CIRCULATION. 143 



obtain the names of subclavian, axillary, and brachial arteries, as 

 they pass under the clavicle, or cross the armpit, or descend along 

 the arm to the elbow, where they divide into two branches, called 

 the radial a,nd ulnar, or cubital arteries. 



3d. The intercostal arteries are several in number, and run be- 

 tween the ribs on each side of the body. 



4th. The ceeliac artery, which is distributed to the stomach, the 

 liver, and the spleen. 



5th. The mesenteric arteries, which ramify upon the intestines. 



6th. The renal arteries, which penetrate into the kidneys. 



And 7th. The iliac arteries, which in a manner terminate the 

 aorta, and which convey blood to the lower extremities, descend 

 along the thighs, and are there called femoral arteries ; then they 

 divide into many branches which terminate in the feet. 



The veins, which receive the blood thus transmitted to all parts 

 of the body, follow very nearly the same course as the arteries ; 

 but they are larger, more numerous, and generally situated more 

 superficially. A great number of these vessels pass beneath the 

 skin, others accompany the arteries, and, at last, they all unite to 

 form two great trunks which empty into the right auricle of the 

 heart, and which have received the names of vena cava superior 

 and vena cava inferior. 



The veins which come from the intestines present an important 

 peculiarity. After uniting in a large trunk, they penetrate the 

 liver, and there ramify like the arteries ; there they again unite into 

 a trunk and terminate in the inferior vena cava close to the heart. 

 This arrangement of the vessels is called the system of the vena 

 porta. 



When the chambers of the heart which open into each other, 

 and which as freely communicate with the great vessels that enter 

 and proceed from them> are forcibly closed, and the blood they con- 

 tain is projected from them, how is one uniform forward direction 

 given to the current ? Why, when the right ventricle contracts, is 

 the blood not sent back into the right auricle, as well as for- 

 ward into the pulmonary artery 1 There is but one mode of pre- 

 venting such an event, which is to place a floodgate between the 

 two chambers ; and there a floodgate is placed, and that floodgate 

 is the valve. As long as the blood proceeds onwards in the direct 

 course of the circulation, it presses this membrane close to the side of 

 the heart, and thereby prevents it from occasioning any impediment 

 to the current. When, on the contrary, the blood is forced back- 



