124 



HUMAN PHYSIOLOGY 



CHAP. X 



auricle and ventricle, and one through which the blood 

 escapes into the aorta. 



These openings are all guarded by gateways, or folds 

 of the thin lining membrane of the heart, called valves. 

 All the valves open one way only, and so permit the 

 blood to flow in one direction, but prevent its return. 

 Guarding the opening between the right auricle and 

 ventricle is a large, three-pointed valve called the 

 tricuspid. The one in the left side is a two-pointed 



valve called the 

 mitral because 

 it looks like a 

 bishop's cap. 

 The valves be- 

 tween the ven- 

 tricles and the 

 arteries are 

 called the semi- 

 lunar valves from their half-moon shape. Sometimes 

 the valves of the heart do not work as they should, and 

 the person suffers from valvular heart disease. 



96. Blood Vessels. The arteries are the vessels that 

 convey blood from the ventricles to all parts of the 

 body. They always lead from the heart. The arteries 

 have thick, strong, tough walls made of muscles, elastic 

 tissue, and a lining of very thin, smooth membrane. 

 The muscles enable the arteries to enlarge whenever 

 more blood is needed in any part of the body and to 

 contract when a smaller amount is required. There are 

 nerves which regulate the size of the arteries, causing 



FIG. 64. Aorta cut open, showing semilunar valves 

 (Zuppke). 



