36 
part of an inch, are clustered round the extremity of every bronchial 
tube, forming a lobulette or lunglet. These air-sacs possess ex- 
ceedingly delicate walls, and do not appear to communicate with 
one another, otherwise than by their common origin from the 
bronchial tube. They generally increase in size slightly towards 
their closed extremity, and often bifurcate.. The internal surface 
of every air-sac, and even of the bronchial tube, for a short distance 
before it terminates, presents an alveolated or honeycombed 
appearance. These minute, shallow, cup-like depressions constitute 
the air-vesicles or air-cells of the lung, and from eight to twenty 
may be counted on the interior of each air-sac. It will thus be 
seen that each terminal bronchial tube is connected with an entire 
group of air-cells, and does not end in a single air-cell, and that 
these fine terminal ramifications of the bronchial tubes never join 
each other. According to Rainey, the minute bronchial tubes when 
they arrive at within about one-eighth of an inch of the surface of the 
lung become changed in structure, and are continued onward under 
the name of intercellular passages, the walls of which are formed by 
the air-cells between which they pass, and by which they are sur- 
rounded. Suggestions for the microscopic investigation of the human 
lung, and the method of preparation of the same, will be interest- 
ing to the members of a microscopical club. In order to examine 
the air-sacs and air-cells, or alveoli (1) a thin slice should be cut 
off the surface of a portion of lung which has been injected, inflated, 
and dried, and the portion itself (not the slice) should then be 
placed under the dissecting microscope. The cut orifices of the 
air-sacs will then be observed ; or (2) soak a piece of lung that 
has been injected, inflated, and dried in spirit for some time, and 
when the piece is well saturated, dissect it under the microscope. 
By imbibition of the spirit, the mass of lung swells, and the air- 
tubes and air-sacs remaining distended, the parts assume nearly 
the same shape and size they have in life ;. or (3), without making 
any section, the ultimate air-tubes may be seen by a magnifying 
power of from 10 to 20 diameters, and they can be followed to their 
termination if the lung has been properly prepared previously. 
This preparative process may be accomplished in several ways, 
namely (1), by inflation of the lung through a bronchus, and sub- 
sequent desiccation ; (2), injection of the air-tubes with mercury or 
wax, &c.; (3), injection of the blood vessels with some opaque 
materials, such as a coloured solution of gelatine, red, yellow, or 
blue, and inflation of the air-tubes, with subsequent desiccation ; 
(4), injection of both vessels and tubes with a transparent substance 
which becomes solid on cooling, such as a solution of gelatine, or 
a mixture of turpentine and wax. The diameter of the air-cells of 
the human lung varies from about the 70th to the 200th part of an 
inch; their shape is described as being angular, globular, and ir- 
regular. Kolliker says that they are invariably polygonal, and that 
