KESPIEATION OF AIR-BREATHING VERTEBRATES. 277 



to the air which is admitted through the spiracles. This 

 arrangement is shown in fig. 46, I. 



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 Eespiration 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 Eeptile. 



325. The lungs of EEPTILES 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. Eeptiles 

 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 



