EESPIRATION. 



227 



fcH 



however, are merely parietal, and communicate directly with tht 

 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. 

 239) and crocodiles the cel- 

 lular subdivisions increase 

 in number and decline in 

 size, andthe common cavity Fig- 238. Portion of the frog's lung 

 is divided by various bands frora within to shew the open parietal 

 and septa stretching across ^~ figure drawn twice the size of 

 it, into a number of mutually 



communicating sacs or pouches; the whole lung thus acquires a 

 more compact orparenchy- 

 matous 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 

 un vascular. 



[ 388. In the class of 



birds we observe, in the p:_ 9 o Q A 



rig. 239. A, several cells from the 



same interesting manner, lung of a Tortoise. A portion of one of 



the general type of the these cells is exhibited in B, magnified 



lung preserved, but the sur- five hundred times part of the septum, 



face of contact is greatlvin- a> a ' which ^' ides this cell from those 



creased bv means of narif next to il> and d ' is seen ' The ves ' 



- P ane ; sels are injected with size'and vermilion, 

 tal cells, which are repeated and form such thick masses, that the 

 again and again. This mo- islets of pulmonic parenchyma betwixt 

 dification is made necessary taem almost disappear. 

 by the larger quantity of blood which is here transmitted to 



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