69 
the Proteus, &c. and larve of frogs and salamanders, 
possess external gills. Amongst the ophidia, and 
perfect chelonia, batrachia, and sauria, we find lungs 
supplied with air through trachee, yet for the most 
part having much nearer analogy with the sacs or 
swimming bladders of fishes than the more deve- 
loped lungs of the superior classes. ‘ The lungs 
of birds are distinguished generally from those of 
all other animals by this peculiarity; they do not 
hang in the cavity of the trunk as loose bags, but 
are fixed to the interior of the back and of the sides, 
surrounding the viscera, and extending to the pelvis 
in the form of two flattened masses of red spongy 
cellular texture. From these, distinct cells convey the 
air to cavities throughout the whole of the bony struc- 
ture, diffusing through them the inspired, changed, 
heated, and rarefied air; thus filling the solids with 
a fluid lighter than the atmospheric, and aiding the 
99 
operation of flight™.” These conditions, however, 
are varied with respect to birds not destined to fly, 
as the ostrich and cassowary, and to those which 
are not only unable to fly, but which pass much 
time under water, as alca impennis and aptenodytes, 
the penguin, northern auk, &c. The superior mag- 
nitude of the vene cave of diving birds is shewn 
by Cuvier, Meckel, and Carus to be important to 
their long continued interruption of respiration. In 
the porpoise, seal, common and sea otter, and in 
tortoises, similar dilatations have been found. . The 
m Carus, by Gore, vol. ii. passim. 
RS 
