184 THE ANATOMY OF VERTEBEATED ANIMALS. 



cavities by an incomplete longitudinal partition. It termi- 

 nates, npon 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 piilmonary arteries, and arteries to the integument; 

 the middle tranks 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 

 trimks 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 inten-upted, its lower part becoming 

 the branchial artery of a giU-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 giUs, but not in 

 the third. 



The adult Axolotl (Siredon) has four pairs of aortic 

 trunks (Fig. 25, E, p. 91) ; the hindermost pair (vi.) gives off 

 the pvdmonary arteries, the three next (v., iv., iii.) supply 

 the external branchiae ; and the anterior trunk passes, above, 

 into an artery which divides into hyoidean and carotid 

 branches. 



In Salamaiulra there are four pairs of aortic trunks in 

 the adult, but the upper moiety of the first, on each side, 

 is obliterated, and remairis 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 ; and it then unites wi.th the second and third to form 

 the root of the dorsal aorta. The basal moiety of the first 



