96 ANATOMY AND PHYSIOLOGY FOR NURSES 



the internal coat of the arteries and in the middle coat of 

 the veins. The capillaries form plexuses, which are very 

 abundant, especially in the lungs, liver, kidneys, and other 

 secreting glands. The skin and mucous membranes con- 

 tribute very freely to this system. In the bones the capil- 

 laries are larger, but not so numerous. 



The arteries are invested in a loose areolofibrous tissue 

 called a sheath. This is considered an important structure 

 in surgical operations; it also contains the accompanying 

 veins, and at times a nerve. 



The coats of the arteries are supplied with blood from 

 the small branches which run in line with such structures. 

 These are the vasa vasomm, or nutrient vessels, and are 

 distributed to the external coat. The small veins which 

 return the blood from the capillaries open into the small 

 veins accompanying the arteries. The arteries are also 

 supplied with nerves. 



THE AORTA 



The aorta is the largest arterial trunk in the body. It 

 arises from the base of the left ventricle and middle of the 

 heart, and at its beginning presents an enlargement, called 

 the bulbus aortae, which contains the semilunar 

 valves. These valves are three in number, and prevent 

 the blood from re-entering the left ventricle, whence it 

 comes. 



After leaving the left ventricle the aorta passes upward, 

 backward, and over the root of the left lung to form the 

 arch. On the right side this arch gives off a single branch 

 about 2 inches in length ; this is the innominate artery, 

 which subdivides into the right common carotid and the 

 right subclavian. On the left side of the arch the left 

 common carotid and left subclavian are given off sepa- 

 rately. 



From the arch the aorta descends through the chest as 



