204 THE HUMAN BODY. 



The two trunks into which the posterior end of the ab- 

 dominal aorta divides (At) are named the common iliac 

 arteries ; each gives off some branches in the pelvis, and 

 then continues along the thigh as the femoral artery (C); 

 this runs to the knee-joint, behind which it is called the 

 popliteal artery (Po). The popliteal artery divides into 

 the peroneal (Pe) and tibial (Ta, Tp) arteries, which sup- 

 ply the lower leg and the foot. 



The Properties of the Arteries. Two fundamental facts 

 must be borne in mind in connection with the arteries : 

 First, that they are highly elastic and extensible ; a large 

 artery is, in this respect, much like a piece of rubber tub- 

 ing of the same size. Second, the arteries have rings of 

 muscular tissue in their walls, and when the muscle con- 

 tracts the bore of the artery (and consequently the amount 

 of blood which flows through it) is diminished. When the 

 muscle relaxes, the bore of the artery is increased, and more 

 blood passes along it to the capillaries in which it ends. 



The Capillaries. The smallest arteries pass into the 

 capillaries, which have very thin walls, and form very close 

 networks in nearly all parts of the body ; their immense 

 number compensating for their smaller size. The average 

 diameter of a capillary vessel is so small that only two or 

 three blood-corpuscles can pass through it abreast, and in 

 many parts the capillaries lie so close together that a pin's 



Into what vessels does the abdominal aorta ultimately divide? 

 What is the femoral artery derived from? To what point does it 

 run? What is the popliteal artery? Into what branches does the 

 popliteal artery divide? What parts do they supply with blood? 



What main facts are to be borne in mind in connection with the 

 arteries? How is the quantity of blood which an artery will let pass 

 regulated? 



What is found when the arteries are followed to the ends of their 

 smallest branches? Describe the structure, arrangement and size of 

 the capillaries. 



