144 STUDIES IN PHYSIOLOGY 



the neck region to supply the right half of the head; the 

 other division passes under the collar bone, down alongside 

 the upper arm bone, and near the elbow divides into the 

 ra'di-al and ul'nar arteries, which lie close to the bones of 

 the forearm, from which they are named. (The pulse is 

 usually taken by feeling the radial artery.) From the left 

 side of the aortic arch two distinct arteries are given off. 

 The left carotid artery takes a course similar to the right 

 carotid described above, thus supplying the left half of the 

 head. The left arm, forearm, and hand are supplied by 

 a separate branch from the left side of the arch. The coro- 

 nary branches from the arch of the aorta, that run into the 

 muscle of the heart itself, have been already described. 



As the aorta passes downward through the chest cavity 

 (the thoracic aorta), it supplies blood to the air tubes and 

 lung tissue (bronchial arteries), to the esophagus, and to 

 the muscles that move the ribs and the spinal column. 



In the abdominal region five principal branches are given 

 off from the aorta. The first (the coe'li-ac axis) soon divides 

 into three parts, which supply the stomach (gastric artery], 

 the spleen (splenic artery), and the liver (hepatic artery) 

 respectively. Two other branches (the upper and lower 

 mes-en-ter'ic arteries) carry blood to the walls of the intes- 

 tines. The two other arteries branch off to the kidneys 

 (renal arteries). In the posterior part of the abdominal 

 cavity the aorta divides into two trunks, which supply the 

 hips, thighs, and lower legs. 



The Systemic Veins. After passing through the capillaries 

 in each of the organs we have mentioned, the blood returns 

 to the right heart through the veins. In general it may be 

 said that the larger veins follow the course of the arteries. 

 Thus a large vein (the jug'u-lar) carries blood downward 

 from the head on either side of the neck ; two others pass 

 upward along the arms ; and these four veins at length unite 

 to form the superior vena cava, which empties the blood into 

 the right auricle. In a similar way the inferior vena cava 



