28 A COURSE ON ZOOLOGY. 



venous blood, but arteries as vessels carrying blood 

 from the heart, while veins return blood to the heart, 

 that blood being arterial or venous accordingly as it 

 comes from the lungs or from the extremities of the 

 body. 



Greater circulation or general circulation is the name 

 given to the second portion of the blood's course; its 

 starting-point is the aorta, and by it the blood is carried 

 from the left ventricle to the right auricle, traversing 

 the entire organism. Almost immediately after leaving 

 the left ventricle, the aorta sends out branches for the 

 nourishment of the heart (coronary arteries) and the 

 lungs (bronchial arteries). It then rises vertically almost 

 to the top of the sternum and bends downward and back- 

 ward, forming an arch, called the arch of the aorta. 

 From the arch arise by two distinct trunks the left 

 carotid and the left subclavian, and by one common 

 trunk the right carotid and right subclavian. 



The carotid arteries distribute numerous ramifications 

 to the neck and head. The subclavian arteries, so called 

 because they pass under the collar-bone, pass to the arms, 

 which they follow throughout the entire length, taking 

 names corresponding to the different parts they pass 

 through. One of their branches, the radial artery, 

 is generally selected by preference in feeling the pulse. 



The descending aorta follows the whole length of 

 the spinal column, distributing in its course important 

 branches to the thorax and to the abdomen. These are 

 the intercostal arteries, for the muscles of the ribs ; the 

 coeliac, for the stomach, liver, and spleen ; the mesenteric, 

 for the intestines, etc. A little below the kidneys a bifur- 

 cation takes place, forming the iliac arteries, which carry 

 the blood to the lower limbs. 



