AMPHIBIA. 



97 



changes, as the branchiae are large in propor- 

 tion, and remain external during the whole 

 period of their existence ; the animal also 

 acquires considerable size before these organs 

 of aquatic respiration are lost. The heart in 

 the early stage of these animals consists of a 

 systemic auricle, which receives the whole of 

 the blood from the system after circulation, and 

 of a ventricle which propels it through a third 

 cavity, the bulbus arteriosus, to the branchial 

 arteries, of which there is one given to each 

 branchial leaf. From the capillary branches 

 of these arteries the aerated blood is received 

 by the branchial veins, which, as in fishes, 

 concur to form an aorta without an intervening 

 ventricle. From the last, or posterior branchial 

 artery, on each side is given off a branch 

 which goes to the rudimentary pulmonic sac, 

 and which ultimately forms the trunk of the 

 pulmonary artery. But the most interesting 

 and important change is that by which the 

 continuous branches of what were originally 

 the branchial arteries combine to form the two 

 trunks of the aorta. This is effected by means 

 of small communicating branches between the 

 branchial arteries and the branchial veins, 

 which, as the branchiae become absorbed, and 

 their minute branches are obliterated and lost, 

 gradually enlarge until they become continuous 

 trunks; and the artery, which was originally 

 branchial, then" becomes the single root of the 

 two descending aortae, and at its base gives off 

 the pulmonary artery. 



The two veins which return the blood from 

 the rudimentary air-sacs gradually enlarge as 

 these cavities become more important, and 

 assume the character of lungs ; and at length 

 they receive the name, as they perform the 

 function, of pulmonary veins. These by de- 

 grees become, as it were, distended at their 

 point of union with the heart, and ultimately 

 form the second auricle. 



This general description will be better un- 

 derstood by a reference to the subjoined 

 figures taken from the tabular views of M. 

 St. Ange, of which an English edition has 

 been published by Mr. Jones.* 



The following detailed description of those 

 figures is necessary to the correct understanding 

 of this intricate but interesting arrangement. 

 Fig. 18. 



The first period, previous to any change having 

 taken place in the branchiae, is given in fig. 18. 

 Four pairs of trunks (1, 2, 3, 4) go off from 

 the heart. The first branch on each side (1) 

 gives off a small anastomotic branch (5) ; 

 after which it becomes divided into numerous 

 branchial filaments (6); these, by their ulti- 

 mate subdivision, terminate in a capillary tissue 

 or network (7), from which arise other minute 

 returning vessels, forming, by their junction, a 

 single large vessel (9), which brings back 

 blood into the general circulation after it has 

 been aerated in its course through the branchiae. 

 The second branch (2) also gives off a small 

 one (14) previously to its subdivision in the 

 second branchial leaflet, which branch enters 

 the returning vessel ; thus producing a com- 

 munication between the two vessels 2 and 9, 

 as in the former case. The returning vessel 

 then terminates in the arch of the aorta, in 

 which the two vessels 13 and 15 also terminate. 

 The third principal vessel (3) is similarly 

 distributed on the third branchial leaflet, and 

 the corresponding returning vessel (16) termi- 

 nates in the aorta, as in the other case. The 

 arch of the aorta, thus formed, gives off a 

 branch (21), which, after receiving the fourth 

 branch from the heart (4), goes into the lungs 

 (19). 



The second period, shewn in Jig. 19, occurs 

 Fig. 19. 



* Tabular view of the circulation in vertebrated 

 animals. 

 VOL. I. 



w 



when the branchiae begin to contract. The 

 anastomotic branch (5), shewn in the former 

 figure, is not much enlarged, and assumes the 

 character of a continuous trunk with 1 . The 

 branches (11 and 12) have increased in size, but 

 the original continuation of 1 going to the bran- 

 chiae, has decreased in the same proportion. The 

 anastomotic branch (14) has acquired the size 

 of the arch of the aorta, whilst the continuation 

 of 2 is diminished, and the branchial leaflet 

 is contracted in a corresponding degree. The 

 branch 3 has become exceedingly small; and 4, 

 which was before the smallest, is now the largest 

 of all. By these changes in the relative di- 

 mensions of the different vessels, especially 

 in the enlargement of the anastomotic branches, 

 the whole system of the circulation is gradu- 

 ally being altered, until, in the third period, 

 (fig. 20,) it has assumed the character of that 

 in the reptile, by the total obliteration of the 

 branchiae and their vessels, and the enlarge- 

 ment of those branches, which, at first only 

 anastomotic, have now become principal. 

 In the adult condition of the animal, there- 



H 



