COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 229 



the auricle. The sinus connects chiefly with the right auricle and is slightly 

 displaced to the right. By cutting open the sinus locate the sin-auricular open- 

 ing guarded by a pair of valves. Cut into one of the auricles and wash out 

 its contents. Looking into the auricle note the inter auricular septum which 

 separates the two auricles; the septum is very incomplete, being perforated by 

 a number of openings. Remove the ventral half of the ventricle and also 

 slit open the conus and bulbus arteriosus by a longitudinal incision. Note the 

 thick spongy walls of the ventricle and the numerous muscle strands in the 

 interior. Locate the single auricula-ventricular opening between the auricles 

 and ventricle. It is on the left side. It is guarded by a pair of valves. In the 

 base of the opened conus arteriosus note the transverse row of three semilunar 

 valves. In the bulbus arteriosus is a longitudinal partition, dividing the interior 

 into right and left channels. 



The heart of Necturus and of all Amphibia receives both arterial and venous blood, and 

 consequently there is a double circulation through the heart. The blood from the systemic 

 veins enters the sinus venosus and is passed on chiefly to the right auricle. The blood from 

 the pulmonary veins, which is aerated blood, returns to the left auricle. Owing to the 

 incomplete nature of the interauricular septum there is some mixing of arterial and venous 

 blood. The blood from both auricles passes into the single ventricle where it is further mixed, 

 and the mixed blood exits by way of the conus arteriosus and ventral aorta. Passing to the 

 gills the blood is aerated and as arterial blood enters the dorsal aorta and pulmonary artery. 

 As the pulmonary artery already contains aerated blood before it reaches the lungs, it is evi- 

 dent that the lungs are but slightly functional in the gilled urodeles. 



9. Comparison of the circulatory system of Necturus and the elasmobranchs. It is 

 evident that the circulatory system of Necturus has undergone some modifications from the 

 condition seen hi elasmobranchs. In the arterial system the chief changes concern the aortic 

 arches. Of the five arches present in elasmobranchs but three have persisted hi Necturus. The 

 persistent arches are the third, fourth, and sixth, the second and fifth having vanished. In 

 addition we note the origin of a new vessel, the pulmonary artery, as a branch from the last 

 (sixth) aortic arch. The aortic arches of Necturus have still the form of an arch which passes 

 through the substance of a visceral arch. In the venous system the changes are more pro- 

 nounced. The anterior cardinal vein still remains the chief vein of the head, but the posterior 

 cardinal vein is much decreased in importance. The functions of the posterior cardinal vein 

 are taken over by a new vein, the postcaval vein, which is absent in fishes (except in the Dipnoi). 

 The origin of the postcaval vein is somewhat complicated but from a study of its relations in 

 Necturus it is evident that the anterior part is derived from the hepatic veins or sinuses of the 

 elasmobranchs (and these in turn are the proximal portions of the vitelline veins), and that 

 the posterior part of the postcaval is formed from the subcardinal veins, chiefly the right one 

 (Fig. 57, p. 224). The posterior portions of the posterior cardinal veins are functioning as the 

 renal portal veins as in fishes. The anterior portions of the posterior cardinals are diminished 

 in importance but have the same relations as in fishes; posteriorly they are connected with the 

 subcardinals (postcaval) as also in elasmobranchs (Fig. 55, p. 205). It will also be noted that 

 the renal portal veins (posterior cardinals) have increased their posterior connections. Whereas 

 in fishes they collect only from the tail, in Necturus they collect from both tail and posterior 

 appendages. This is due to a union between the renal portal and abdominal veins (Fig. 57). 

 Meantime the ventral abdominal vein (same as the lateral abdominal veins of elasmobranchs) 



