AMPHIBIA. 



99 



attached to the cranium ; (fg. 25, ,) the single 

 piece (/>,) and the two rhomboidal pieces (r, c ,) 

 in the meantime become united and extended, 

 (jigs. 25, 26,) and gradually lose by absorption 



Fig. 25. Fig. 26. 



d 



the branchial arches, and ultimately form a 

 broad disc, the body of the os hyoides, the 

 anterior margin of which on each side is di- 

 lated into a scutiform process, and the posterior 

 margin bears two bony appendages, which are, 

 in fact, the posterior cornua of that bone. 



Such are the changes which this bone un- 

 dergoes during the gradual passage of the 

 amphibious animal from the tadpole state, in 

 which it represents the class of fishes, to its 

 perfect or reptile condition; and it affords a 

 most interesting instance of the manner in 

 which the true nature of an organ, existing 

 under ambiguous circumstances in one class of 

 animals, is often clearly illustrated by its cha- 

 racters, or, as in the present instance, by its 

 transformations, in another. 



The minute filiform branchiae, which are 

 appended to the tadpole of the frog im- 

 mediately behind the head, have essentially 

 the same structure as is observed in the gills 

 of the perennibranchiate family, as the siren 

 and the proteus, though in a different form. 

 In the proteus each branchia consists of three 

 principal divisions or branches, from each of 

 which proceed seven or eight leaves, again sub- 

 divided into numerous regular leaflets form- 

 ing the ultimate divisions of the branchiae, on 

 which the extreme capillary branches of the 

 vessels ramify, and in which the blood under- 

 goes its necessary change. A minute rami- 

 fication of the branchial artery, conveying the 

 impure blood from the heart, enters each leaf- 

 let at its base, (jig. 27, a.) and passes, along 



Fig. 27. 



fr- 



its shorter or inner margin, giving off capillary 

 branches in its course, which, after meandering 

 over the surface of the leaflet, and commu- 

 nicating with each other in various directions, 

 pass over to the opposite margin of the leaflet, 

 and reunite in a corresponding ramification of 

 the branchial vein (6), which passes out at 

 the base to combine with the corresponding 

 branches from the other leaflets, and convey 

 the aerated blood back to the heart. This is 

 the general structure, modified however in the 

 different genera, by which this important func- 

 tion is effected in all the amphibia, as long as 

 they are confined to their aquatic life ; and 

 whilst the higher groups lose these organs as 

 they advance, and acquire the necessary organs 

 for atmospheric respiration, those of the lower 

 forms retain them throughout life, coexistent 

 with rudimentary lungs ; and thus probably 

 exhibit the remarkable phenomenon of a two- 

 fold mode of respiration at one and the same 

 time in the same individual. 



Such, then, is the general structure of the 

 organs of aquatic respiration, whether in the 

 early and transitory form in which it is seen in 

 the frog and the salamander, or in the perma- 

 nent character which belongs to it in the peren- 

 nibranchiate group of the siren, the axolotl, the 

 menobranchus, and the proteus. But as the 

 former of these groups acquires gradually a per- 

 fect and unmixed atmospheric respiration, and 

 as the pulmonary cavity serving this office is 

 only slowly developed, so we find in the pe- 

 rennibranchiate forms that the lungs also exist, 

 though in little more than a rudimentary state. 

 The early condition of the lungs in the cadu- 

 cibranchiate genera, in which they ultimately 

 exhibit a somewhat advanced structure, is that 

 of a mere rudimentary sac, without internal 

 cells or any appearance of even the commence- 

 ment of that more perfect structure which they 

 afterwards acquire. Gradually, however, the 

 inner surface is furnished with small processes, 

 forming little sacs or cells, on which the capil- 

 lary branches of the pulmonary vessels ramify, 

 and through the infinitely attenuated surfaces 

 of which the impure blood undergoes its essen- 

 tial process of depuration. 



In the lower forms of the class, as in it\e pro- 

 teus anguinus for instance, the air-bags, for they 

 scarcely deserve the name-of lungs in this state, 

 never arrive at this advanced stage of develop- 

 ment, but remain permanently in the condition 

 of simple membranous sacs. Every part of 

 the apparatus belonging to that organ is equally 

 rudimentary. The glottis consists of nothing 

 more than a small slit in the lower part of the 

 fauces, placed between the branchial apertures 

 of each side. The margin of this little opening, 

 which has no cartilaginous ring to support it, is 

 furnished with a small soft pair of muscles, by 

 which it is opened. The tube leading from this 

 opening speedily bifurcates, and one passes to 

 each air-bag. In this rudiment of a trachea 

 and of bronchi, there is no appearance of car- 

 tilaginous rings ; it is a mere membranous 

 canal, each branch of which opens without any 

 other apparatus into its air-cell. From the 

 perfect condition of the branchiae, and the very 



H 2 



