ORGANS OF RESPIRATION. 557 



of lateral branchial arteries, have been often observed in 

 the embryo of the common lacerta agilis] five pairs of 

 branchial arteries were perceived by Baer; three bran- 

 chial openings have been observed, at the same time, on 

 each side of the neck, in saurian, as in ophidian reptiles ; 

 a smaller fourth opening makes its appearance at a later 

 period. In the substance of the first or most anterior of these 

 pharyngeal folds, thus formed by the successive fissuring of 

 the outer serous layer of the neck, in the embryo of verte- 

 brata, are developed the two lateral portions of the lower jaw ; 

 in the second fold, are formed the cornua of the os hyoides ; 

 and in the succeeding folds are developed, in- branchiated 

 animals, as fishes and amphibia, the true branchial arches and 

 gills for aquatic respiration. 



The lungs of chelonian reptiles still extend over the whole 

 dorsal part of the trunk as far as the pelvis ; they are fixed 

 by the pleura to the ribs, which also separates them from the 

 cavity containing the digestive and generative organs ; they 

 are symmetrically developed on the two sides, largely cellular 

 internally, very capacious, and still composed of a single 

 distinct undivided lobe on each side. The large-celled 

 capacious lungs of turtles, expanded over the whole dorsal 

 region of the trunk, serve most advantageously to poise their 

 heavy body and its members while swimming, or floating 

 asleep in the water, like the air- sac of fishes, or like the 

 extensive pulmonary organs of birds, which poise their 

 suspended heavy parts while swimming in the light air. 

 The extent of respiration corresponds, not with the quantity 

 of air taken into the body of an animal, but with the extent of 

 surface over which the aerated blood is spread, the rapidity 

 of the blood's course over that surface, and the frequency of 

 the renewal of air in the pulmonary organs. The small, but 

 minutely subdivided cellular lungs of a rabbit, present a 

 much more extensive surface for the distribution of pulmo- 

 nary blood-vessels, than the capacious lungs of a turtle ten 

 times its size ; and although the quantity of air taken in by 

 the quadruped is less, it is entirely employed in effecting the 

 aeration of the blood, and hence its higher temperature, 

 activity, and general development. As the ribs of chelonia 

 are immoveable, the lungs fixed to their interior, and the 

 diaphragm consists only of a few muscular bands extending 



