PRINCIPAL ARTERIES. 107 



which serve to prevent the reflux of the blood, and which 

 completely close the vessel when expanded. 



The aorta runs upward and to the right, and is here the 

 ascending aorta; then it turns to the left, passing in front of 

 the spinal column, and taking a new turn downward, it forms 

 the arch of the aorta; it runs along and to the left of the 

 spine through the posterior mediastinum, and is called the 

 descending aorta, and passes through the opening in the dia- 

 phragm. On reaching the abdomen it becomes the abdo- 

 minal aorta, up to about the fourth lumbar vertebrae, where 

 it bifurcates and forms the two primitive iliac arteries. 



From its upper portion the aorta throws off important 

 branches, of which the principal are the following: The 

 brachio-cephalic or innominate artery, which springs from the 

 arch, and is its representative on the right side of the chest. 

 This trunk gives off the right common carotid and subdavian; 

 the left common carotid and subdavian spring directly from 

 the arch of the aorta. 



The common carotid arteries run upward along the outside 

 of the neck on a level with the upper border of the thyroid 

 cartilage, and each divides into the external and internal 

 carotid. 



The external carotid gives off the superior thyroid, the 

 facial, lingual, and occipital arteries. At the level of the con- 

 dyle of the jaw it divides into the temporal and internal 

 maxillary. 



The internal carotid runs upward along the cervical ver- 

 tebrae, enters the skull, gives off the ophthalmic artery, and is 

 distributed through the brain. 



The subdavian runs outside, behind and below the clavicle, 

 as its name indicates, and gives off, among other branches, the 

 vertebral artery, and the internal mammary; and on reaching 

 the arm-pit (axilla) it takes the name of the axillary artery, 

 and gives off important vessels to the shoulder and chest; 

 and then descending along the humerus under the name of 

 the brachial artery, it divides below the elbow, and forms the 

 radial and ulnar arteries, which furnish the vessels of the 

 fore-arm and those of the hand. 



Among the arteries arising from the descending aorta we 



