37 
their external sides are always nearly plane. Jones and Sieveking 
say that the vesicles are never angular nor polygonal until subjected 
to pressure from some pathological process. It has been calculated 
by Rochoux that the total number of air cells in the human lung 
amounts to 600 millions. Keil computes the number at three 
billions and 488 millions, and the expanse of membrane represented 
by them Lieberkuhn reckons as equal in area to 1,500 square feet. 
Hales gives an area of 289 square feet, and Keil an area of 21,906 
square inches ; but nothing in the way of number or measurement 
is really reliable ; this fact, however, is certain: that the extent of 
surface is very great. It is in these air-cells that the deposit of 
tuberculous matter takes place primarily and chiefly—first, on their 
lining membrane or wall, and then gradually extending to the 
entire cell, and probably to the minute bronchial tubes and the in- 
testinal tissue of the lungs. In Dr. Clarke’s preparations, illustra- 
tive of the seat of tubercle, he shews that the deposit extends to 
the walls of the air vesicles, the areolar tissue around the blood 
vessels and bronchi, and between the lobules cnly, at an advanced 
period of growth, and that it does not occur indifferently at any 
part external to the blood vessels. The observations of Jones and 
Sieveking imply that the primary deposit of tubercle takes place 
in a semi-fluid form in the vesicular cavity, which it distends so as 
to form a round point, of the size of a small pin’s head, visible to 
the naked eye. Rainey says that in tuberculous lungs, success- 
fully injected, it can be seen that the tuberculous matter is poured 
from the free surface of the pulmonary membrane into the interior 
of the air-cells ; the pale colour of the tuberculous matter contrasts 
so strikingly with the red capillaries, that the exact form and limit 
_ of the former can be seen if only one, or part of one, cell be filled. 
It is right, however, to state that opinions are now somewhat 
divided on the subject, and future investigations may, to some 
extent, modify the foregoing statements. Lach air-cell contains 
more or less pigmentary matter, and has a pavement of epithelium 
without cilia, composed of minute polygonal cells, which forms a 
simple layer, and rests immédiately on the lining membrane which 
forms the true wall of the air-celi. These epithelial cells derive 
_ additional interest from the fact that, in them, by fatty degeneration, 
the first elements of tubercle are made manifest. A beautiful net- 
~ work of yellow elastic fibrous tissue, to which the elasticity of the 
whole lung is due, connects and yet separates the air- -cells, forming, 
in fact, their defined and protecting frame-work, and giving support 
to the blood vessels which form minute capillary plexuses (each 
_air-cell having its own plexus) so arranged between the walls of 
each cell, that the contained air shall have the fullest opportunity 
4 thoroughly to aerate the blood. This net-work of pulmonary capil- 
aries is spread out so densely that the interspaces or meshes are 
eyen narrower than the vessels, which are, on an average, the three 
o 
