112 HUMAN BIOLOGY 



auricle and ventricle tough flaps of membrane are provided 

 which close the opening while the ventricles are contracting. 

 Connected with each of these flaps are tough cords of tissue 

 that are attached to the muscular walls of the ventricle. 

 These cords prevent the valves from being forced up into the 



A = positions of valves before B = position of valves at the 

 the contraction of the ven- beginning of the contrac- 



tricle. tion of the ventricle. 



FIG. 34. Diagrams to show the action of the valves of the heart. 



auricle (Fig. 34) . When the ventricles cease to contract, the 

 blood entering the auricles presses these valves downward 

 and so enters the ventricles. 



IV. THE BLOOD VESSELS 



158. Position of arteries and the pulse. We have de- 

 fined an artery as a blood vessel carrying blood from the 

 heart. Every time the ventricles contract, the arteries 

 leading from them are expanded, and this is true of every 

 artery in the body. Most arteries lie beneath thick layers 

 of muscle or bone, which protect them from possible injury; 

 but in certain regions of the body they lie close to the sur- 

 face. If one places the fingers on the wrist two inches or 

 more below the ball of the thumb, it is possible to feel a 



