98 



AMPHIBIA. 



Fig. 20. 



fore, the heart consists of a single ventricle, 

 and of two auricles. The existence of a se- 

 cond auricle was first demonstrated in the 

 higher forms, the frogs and toads, by Dr. Davy,* 

 and, although in the latest works of Cuvier 

 and Meckel the auricle in these forms is de- 

 scribed as single, yet the more complicated 

 structure has since been amply confirmed by 

 many other anatomists. Weberf especially 

 has described the biauricular structure in a 

 large American frog ; but he failed to demon- 

 strate it in the perennibranchiate amphibia. 

 From a very interesting paper by Mr. Owen, 

 in the first volume of the Zoological Society's 

 Transactions,^ it appears that the biauricular 

 structure of the heart was ascribed by Hunter 

 to all the amphibia except the perennibranchiate 

 forms ; in which, however, the existence of 

 the left auricle has been satisfactorily deter- 

 mined by Mr. Owen, who has also given some 

 very interesting illustrations of the mode in 

 which the coexistence of branchiae and rudi- 

 mentary lungs is associated with certain pecu- 

 liarities of the circulation. The circulation in 

 the adult amphibia, then, assumes exactly the 

 character which we find in the reptilia, but 

 in the most simple form. 



The little pulmonic auricle receives the 

 blood perfectly aerated from the lungs by 

 means of the pulmonary veins. The systemic 

 auricle at the same time receives the impure 

 blood from the system by the vena; cavae. The 

 blood from the two auricles is sent together 

 into the single ventricle where it becomes 

 mixed, and this mingled arterial and venous 

 blood, thus but half purified, is propelled by 

 the same impulse, partly into the pulmonary 

 arteries to be more perfectly purified, and the 

 remainder through the aorta and the whole 

 circulating system to the different organs of the 

 body. The aeration of the blood, therefore, 

 is but imperfect; a condition which is met 

 with equally in the whole of the reptilia. 



VI. Respiration. The preceding observa- 

 tions on the circulation have in some measure 

 necessarily anticipated the account which we 

 have to offer of the correlative function of re- 

 spiration, and the character of those changes 



* Zool. Journal, vol. ii. 



t Beitrage von dem Herzen der Batrachier, 8vo. 

 1832. 



f Part iii. p. 213. 



to which its organs are subjected in the transit 

 of the amphibia from the pisciform to the 

 reptile state. Breathing water, in the first 

 instance, exclusively, these animals are fur- 

 nished in the tadpole condition with branchiae 

 or gills, of a leaf-like form, considerably sub- 

 divided, though far less so than in the fishes. 

 These branchiae are, in the first instance, in all 

 cases external ; but in the higher forms of the 

 class they remain so situated only for a brief 

 space, becoming, as in the frogs and toads, 

 internal at a very early period of their ex- 

 istence. They are supported by cartilaginous 

 or osseous arches, connected with the os hy- 

 oides, and the changes which they undergo are 

 accompanied by alterations in the form of that 

 bone, to which allusion has already been made, 

 and an account of which will now be given. 



At that period of the tadpole's existence, 

 at which its branchiae are in full action, and 

 the lungs still restricted to the state of a black- 

 ish, rudimentary tissue, we find the tympanic 

 bones, (Jigs. 21, 22, e,} developed to a great 



Fig. 22. 



extent, and forming the basis to which the 

 branchial apparatus is suspended, by means of 

 a rather thick angular portion, (Jigs.21, 22, a.) 

 This has been shewn by Cuvier to represent 

 what in the fishes is composed of three bones, 

 and is the medium by which in them the 

 whole branchial apparatus is suspended to the 

 temporal, and which bears also the branchi- 

 ostegous rays. Between these two lateral 

 branches is a single piece, (Jigs. 21, 22, 6,) 

 which, according to the same authority, cor- 

 responds to the chain of bones placed in most 

 fishes between the two first branchial arches. 

 To the posterior point of this bone are attached 

 two rhomboidal portions, (c, c,) to the external 

 margins of which are suspended the arches on 

 which the branchiae are supported, and which 

 represent the chain of bones in fishes, bearing 

 the two last branchial arches. 



As the age of the tadpole increases, and its 

 metamorphosis is proceeding unseen, (Jigs. 

 23, 24,) we find the branches which support 



Fig. 23. Fig. 24. 



d 



the branchial apparatus (a) gradually lengthen- 

 ing, and becoming more and more slender, 

 and at length exhibiting the two long cartila- 

 ginous pieces, by which the os hyoides is 



