230 STRUCTURE OF THE VERTEBRATES 



conus arteriosus is muscular and is usually guarded by several 

 rows of valves. 



The reptiles as a group differ from the amphibia in several 

 important points: (1) the sinus venosus is small and usually not 

 visible on the outside of the heart; (2) the septum of the auricle 

 passes through the auriculo-ventricular opening, cutting the 

 valves into two groups; and (3) the ventricle is always more or 





Ventral aorta 

 Ventricle 



uricle 



Auricular septum 



A. Dogfish 



Auricular septum 



B. Crossopterygian 



Pulmonary 



artery 



Ventricular septum 



Aorta 



C. Amphibian 



D. Reptile 



E. Mammal 



Fig. 132. Comparative Anatomy of the Heart (Diagrammatic). The twist- 

 ing of the heart has been ehminated, the diagrams showing the hearts in 

 their primitive position with the ventricle and the auricle posterior. Xote 

 the growth of the septum, with the eventual separation of the ventral aorta. 



less divided by a septum into two chambers. The degree of sep- 

 aration in the ventricle varies widely in the different orders. In 

 the turtles and lizards the ventricular septum is a semicircular 

 band of tissue incompletely dividing the chamber. Blood from 

 the right auricle is directed by flaps of the valves into the right 

 (pulmonary) cavity, the aerated blood entering the left chamber 

 of the ventricle. As the heart contracts the opening in the septum 



