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 Eabbit 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. JSTow 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. Now 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 



