480 SANGTJIFEROUS SYSTEM. 



During the progress of the metamorphosis from the tad- 

 pole or larva state, the vessels proceeding to the head and 

 neck enlarge, as seen in the second stage of the triton cris- 

 tatus, (Fig. 137- / m.n. 0,) the anastomosing canals (137- b.b. 

 g. g.) between the trunks of the branchial arteries and veins, 

 increase in size, the capillary ramifications of the gills (137. c. 

 c. c.) become diminished, obliterated, and absorbed ; and the 

 small branches on each side proceeding from the posterior 

 branchial arteries to the lungs, form considerable pulmonary 

 arteries, (137. i- *) which ramify over the increasing surface 

 of these now important respiratory organs, (137. # #) The 

 venous blood of the system is now therefore conveyed in greater 

 quantity and by a more direct course, from the bulbus arte- 

 riosus (137. a,) through the communicating trunks of the 

 branchial arteries and veins (137- b. b. g. g.} to the branches 



FIG. 137. 



and trunk of the descending aorta. (13?. d. d. e.) The two 

 arterial trunks formerly (136. h. h.} conveying arterialized 

 blood from the branchial veins to the air-sacs or rudimentary 

 lungs, now become small ductus arteriosi (137- h. h.) con- 

 necting the pulmonary arteries (137. - iy) with the branches 

 (g. d. g. d.) of the abdominal aorta, (137* .) This is nearly 

 the condition in which the perennibranchiate species have 

 the development of their vascular system permanently ar- 

 rested, where the respiration is effected equally by pulmonic 

 and branchial apparatus in the adult state. By the further 

 metamorphosis of these vessels during the development of 

 the caducibranchiate amphibia, it is manifest that the ante- 

 rior pair of branchial arteries are gradually converted into 



