THE CIRCULATION OF THE BLOOD 189 



amount of fibrous connective tissue. They readily 

 collapse when empty, do not pulsate, and possess 

 elasticity and contractility, but not as marked as 

 seen in the arteries; however, these properties aid in 

 forcing the column of blood toward the heart, after 

 leaving the capillaries particularly after any obstruc- 

 tion to the flow of the blood stream. The veins collect 

 the blood from the capillary area throughout the 

 body, and return it to the right side of the heart. 

 They start as very small vessels called venules, and 

 then become veins, which increase in size as they 

 course toward their main trunks. 



Veins possess valves, arranged in pairs, composed 

 of folds of the lining membrane intima. They always 

 project toward the heart, and are flattened against the 

 wall of the vessel if the blood is flowing unobstructed 

 toward the heart; but when any obstruction takes 

 place they are distended into the channel, preventing 

 a return flow, or regurgitation of the blood. 



The veins distend under these conditions due to 

 their elasticity and accommodate the blood, until the 

 obstruction is removed, when the muscle fibers in 

 the wall contract and force the column of blood 

 onward. With the congestion relieved, the valves 

 again collapse against their walls. 



The Pulse. The pulse is the regular beat which 

 is transmitted to the examining finger w r hen placed 

 on an artery. It is felt best in the radial artery by 

 making gentle pressure with the tip of the index 

 finger in the region of the artery at the lower fourth 

 of the forearm on the outer side when the palm 

 is turned upward or supinated, just a little above 

 and to the inner side of the prominence of the 

 styloid process of the radius. By pressing downward 

 the beat or pulsation can be felt. The artery lies 

 on a firm bed formed by the flexor longus pollicis, 

 and pronator quadratus muscles, beneath which is 

 the bone. The expansion of the artery is due to the 



