BLOOD SYSTEM. 



383 



thinner. The veins are found in two positions: (1) 

 deep-seated and accompanying the arteries, and (2) 

 superficial or subcutaneous. It is the subcutaneous 

 which show bluish through the skin, especially in 

 blondes. The greater size and number of the veins is 

 necessary because the blood current is sluggish in them 

 as compared with the arteries. 



Valves. — The comparative sluggishness of the current 

 is also the reason for the existence of semilunar valves 

 in veins. These are not in triplets, as at the opening 

 of the great arterial trunks into the heart. They are 



Fig. 263. — Vein : A, cut open so as 

 to show the valves, v \ B, section 

 through the valves. 



Fig. 264. — a, arteriole ; v, veinlet ; 

 c, capillary network. The ar- 

 rows show the course. 



scattered along their course irregularly and singly. 

 They can not, therefore, arrest, but only retard the 

 backward flow of the blood. The knotted appearance 

 of the subcutaneous veins when gorged with blood (as 

 when the arm is corded) is due to the filling of these 

 valves (Fig. 263). 



3. Capillaries. — The blood system is a closed system of 

 pipes. There is no discharge of the blood from the 

 arteries into the tissues and taking up of it from the 

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