SANGUTFEROUS SYSTEM. 483 



sacs (119. A. q.} 9 and they communicate by distinct ductus 

 arteriosi with the next anterior or second branches of the 

 aorta. The second and third aortic arches, of larger size, 

 unite above the oesophagus (119. A. o. o.), like the branchial 

 veins of fishes, to form the common dorsal trunk of the 

 aorta (119. A./*. /I), and their united trunks on each side 

 send off cephalic branches (119. A. o. o.) to the back part of 

 the head, as in the former class. The fourth or most ante- 

 rior pair, after giving off branches to the mouth, unite, 

 behind the aesophagus, with the cephalic branches to be 

 distributed on the head. The great aortic branches are 

 nearly similar in the amphiuma, where the proximal arches 

 are distributed chiefly on the long cancellated lungs, and 

 the two succeeding arches unite above the oesophagus to 

 form the descending aorta. 



The heart of the larvae, before the development of the 

 lungs, consists, like that of the fishes, of a single auricle 

 which receives the venous blood of the system, and of a 

 single ventricle which propels it into the bulbus arteriosus 

 and the branchial or aortic arches ; but as development 

 advances, and the air-sacs assume the functions of lungs, 

 the arterialized blood collected from these pulmonary ca- 

 vities, developes a small distinct sinus or left auricle on the 

 united trunks of the two veins which return it to the ven- 

 tricle. The existence of this smaller left auricle in the 

 adult amphibia was first pointed out by Dr. Davy in the 

 caducibranchiate species, as the frogs and toads, and the 

 same structure of the heart was discovered by Weber, to 

 pervade also the perennibranchiate amphibia, as the axolotus 

 and the proteus. The left auricle, smaller than the right, 

 is separated by a thin transparent septum from the larger 

 systemic auricle, and is provided, like it, with distinct 

 valves at its entrance into the ventricle. The venous blood 

 of the system is generally collected into a distinct sinus 

 venosus in amphibia, as in fishes, before it is transmitted 

 to the large thin right auricle of the heart. The sinus ve- 

 nosus of the triton is a large round contractile cavity like 

 the right auricle. The blood from the right and left auricles 

 is mixed in the ventricle, by permeating the loose columnar 

 structure of its parietes, and is prevented from returning 

 into either aurcle, by small semilunar valves defending the 



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