THE CIRCULATION. 157 



common iliac. This in each case divides into an external 

 iliac and an internal iliac. The internal iliac supplies 

 blood to the organs of the pelvis, while the external iliac 

 leaves the trunk and descends through the thigh as the 

 femoral artery. At the knee it is called the popliteal artery, 

 which then divides into the anterior and posterior tibial 

 artery. 



The arterial supply of the brain deserves special atten- 

 tion. At the base of the brain surrounding the optic tracks 

 and the pituitary body there is a circular blood vessel into 

 which all the arteries that supply the brain empty. These 

 supplying arteries are the two internal carotids and the two 

 intra- vertebral arteries. The two intra- vertebral arteries, 

 however, unite when they reach the medulla and form the 

 basilar artery which runs from the middle of the medulla to 

 reach the circular blood vessel. This circular blood vessel 

 is called the Circle of Willis. From it in turn arise the 

 numerous arteries which finally supply the brain. By this 

 rather remarkable arrangement several things are accom- 

 plished. Every part of the brain may receive blood brought 

 to it in as many as four different channels. Further, the 

 amount of the blood supply of the brain will be constant 

 throughout its entire substance; and lastly, the pulse of the 

 carotid and intra-vertebral arteries is materially rediiced in 

 the Circle of Willis, and so the blood reaches the brain 

 largely relieved of these rhythmic pulsations. 



THE VENOUS SYSTEM. 



The blood returns from the head and neck on each side 

 in three vessels called the external and internal jugular 

 veins, and the vertebral vein. From the arm the blood is 

 returned through the subclavian vein. The subclavian veins 

 on each side have emptying into them the external jugular, 

 then the vertebral, and then join with the large internal 

 jugulars to form the innominate veins. There are thus two 

 innominate veins, although only one innominate artery. 

 The two innominate veins, after receiving several small 



