226 On the Structure and Functions of the Organs of Respiration. 



from the attack of either birds or fishes. The air bag is wanting in those 

 fishes which lie in the bottom of the sea, or in mud, such as lampreys, 

 rays, flounders, &c. From the great vascularity of this organ, where it 

 exists, joined to other circumstances, and especially its analogies with the 

 respiratory apparatus of some of the amphibia, it appears probable that it 

 is in some way subservient to respiration, although its precise function has 

 not been ascertained. In many fishes, the membrane lining the alimentary 

 canal appears to be likewise subservient to respiration ■. for they may some- 

 times be perceived to swallow air, which is ejected as carbonic acid gas. 



There are few, if any, instances in the animal kingdom, where we may 

 so clearly perceive the transition between two great classes, differing ex- 

 tremely in the relative development of their various systems, and in the 

 media they are destined to inhabit, as we may between fishes and amphibia. 

 We are not to suppose, however, that this transition is really more gradual 

 than in other cases ; but it is more evident, and in its general outlines is 

 familiar to every one. The embryo of each of the higher orders passes 

 through a succession of these forms ; but they can only be traced by the 

 hand of a skilful anatomist. 



All of the class amphibia, (otherwise called Batrachian reptiles,) when 

 young and imperfect, inhabit the water solely, and are therefore provided 

 with organs for aquatic respiration, precisely analogous to those of fishes. 

 In all instances the branchiae are at first external, hanging like tufts from 

 the neck ; and this state continues through life in the proteus, siren, axo- 

 lotl, and other genera of the family of perennibranchia. In those, how- 

 ever, whose development proceeds further, we soon observe that these 

 external gills disappear, and internal branchiae are formed precisely similar 

 to those of fishes. As the animal advances towards its final change, these 

 disappear in their turn, and lungs are developed by which it breathes air 

 for the remainder of its life. These changes are of course accompanied 

 by great alterations in the circulating apparatus ; and indeed all the sys- 

 tems undergo a very rapid transformation. Dr. Edwards has shewn that 

 a tadpole may be prevented from becoming a frog at the usual period, by 

 diminishing the quantity of light and heat to which it is exposed ; and 

 that by the want of these stimuli, it will continue to grow in its imperfect 

 state, without advancing itself to a higher form. 



It appears as if in the family of perennibranchia, the process of develop- 

 ment had been stopped at an early stage ; so that the adult state of these 

 animals corresponds with the larval condition of the frog. We find them 

 possessed of rudimentary lungs, in the form of simple vascular membranous 

 sacs, very similar to the air bags of fishes ; and the tube by which they 

 open into the mouth bears a much greater analogy in structure to the 

 ductus pneumaticus, than to the tracheae of higher animals. It would not 

 be difficult to trace the gradual ascent in the openings of these air bags or 

 pulmonary organs, from their being completely closed sacs in many fishes 



