VEINS 



319 



clear to the ventricular wall. In systole the edge of the septum is forced forward, 

 completely separating the two atria. No corresponding septum is developed 

 in the ventricle, but numerous muscular trabeculae extend through the 

 ventricular cavity which tend to prevent the mingling of arterial and venous 

 blood. Conditions dififer in the lungless urodeles (p. 277), there being no atrial 



septum, the whole heart being venous as in 

 fishes. In Proleiis, Cryplobranchus, and 

 caecilians the arterial bulb and trunk are 

 6 ^ /v. simple, but in many other urodeles and in 



anura they are spirally twisted and the bulb 

 contains a spiral fold or septum (fig. 340) 

 which possibly is the result of fusion of 

 valves. This fold separates the trunk into 

 two tubes. This division is continued in 



FxG. 339. Fig. 340, 



Fig. 339. — Heart and adjacent parts of Prolopterus, after Rose, a, atrium; aoe, 

 oesophageal artery; /, air bladder (lung); c, conus; h, hepatic vein; j», is, superior and 

 inferior jugular veins; oe, oesophagus; pa, pulmonary artery; pc, postcardinal vein; ph, 

 pharyngeal artery; s, sinus venosus; sc, subclavian vein; 1-4, afferent branchial (aortic) 

 arteries. 



Fig. 340. — Heart of frog laid open, based on figures by Bourne, av, atrioventricular 

 valve; c, carotid; la, left atrium; p, opening of pulmonary vein into atrium; pa, pul- 

 monary arch, connected with bulb at po; ra, right atrium; s, opening of systemic veins 

 into atrium; sv, spiral valve in bulb; v, ventricle. 



the anterior part of the truncus by a horizontal septum (short in urodeles, longer 

 in anura) separating aortic and pulmonary trunks, the former subdivided in a 

 similar way a little farther forward into carotid and aortic portions. 



In the early amphibian larvaj each fully developed aortic arch except the 

 last (morphological sixth) extends into the gills, but as the branchia; begin to be 

 absorbed, a small vessel connecting the bases of the afferent and efferent arteries 



