ORGANS OF CIRCULATION. 165 



great, contractile venous reservoir (sinus venosus), from which the 

 blood is driven into the right auricle, of considerable size also. The 

 blood of the two auricles becomes mixed in the ventricle, and its 

 regurgitation is prevented by means of valves. From the ventricle 

 arises the truncus arteriosus, which soon dilates into a contractile 

 bulb, and is furnished with two pairs of superincumbent valves. 

 Three branchial arteries are always given off from this trunk, to 

 ramify upon the gills ; while from the latter three branchial veins 

 take their rise, and constantly unite into a common trunk, which, 

 anastomosing with that of the opposite side, forms the descending 

 aorta. The trunk of the pulmonary arteries is given off in a re- 

 markable manner from the most posterior of the branchial veins, 

 while the most anterior of these last furnishes the trunk of the 

 carotid. The pulmonary veins pour their blood into the left 

 auricle. 



Those Ichthyodea, as the Menopoma, Amphiuma, and Meno- 

 branchus, which in their perfect condition have no gills, but only 

 branchial fissures, depart in some degree from the type of structure 

 just described. The trunks of the venae cavas, with the auricles 

 and ventricle, are similar in character to those of the Perenni- 

 branchiate Ichthyoidea, but the number of valves in the bulb of the 

 aorta is increased, and from it there arise upon either side two 

 main arches, which unite together behind the oesophagus to form 

 the abdominal aorta, after having given off branches for the supply 

 of the head. 



In the larvae of Batrachia, the heart consists at first of a single 

 ventricle and auricle, with a sinus venosus, and bulbus arteriosus 

 which gives off branchial twigs, as in the Ichthyodea; the left au- 

 ricle is formed as the lungs become developed, and then the vas- 

 cular system resembles very much, e. g. in the larvae of the Sala- 

 manders, that of the fish-like Reptiles with persistent gills, e. g. 

 Proteus and Siren. 



In the Batrachia, when perfectly developed, we always meet with 

 two auricles, not separated externally, but divided within from each 

 other by a membranous partition, and a single ventricle. From 

 the latter arises a single long arterial trunk, which is divided inter- 

 nally at its origin by an imperfect septum into two halves, and 

 then splits into two branches, each of which again subdivides into 

 an arch for the aorta, and an artery for the lungs ; these two 

 trunks communicate throughout life by a pair of ductus artcriosi, 

 so that a mixture of two different kinds of blood takes place in 



