152 CIRCULATORY SYSTEM 



of osmosis, since the blood vessels are closed tubes. Blood sup- 

 plies the cells with building and fuel materials and collects wastes 

 from them ; therefore, some blood vessels must be tiny enough 

 to get near to all the cells. These microscopic blood vessels, the 

 capillaries, connect with larger and larger vessels, the arteries and 

 veins, until the largest arteries and veins attain a size of one half 

 inch in diameter. 



Problem. Study of the heart. 



Secure from the butcher, the heart of an ox or of any other large animal. 

 If the school is near a slaughterhouse, it may be possible to secure enough 

 specimens so that each group of four students may have one. Cut the hearts 

 longitudinally. (These hearts may be kept in formaldehyde for further study.) 



I. Describe the covering of the heart (the pericardium) . 



A. What is its purpose ? 



B. Explain all adaptations for this purpose. 



II. Count the number of chambers or spaces in the heart. 



A. The upper spaces are the right and left auricles; the lower spaces, 

 the right and left ventricles. Describe the structure, location, and size of 

 auricles and ventricles. 



B. Describe the structure that separates the left side of the heart from 

 the right side. 



C. Find the movable flaps, known as valves, that separate the auricles 

 from the ventricles. 



1. Describe the valves in the left side of the heart, including in 

 your description the number of flaps, method of attachment, and func- 

 tion. Observe and describe the valves in the right side. 



a. When blood passes from the auricles to the ventricles, what 

 effect would you expect it to have on the valves? 



b. If blood attempts to pass from the ventricles to the auricles, 

 what effect would it have on the valves ? 



c. What must be the normal direction of the flow of blood ? 



III. Name the tissues that largely comprise the walls of the heart. 



A. Compare the thickness of auricle walls with those of the ventricle. 



B. Compare the thickness of the wall of the left ventricle with other 

 walls of the heart. 



C. What is the relation of thickness of heart muscle to its activity ? 



