SANGUIFEROUS SYSTEM. 481 



the great arterial trunks which proceed to the head and neck, 

 as seen in the third stage of the triton (Fig. 138. n. n. o. o.) 

 where the branchiae have been entirely absorbed, and these 

 p JG ia g cephalic vessels have acquired a 



less tortuous course. The second 

 pair of branchial arteries are con- 

 verted into the right and left 

 branches of the abdominal aorta 

 (138. b. d. b. d.), and the poste- 

 rior pair of branchial arteries be- 

 come the two pulmonary arteries 

 (138. i. k. i. k.) conveying mixed 

 blood from the heart to the lungs. 

 The development of the great 

 aortic trunks, from the previously 

 formed branchial arches, is effected in a similar manner, but 

 according to a simpler plan, in all the higher classes of ver- 

 tebrata, as in them the capillary ramifications for an aquatic 

 respiration are not developed from the branchial arches. 

 The anastomosing canals, therefore, observed in nearly all 

 the amphibia, between the trunks of the branchial arteries 

 and branchial veins, may be regarded as the continuations 

 of the primitive aortic arches, temporarily reduced in size 

 during the presence of the gills in the caducibranchiate 

 forms, and permanently kept open in those species which 

 preserve the branchiae through life; whilst in fishes, and 

 apparently in the siren, the aortic arches are entirely divided 

 into branchial capillaries, through which the whole blood of 

 the system is necessarily conveyed to increase the extent 

 of their aquatic respiration. The capillary branches of the 

 branchial arteries in the external gills of the tadpoles, do 

 not form ramifications as in the corresponding vessels of 

 fishes, but minute arches or loops which pass singly around 

 each small leaflet of the ramose branchiae. Although the 

 branchial arteries, after the metamorphosis of the frogs, and 

 after the functions of these vessels have changed, appear to 

 coalesce, and to originate on each side of the bulbus arterio- 

 sus by a common trunk, they are yet found in the adult 

 state to preserve their three canals distinct to their origin, 

 being separated from each other by internal septa which are 

 discovered on opening the two primary trunks of the aorta. 



PART T. II 



