280 RESPIRATORY APPARATUS OF MAMMALS, ~ 
this cavity, by their summit or apex; and are covered by a 
serous membrane termed the plewra, which also lines the 
thorax, being reflected from one surface to the other precisely 
in the manner of the pericardium (§ 43). Thus the pleura of the 
outer surface of the lung is continually in contact with that 
which forms the inner wall of the thorax ; they are both kept 
moist by fluid secreted from them ; and they are so smooth, 
as to glide over one another with the least possible friction. 
The lungs themselves are 
yi very minutely subdivided ; 
and thus expose a vast ex- 
tent of surface in proportion 
to their size. The air-cells of 
the human lung, into which 
the air is conveyed by the 
branches of the wind-pipe, and 
on the walls of which the 
blood is distributed, do not 
average above the 1-100th of 
an inch in diameter. In the 
accompanying figure is repre- 
sented, on one side, the lung, 
d, presenting its natural ap- 
pearance ; and on the other, 
the ramifications of the air- 
passages or bronchial tubes, 
c, ¢, by which air is con- 
veyed into every part of the 
Fig. 162.—A1r-rusEs AND LuNG or Man. lungs. The trachea or wind- 
pipe, 6, opens into the 
pharynx or back of the mouth, by the larynx, a. The con- 
struction of this is especially destined to produce the voice, 
and will be explained under that head (Chap. xm); but 
it may be here mentioned that the entrance from the 
pharynx into the larynx consists of a narrow slit, capable of 
being enlarged or closed by the separation or approximation 
of its lips, which form what is called the glottis. The aper- 
ture of the glottis is regulated by the muscular apparatus of 
the larynx ; the actions of which are not under the direct 
control of the will, but are automatic, like those concerned in 
swallowing ($ 194) ; and the purpose of this provision is to 
