572 



KEPTIL1A. 



branchial artery (Jig.Q58, ), is made to circulate over the branchial 

 fringes (&), and, being again collected into the branchial vein (c), 

 in a purified condition, it is poured into those large trunks, the re- 

 presentatives of the vessels r, r, (Jig. 257,) which form the aorta. 



But, besides the branchial circulation, these creatures likewise 

 possess lungs (Jig. 254, z 9 t), and a pulmonary circulation of 

 greater or less importance in different genera. Nevertheless, the 

 pulmonary artery is merely a small twig given off from the aortic 

 system of vessels, through which semi-arterial ized blood passes to 

 the lungs, to be returned in a still purer condition to the left 

 auricle of the heart. 



(630.) If the student has fully comprehended the permanent 

 condition of the blood-vessels as it exists in the perfect Reptile 

 and in the Perennibranchiate Amphibian, he will have little 

 difficulty in understanding the changes which occur in the dis- 

 tribution of the vascular system during the metamorphosis of the 

 CADUCIBRANCHIATA. 



In the Salamander, when the lungs begin to be developed and 

 are co-existent with the branchial apparatus, the arrangement of 

 the circulating system is precisely similar to that described as being 

 permanent in the Perennibranchiata ; as may be seen by a reference 

 to the appended diagram, which would equally illustrate the dis- 

 tribution of the blood-vessels in both cases. 



In this early stage of the tadpole's life, the contraction of the 

 heart and bulbus arteriosus drives the greater part of the blood through 

 the branchial veins (Jig. 259, 

 a, a, a) to the gills, from which 

 it is returned in a purified con- 

 dition by the branchial veins 

 (f, y,y, ), which by their union 

 at length form the aorta, as 

 in fishes. At this period the 

 pulmonary artery (b), which is 

 very small in correspondence with 

 the as yet rudimentary condition 

 of the lungs, is merely a branch 

 derived from the aortic system, 

 and reinforced by a vessel (c) 

 given off from the bulbus arte- 

 riosus. The greater propor- 

 tion of the blood, therefore, 



Fig. 259. 



