238 THE BIOLOGY OF THE ANIMAL 



which direction does it extend ? Examine its sur- 

 face with a lens. Do you find blood-vessels in its 

 wall? How do you distinguish the aortic arches 

 from the surrounding parts ? In what direction 

 do the arches run ? Make a drawing of the heart, 

 showing all of the parts thus far examined. 



Cautiously bend the heart backward by raising 

 its tip. On the dorsal side of the heart find the 

 sinus venosus. "What is its shape ? How is it 

 attached to the heart ? How much of the latter 

 does it cover? What is the texture of its wall? 

 Find three veins, the venae cavse, one posterior 

 and two anterior, leading into the sinus. At 

 what points do they enter? Dissect away the 

 ventral waU of the sinus and look for the open- 

 ing, the sinu-auricular aperture, by which it 

 communicates with the right auricle. "What is 

 the position of the aperture? Shape? Note the 

 lip-like folds or valves which guard the opening. 



II. The veins, 

 a. The anterior vense cavae. — On either the right 

 or left side trace the caval vein of that side by 

 carefully dissecting away the muscles and mem- 

 branes. It wiU be seen to be formed by the 

 union of three veins — one, the subclavian, com- 

 ing from the direction of the fore-limb ; another, 

 the external jugular, from the outer edge of 

 the hyoid bone ; and the third, the innominate, 

 coming in between the other two. "Which of 

 these veins is the largest? Trace forward the 

 two branches of the external jugular vein. One 

 branch, the lingual vein, comes from the 

 tongue and the floor of the mouth ; the other 



