another, are of the same shape and dimensions as those which exist between the first set of cells and the bronchial tubes ; they can be very distinctly seen by looking down upon the air-cells from the intercellular passages, focus sing the microscope at the same time. Since these openings are not necessarily in a straight line, the exact number of cells which com municate cannot in this manner be determined; but the number will depend upon the dis tance which intervenes between any given part of the bronchial passage and the surface of the lobule : so that when a bronchial passage arrives nearest the surface it will be separated from it only by a single terminal cell. The dimensions of the cells in different animals present many diversities. In the lung of the kangaroo, especially in those parts remote from the surface, the air-cells are very small, and disposed with the greatest irregularity. The lining membrane is also proportionally imperfect, being perforated in many places opposite the areolx of the plex uses, so as to admit the air passing through them to come into contact with the coats of the vessels, as in the lung of the bird. In this mammal the minuteness of the air-cell is such, that it is too small to contain a single ciliated epithelium (Rainey). In the lung of the rat and mouse the air-cells are still more minute, and certainly many of them are not of a sufficient size to receive even an indi vidual particle of the dimensions of the bron chial ciliated epithelium. The air-cells are disposed with the same kind of irregularity, and the pulmonary membrane is deficient, as in the lung of the kangaroo. In the lung of the hare, the air-cells are very small, but perhaps not so much so as in the preceding species. The lung of the rabbit resembles that of the hare, but its air-cells are rather larger. In the lung of the dog the air-cells are larger than in the rabbit ; but still in the more central parts of the lung they are very minute, too minute, indeed, to be capable of having a lining of ciliated epithelium without being wholly unfitted for the purposes of respiration. In the monkey, the air-cells are large, and resemble those in the human lung. In the lung of the sheep and ox they are upon the whole, about the same size, and' very minute in both.* The diameter of one of the intercellulat * As the following passage expresses the views of S. van der IZolk, Harting, Ktilliker, as well as that of the writer, Adriani, I append it here at length : — passages ranges from to of an inch, and that of the cells from ,24„ to ,h (Todd and Bowman). In the lung of the calf these cells do not exceed By Dr. W. Addi son they are stated, in the human lung, to measure from Th tO 4-6 of an inch.
Minute Structure of the Air- Cells.— Three
anatomical elements enter into the composi tion of the air-cells : the epithelium, the blood plexus, and the elastic tissue. The inter lobular tissue is not here considered.
1st. The Epithelium of the Air-Passages and Cells.— It was first surmised by Dr. Thomas Addison, from the phenomena of the dif ference between pneumonia and bronchitis, that the air-cells of the lungs must be desti tute of epithelium. Dr. W. Addison contends that the air-cells "possess an epithelium in form of large round nucleated scales, and from one to fifteen or more nuclei may be counted in a single scale. A great many nuclei without any epithelial envelope may be seen upon them ; but I have never satisfied myself that they possess the ciliated cylinder epithelium so abundant in the trachea and bronchi."* Mr. Rainey denies the presence of epithe lium of any description on the interior of the air-cells, the vascular plexus being lined only by a " pulmonary membrane." Rossignol is the only subsequent writer who has sup ported this view :—" Neither does the ciliated epithelium lining the bronchial tubes extend into the intercellular passages, and from thence into the air-cells, or rather air-spaces (speak ing of the bird's lung), but it ceases where the bronchial membrane terminates. In the mammal, but especially in man, in whom the air-cells are very large, the fact of their having no epithelial lining can only be proved by a careful examination of the parts with the microscope, and therefore, with no other means than those of deciding this question, it might always remain sub jztdice, so long as persons are found who are more ready to confide in the assertions of others than submit to the pains and difficulty of examining the point for themselves."f Rossignol says : " Les parois alveolaires sont formees : 1° par une charpente de fibres qui laissent entre elks des espaces vides ou areoles ; 2° par une nzent brane transparente, qui n'offre aucune trace de fibres, qui recouvre la charpente preceden te; et remplit les espaces vides." In this passage M. Rossignol has evidently adopted without inquiry the conclusion of Mr. Rainey, with whose writings he seems well acquainted. The opinion of all German and English anatomists is now finally formed with reference to this point. Carpenter, Quain and Sharpey, Kirkes and Paget, KOHiker, S. Van der Kolk, Harting, Adriam, and Schultz describe a pavement epi thelium on the interior of the air-cells of the lungs ; and the author who has devoted many special examinations to this particular point is now convinced that a fine pavement epithe lium does cover these parts NI, hich he proposes to distinguish as the " hyaline epithelium." Messrs. Todd and Bowman, like Rossignol, adopt the views of Mr. Rainey, and teach that the air-cells have no epithelium of any kind. The adjoined is the illustration of the epithelium given by Schroeder Van der Kolk in Adriani's Essay.t furnished with a nucleus and minute granules They are adjusted accurately, as a single layer, edge to edge. The description given by 'Coi ner coincides with the preceding.* By this distinguished observer it is repre sented under the character of transparent pavement epithelium, the cells of which are By Kalliker the cells of this epitheliutn (a, ,fig.220.) are stated to consist of polygonal par ticles of from 1-1600th to 1-2250th of an inch in diameter, and from 1-2800th to 1-3800th of an inch in thickness. They repose imme diately upon the fibrous layer. They are normally shed ; though not readily detected in health, it is easy to discover these epithelia in disease. This epithelium lines every part of the air-passages and cells except the bronchi. These latter tubes are furnished with a thick layer of ciliated epithelium, which, as formerly stated, terminates abruptly at the commence ment of the intercellular passages. It may then be acceptea as a fixed conclusion in the histology of the lungs that the air-cells are lined internally by a single layer of " hyaline epithelium." This conclusion is corroborated by the minute structure of the respiratory organs in all aninials. In none are the vessels absolutely naked.