right anterior cardinal vein (precava) 



cardiac artery 

 from left ventricle^ 



carotids 

 pulmonary arch 



sinus venosus 



right ventricular chamber. 



left ventricular chamber 



B 



semilunar valve 



arrow through 

 foramen of Ponizza 



eft atrium 



Figure 1 1-3. Dorsal view of heart and main blood vessels of the alligator. A, as seen from below, 

 B, and details of foramen of Panizza, C. 



systemic blood passes through the right trunk, which leads 

 into the carotid arches, and the right systemic arch. 



Embryological development The development of the rep- 

 tile's heart is much like that of the mammal's. The heart tube 

 is formed of bilateral components and flexed. Posteriorly the 

 vitelline veins form a sinus venosus. Anterior to the sinus, con- 

 strictions delimit an atrium, a ventricle, and a truncus. The 

 flexion continues until the atrium lies dorsal to the ventricle. 

 The atrium becomes divided by an interatrial septum into 

 right and left chambers connected by perforations through 

 the septum. There is no evidence of an interventricular 

 septum (in Lacerla), but a fold arises from the ventral ven- 

 tricular wall to separate the cavum pulmonale from the 

 main ventricular cavity. This fold may represent a modified 

 interventricular septum. 



The truncus of the reptile is subdivided into three parts 

 rather than two. A series of folds appear which spiral around 

 the wall of the truncus in a clockwise direction from front 

 to rear. These folds arise at the base of the arches and ex- 

 tend posteriorly. At the heart end, there is a large dorso- 

 lateral fold on the left side and a smaller ventrolateral fold 

 on the right. Smaller folds now appear dorsolaterally on the 

 right and ventrolaterally on the left. The large left dorso- 

 lateral fold extends gradually across the chamber and meets 



the opposite fold to subdivide the truncus into pulmonary 

 and systemic channels. This septum extends the length of 

 the truncus from the arches to the ventricle. The right dor- 

 solateral fold also develops and subdivides the systemic 

 channel into right and left halves. This subdivision extends 

 only through the proximal part of the truncus. Semilunzu" 

 valves form at the base of these trunks. The several arches 

 at the end of the truncus are some distance beyond the ends 

 of the spiral channels within the truncus. 



In the bird heart, the sinus venosus is greatly reduced. 

 The right atrioventricular valve is muscular as contrasted 

 with the membranous valve of the lizard or the alligator. 

 The systemic channel is not subdivided into right and left 

 parts. The left systemic arch disappears so that the entire 

 channel, which has no connection with the right ventricle, 

 serves the right systemic and carotid arches (Figure 11-11). 

 These apparent changes in relationship between the trunk 

 openings and the interventricular septum help explain the 

 difference between the lizard, or turtle, and the crocodile. 



Reduction of the sinus venosus in the reptiles appears to 

 be a parallelism to the situation in the mammal. The re- 

 semblance of the four-chambered avian heart to that of the 

 mammal is superficial and misleading. A comparison of the 

 avian and crocodilian heart shows that they are built on 

 the same plan and agree in almost every detail. The croc- 



342 . THE CIRCULATORY SYSTEM 



