RESPIRATION OF AIR-BREATHING VERTEBRATES. 277 
to the air which is admitted through the spiracles. This 
arrangement is shown in fig. 46, l . 
324. Hitherto it has been seen that the respiratory appa¬ 
ratus is not connected with the mouth, which in the Inverte- 
brated classes has the reception of food as its sole function. 
On this account, we cannot regard the air-sacs of Insects as 
bearing any real analogy to the lungs of Vertebrata. The 
simplest condition of the true lung is that which constitutes the 
air-bladder (or “ sound ”) of Fishes. This we sometimes find in 
the condition of a closed bag, lying along the spine; and its use 
cannot be that of assisting respiration, since air is not ad¬ 
mitted to it from without. But in other cases we find it 
connected with the intestinal tube, by means of a short wide 
duct; and since many Fishes, as the Loach, are known to 
swallow air, which is highly charged with carbonic acid when 
it is again expelled, it seems probable that their air-bladder / 
effects this change in precisely the same manner as a lung 
would do—the air being transmitted to it from the intestine. 
There are some Fishes in which the resemblance of the air- 
bladder to a lung is more decided, and its connexion with the 
function of Respiration is evidently more important. The 
canal by which it communicates with the alimentary canal 
opens into the latter above the stomach, and even, in some 
instances, at the back of the mouth; so that a gradual 
approach is seen to the arrangement which exists in air- 
breathing animals. In these Fishes, as in the Amphibia that 
retain their gills (§ 87), it would appear that the respiration 
is accomplished partly by the lungs, and partly by the gills ; 
this is the case in the curious Lepidosiren (fig. 41), which, as 
formerly mentioned, is regarded by some naturalists as a Fish, 
and by others as a Reptile. 
325. The lungs of Reptiles are for the most part but little 
divided ; so that, although they hold a very large quantity of 
air, this does not act advantageously upon the blood, in con¬ 
sequence of the small surface over which the two are brought 
together (§312). In Serpents we find but a single lung, that 
of the other side not being developed (fig. 34); and this lung 
extends through nearly half the length of the body. Reptiles 
have no power of filling their lungs by a process resembling 
our inspiration , or drawing-in of air; but they are obliged to 
swallow it, as it were, by the action of the mouth. The skin 
