[38 HUMAN PHYSIOLOGY CHAP. 



SUMMARY' OF THE MAIN POINTS 



1. The heart is the central organ of circulation. It is a 

 hollow, muscular organ situated in the chest, between the lungs, 

 and covered with the pericardium. 



2. The heart is five inches long, three and one half inches 

 wide, weighs about ten ounces, and consists of four chambers, 

 two auricles and two ventricles. 



3. The tricuspid valve is on the right side between the auri- 

 cle and ventricle, and the mitral on the left ; and the semi- 

 lunar valves are at the beginning of the arteries. 



4. All blood vessels leading from the heart are called arteries. 

 They have heavy, elastic, muscular walls, and carry blood to all 

 organs and tissues of the body. 



5. The arteries grow smaller and smaller until they shade 

 off into the very thin walled capillaries. These are so numerous 

 everywhere that you cannot pierce the skin with a fine needle 

 without injuring several of them. 



6. The capillaries unite and form larger vessels, called veins, 

 which grow larger and larger until there are only two great 

 trunks. The veins return the blood from the capillaries to the 

 right auricle. 



7. Lymphatic capillaries begin in lymph spaces all over the 

 body and serve as drainage tubes to return most of the wastes 

 into the blood. 



8. The thoracic duct returns the lymph from the entire 

 body below the diaphragm and from the left half above the 

 diaphragm. The right lymphatic trunk is much smaller ; it 

 returns the lymph from the right half of the body above the 

 diaphragm. 



9. A moderate amount of daily exercise is essential to the 

 proper circulation of both blood and lymph. The wastes stag- 

 nating in the lymph spaces lead to disease. 



