162 
MR. ADDISON ON THE AIR-CELLS OF THE LUNGS. 
and allowed it by its own weight to trickle down the bronchial tubes which were 
filled with the metal, and were very conspicuous at the thin margin of the organ. I 
placed a portion of the injected thin margin of the lung between two plates of glass, 
and on using slight pressure, the mercury at the extremities of the injected branches 
was forced into still smaller nodulated branches, which divided and subdivided in an 
unvaried branched order, (a) figs. 5 and 6 ; by using a little more pressure their 
numbers increased, the symmetry of the branches being readily detected by the mi- 
croscope ; but ultimately, by continuing the pressure, all symmetry was lost in a mass 
of minute globules ( a ). The same effects were produced by pressing the mercury in 
the bronchial tubes onward between the finger and thumb without using the plates 
of glass. 
With a good .light and a power magnifying 120 times linear, the globules of mer- 
cury are seen to be inclosed in cellular cavities formed laterally on branched passages. 
In the small branches the metal appears in round globules ; it is less globular in the 
larger branches, and a disposition to the formation of cells may be detected by the 
depressed lines seen on the column of mercury in branches of still greater magnitude. 
I have in my possession a preparation containing lobules from the thin margin of 
a foetal lung which were partially distended by air, sufficient to show the regular 
branched symmetry of the air-tubes to their ultimate terminations; establishing a 
perfect analogy between them and the secreting tubes of glandular organs. The air 
with which the tubes were partially distended has been absorbed by the fluid in 
which the preparation is preserved, and membranous septa are everywhere visible in 
their interior. The pulmonary cells are evidently formed by the pressure of the air 
against the sides of the tubes in the intervals between these folds (fig. 8.). A careful 
examination of the membranes of the air-cells by the microscope in a thin section of 
dried inflated lung will be sufficient to convince any one that they do not form round 
nor even rounded cells, but that they are perfectly flat membranous plates, circum- 
scribing polyhedric spaces. When healthy and recent they are exceedingly tough and 
elastic. I have often found that the tissue of the lungs may be stretched to twice 
its dimensions without rupturing them. They will bear the scrutiny of the highest 
powers of the microscope, and are characterized by several peculiar ovate bodies 
which form a part of their structure. They are also marked by numerous delicate 
lines, which are, no doubt, uninjected vessels of the capillary network. They possess 
an epithelium in the 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 
epithelium 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 the 
bronchi. 
The dimensions of the air-cells, as might be expected from the preceding details of 
their structure and formation, gradually increase with age, but in healthy women 
they are always smaller than in healthy men at the same period of life. Taking the 
recent lung of a healthy man, aged forty-five, as the mean between the small cells of 
