HUMAN STRUCTURE AND LIFE-ACTIVITIES 487 



man and other vertebrates the blood always flows in one 

 direction from the heart in blood-vessels, and that these are 

 so arranged that blood ultimately comes back to the heart, 

 i.e., makes a complete circuit. This circulation is remark- 

 ably rapid; it is believed that blood goes from the heart 

 through the capillaries in such a distant organ as a foot and 

 returns to the heart in about half a minute. (How many 

 times would the blood complete its circuit in a day?) 



410. Structure of the Heart. (D or L) Procure from the market a 

 sheep's heart with the lungs attached, and with the membrane 

 (pericardium) surrounding the heart. Insert a large tube in the 

 trachea and inflate the lungs by blowing into them. Note the rela- 

 tion of lungs and heart. Carefully dissect away (with forceps and 

 scissors) the fat which adheres to the heart, taking care not to cut off 

 any arteries or the thin-walled veins. It is best to stuff the veins 

 with cotton or insert a small roll of paper. 



The general form and external structure of the sheep's heart is 

 similar to that described and illustrated for the human heart in many 

 books on anatomy and physiology. Examine pictures in such books ; 

 note positions of the two auricles and two ventricles of the sheep's 

 heart. Notice that a probe (e.g., a rounded stick) inserted into a vein 

 enters an auricle. The connection of the arteries with the ventricles 

 can be seen later (next paragraph). Cut across the heart transversely 

 about an inch from the pointed end (apex). This will open the two 

 ventricles. The left one is a rounded cavity, the right is crescentic 

 in outline. Note the relative thickness of the muscular walls of the 

 two ventricles. Now, take a blunt stick about the size of a pencil, and 

 inserting it into the left ventricle, probe carefully until it emerges out 

 of the largest artery. This is the aorta, whose branches are arteries 

 leading to all the organs except the lungs. Insert a similar stick 

 into the right ventricle and out through its artery. This is the pul- 

 monary artery, whose branches conduct blood to the lungs. 



The action of the valves in the aorta or pulmonary artery in pre- 

 venting blood from flowing back into the ventricles can be demon- 

 strated on a heart with the apex removed, as follows. Insert a large 

 glass tube (about \" or f " caliber) into one of these arteries held 

 upright, and fill it with water. Or connect the artery with a large 

 funnel. If the valves are still in good order, the water will remain in 

 the tube or funnel. 



