It is, then, important to remember that the intercellular passages are open spaces between the ultimate cells, their walls being constituted of these latter. Like the ultimate cells, there fore, they participate actively in the process of respiration. They are not merely convective conduits. Since they proceed at every plane and angle from the centre of the lobule, a section of the latter in any direction will cut these passages both transversely and longi tudinally.
Ultinzate Air-Cells of the Lungs.— Vesicuke, 8. cellake derecu, s. Malpighiance, alveoli pul monunz ; Rossignol.—An air-cell in the human and mammalian lung is a space circumscribed by a single wall of reticulated capillaries, and varying infinitely in figure, and presenting in different parts of the lung numerous varieties of size ; each cell having an opening embracing a section, more or less considerable, of its circumference. The cells on the walls of the intercellular passages (the sides of the in fundibulum of Rossignol) may be defined as mere cup-shaped de'pressions, sometimes perforated at the bottom by a large foramen opening into one or more cells. Under the pleura the air-cell occurs as a four or six-sided chamber, of which the bottom, presenting under the pleural membrane, is rounded, and might readily be mistaken for the fundus of a pear shaped vesicle, the apex running into a bronchial tubule. If a cell, situated in the central parts of a lobule, be selected for examination, it will be found as a polyhedral alveolus, one or two or more of whose sides are deficient or con verted into a foramina, through which its enclosed space communicates with those of contiguous cells. No cell is a perfect geome trical figure—such, that is, as would be formed by regular plane sides ; because ridges and partial partitions, from the encroachment of the angles of neighbouring cells, project into and multiply its interior. It is not often that the eye falls upon a unilocular cell having only one opening : they occur most frequently as ir regular, angular spaces, with one or more im perfect sides, fig. 219. Those cells which com municate directly with the bronchial tubes and intercellular passages open 'into them by large circular apertures ; and they are themselves similarlyperforated, to communicate with other vesicles, which again open into others beyond them ; so that each of the openings in the air passage leads to a series of cells, :extending from it to the surface of the lobule. The vesicles which communicate directly with the air-passages are more niinute, and have a closer vascular network than those which lie nearer to the surface of the lobule ; an arrangement which is in beautiful harmony with the relative facility by which the air in them respectively is renovated. The dia meter of the human air-cells is about twenty times greater than that of the capillaries which, are distributed upon their parietes, varying, according to the measurement of Weber, from the 1 to the ,,11, of an inch.
v-siT It has been calculated by M. Rochoux that as many as t7,790 air-cells are grouped round each terminal bronchus ; and that their total number in the lungs amounts to no less than six millions. The dimensions of the air cells given by M.Moleschot*, are very much less than those of Rainey and Kolliker. Accord ing to the former observer, they range from 1 th to ."1,Thth of an inch : those of Car TT-6 penter and Kolliker correspond with those of Weber already stated. They continue to increase in size from birth to old age, and present in man a greater capacity than in woman. Dr. W. Addison supposed that the air-cells did not exist before birth, that they were mechanically formed by the first act of inspiration, and that the foramina between the cells were really ruptured partitions caused by the pressure of the atmosphere.
It was, however, first proved by Mr. Rainey, and by Professor Harting more lately, that they exist nearly as perfect in contour before as after birth. Neither the form, the num ber, nor the disposition of the air-passages and cells can any longer be held as the off spring of chance, but as the nicely adjusted products of marvellous foresight and design.
The preceding statement will enable the reader to understand the sources of the dif ferences by which the views of different writers upon the structure of the lungs are marked. It is easy to make a" labyrinth," a "passage," or a "group of vesicles," or 44 a funnel-shaped ar rangement of cells," out of the complex ap pearance which a section of an inflated and dried lung presents. It is important to observe that the classification of the cells into the pa rietal and terminal, as suggested by Rossignol, is calculated to lead to a false idea as to the real arrangement of the air-cells within the lo bule. The capsule of the lobule encloses a pear-shaped space ; but this is not the infun dibulum of Rossignol. This ingenious author applies this term to those parts which Mr. Rainey and Dr. W. Addison have distin guished as the air-passages surrounded and terminated by secondary passages and air cells. The septa bearing alveoli which pro ject everywhere into the funnel of Rossig nol, render the word parietal, as applied to them, altogether unmeaning. Every recent observer admits that the air-cells open every where into one another, such that the air entering one intercellular passage at one part of the lobule would traverse its entire extent through the intervening labyrinth of cells, and return throug,h another air-passage into the same peduncular bronchus.