266 STRUCTURE OF RESPIRATORY ORGANS. 
its structure modified for the advantageous performance 
of this function. This modification consists in the peculiar 
vascularity of this membrane, that is, in the large number of 
vessels that traverse its surface; and also in the thinness of 
the membrane itself, by which gases are enabled to permeate 
it the more readily. Moreover, we always find this membrane 
so arranged, that it exposes a very large surface to the air or 
water which comes into contact with it; and this surface may 
be immensely extended, without any increase in the size of 
the organ. Thus the small lungs of a Babbit really expose 
a much larger respiratory surface to the air, than is afforded 
by the large air-sacs of a Turtle which are ten times their size. 
This is effected by the minuteness of the subdivision of the 
former into small cavities or air-cells, whilst the latter remain 
as almost undivided bags. 
312. It is desirable to possess a distinct idea of the mode 
in which the surface is thus extended by subdivision. We 
may, for the purpose of explanation, compare the lung to a 
chamber, on the walls of which the blood is distributed, and 
to the interior of which the air is admitted. This chamber, 
for the sake of convenience of description, we shall suppose to 
have two long and two short sides, as at a. How if a parti¬ 
tion be built-up in the direction of its length, as at B, a new 
surface will be added, equal to that which the two sides 
previously exposed; since both the surfaces of this partition 
are supplied with blood, and are exposed to the air. Again, 
if another partition be built-up across the chamber, as at c, a 
new surface will be added, equal to that which the ends of the 
chamber previously exposed. And thus, by the subdivision 
of the first chamber into four smaller ones, the extent of sur¬ 
face has been doubled. How if each of these small ones were 
divided in the same manner, the surface would again be 
doubled; and thus, by a continual process of subdivision, the 
