80 THE STRUCTURE OF THE FOWL 



caudal caval opening. The opening from the 

 right atrium into the right ventricle is cres- 

 centic in outline, and is provided with a strong 

 muscular plate, which replaces the membranous 

 tricuspid valve of mammals. 



In the left atrium is a common opening 

 through which the two pulmonary veins pour 

 the blood after it has passed through the 

 lungs. The opening from this atrium into the 

 left ventricle is circular in outline and is guarded 

 by membranous flaps corresponding to the 

 bicuspid valve of mammals. The septum 

 between the two atria is thin and has a still 

 thinner oval area — the fossa ovalis — near its 

 centre. 



The wall of the right ventricle is much thinner 

 than that of the left, and the septum between 

 the two cavities bulges towards the right, with 

 the result that, if a transverse section be made 

 of the ventricular mass, the left cavity is 

 circular in outline while the right is crescentic. 

 At the base of the right ventricle is the opening 

 into the pulmonary artery guarded by three 

 pocket-like semilunar valves, the open mouths 

 of which look away from the heart. 



The left ventricle has thick walls except at 

 the apex. The aorta leaves the base of the 



