THE CIRCULATION OF THE FROG. 159 



lulbus arteriosus. A venous sinus, the walls of which are 

 rhythmically contractile, receives the venous blood from the 

 body, and opens into the right auricle. In Proteus, Menohran- 

 chus, and Siren, the septum of the auricles is less complete 

 than in the other Amphibia. The left auricle is much smaller 

 than the right, and a single pulmonary vein opens into it. 

 The interior of the ventricle is more like a sponge than a cham- 

 ber with well-defined parietes. The walls of the long bulbus 

 arteriosus contain striated muscular fibres, and are rhythmi- 

 cally contractile. Valves are sometimes placed at each end 

 of it, and it may be imperfectlj' divided into two cavities by 

 an incomplete longitudinal partition. It terminates, upon 

 each side, in either three, or four, trunks, which ascend upon 

 the branchial arches. The most anterior of these trunks give 

 off the carotid arteries, the most posterior the pulmonary ar- 

 teries, and arteries to the integument ; the middle trunks form 

 the principal roots of the dorsal aorta. 



In Proteus, where there are three branchial arches, the 

 bulb of the aorta splits into two trunks ; each of these divides, 

 at first, into two branches, and then the posterior branch, on 

 each side, again subdivides into two others. Thus, three pairs 

 of aortic trunks are formed, which ascend upon the branchial 

 arches. The two anterior pairs of aortic trunks pass directly 

 into the roots of the dorsal aorta, but each gives off a vessel 

 which enters one of the external gills, the blood from which is 

 brought by an efferent canal into a higher part of the same 

 aortic arch. The third aortic trunk, on each side, is inter- 

 rupted, its lower part becoming the branchial artery of a gill- 

 tuft. The blood is carried out of this branchia by a venous 

 trunk, which opens into the root of the dorsal aorta, and is, in 

 reality, merely the upper part of the third aortic trunk. The 

 facts may be expressed in another way, by saying that the 

 bases of the branchial artery and vein anastomose in the first 

 two gills, but not in the third. 



The adult Axolotl [Siredon) has four pairs of aortic trunks 

 (Fig. 25, E, p. 83) ; the hindermost pair (n.) gives off the 

 pulmonary arteries, the three next (v., iv., ni.) supply the ex- 

 ternal branchise ; and the anterior trunk passes, above, into an 

 artery which divides into hyoidean and carotid branches. 



In Salamandra there are four pairs of aortic trunks in the 

 adult, but the upper moiety of the first, on each side, is oblit- 

 erated, and remains as a mere ductus Botalli. The fourth 

 trunk gives off the pulmonary artery; some twigs for the 

 oesophagus, and a few cardiac branches, next arise from it; 



