BESPIEATION. 



227 



Fig. 238. Portion of the frog's lung 

 from within, to shew the open parietal 

 cells figure drawn twice the size of 

 nature. 



however, are merely parietal, and communicate directly with the 

 middle cavity of the lung, 

 which is filled with atmo- 

 spheric air, and upon the 

 membranous walls of which, 

 as well as upon their bot- 

 toms, the blood-vessels ra- 

 mify. In the turtles (fig. 

 and crocodiles the cel- 

 lular subdivisions increase 

 in number and decline in 

 size, andthe common cavity 

 is divided by various bands 

 and septa stretching across 

 it. into a number of mutually 

 communicating sacs or pouches ; the whole lung thus acquires a 

 more compact or parenchy- 

 niatous appearance. In the 

 serpents (fig. 240), in which 

 one only of the two lungs is 

 ever completely evolved, this 

 at the upper part is covered 

 with small parietal cells ; 

 but these gradually become 

 smaller and smaller, less 

 and less distinct, and finally 

 disappear entirely, so that 

 the lower part of the lung 

 is completely vesicular and 

 unvascular. 



[ 388. In the class of 

 birds we observe, in the 

 same interesting manner, 

 the general type of the 

 lung preserved, but the sur- 

 face of contact is greatly in- 

 creased by means of parie- 

 tal cells, which are repeated 

 again and again. This mo- 

 dification is made necessary 



Fig. 239. A, several cells from the 

 lung of a Tortoise. A portion of one of 

 these cells is exhibited in B, magnified 

 five hundred times part of the septum, 

 a, a, which divides this cell from those 

 next to it, c and d, is seen. The ves- 

 sels are injected with size]and vermilion, 

 and form such thick masses, that the 

 islets of pulmonic parenchyma betwixt 

 them almost disappear. 



by the larger quantity of blood which is here transmitted to 



Q2 



