THE SYSTEMIC CIECTJLATION 199 



the chest cavity, finally piercing the diaphragm and en- 

 tering the abdominal cavity. The change in direction as 

 it leaves the heart results in an arch called the arch of the 

 aorta. From the top of this arch on the right side is given 

 off a large branch which, in turn, soon divides into two 

 parts. One of these two parts (the right carotid artery) 

 suppUes blood to the right side of the head. The other 

 part passes under the collar bone, down the arm, and at 

 the elbow divides into the radial and ulnar arteries thus 

 supplying the right arm with blood. It is the radial artery 

 whose pulsings are felt in the wrist. 



From the top of the arch on the left side are given off 

 two branches, one of which (the left carotid) supplies blood 

 to the left side of the head, while the other branches as on 

 the right side and suppUes blood to the left arm. 



From the aorta, near the heart, arise also the coronary 

 arteries which, branching over the surface of the heart, 

 supply the tissues of that organ with blood. As the aorta 

 curves downward through the chest cavity it gives off 

 branches which supply the lung tissues, the esophagus, and 

 the muscles of the ribs and spinal column. 



From the aorta in the abdomen are given off five main 

 branches. The first {coeliac axis) divides into three parts 

 which supply the stomach, spleen, and liver respectively. 

 Two other branches (the mesenteries) carry blood to the 

 walls of the intestines. The remaining two branches carry 

 blood to the kidneys. The main trunk of the aorta con- 

 tinues to the rear of the abdominal cavity and finally 

 divides into two branches which each enter a leg and 

 supply these extremities with blood. 



All these branches of the aorta subdivide first into smal- 

 ler arteries and then into capillaries, which are in turn col- 



