DISTEIBUTION OF THE SYSTEMIC ARTERIES 449 



common aorta, is about 2 to 2^ inches in length. On leaving its parent 

 trunk it passes in a forward direction beneath the trachea or windpipe, 

 and soon divides into two unequal branches, one passing to the right and 

 the other to the left. The right one is the arteria iimominata, and the 

 left one the axillary artery. The right branch is much the larger of the 

 two, for in addition to giving blood to the fore -limb and the anterior 

 part of the trunk, like its fellow, it also supplies the head by means of 

 a large branch, the cephalic artery. Each of these vessels, on leaving 

 the anterior aorta, gives off eight branches of considerable size, as 

 follows : — 



1. Dorsal. 



2. Superior Cervical. 



3. Vertebral. 



4. Internal Thoracic. 



5. External Thoracic. 



6. Inferior Cervical. 



7. Supra-scapular. 



8. Subscapular. 



These vessels divide and subdivide in their course, and furnish blood to the 

 withers, the neck, the shoulders, and the walls of the chest. The vertebral 

 artery requires special notice, inasmuch as it courses along the neck, partly 

 enclosed in small bony canals in the transverse processes of the six anterior 

 cervical vertebrae. In its course it supplies many branches to the deep 

 cervical muscles, and others which enter the spinal canal and are distributed 

 to the spinal cord. 



After giving off the arteries above-named, the axillary descends along 

 the inner aspect of the upper arm, where it takes the name of the brachial 

 or humeral artery. 



Brachial or Humeral Artery. — This vessel descends on the inner 

 aspect of the humerus and distributes branches to the muscles of the upper 

 arm and other adjacent structures, the larger and more important of which 

 are — 



1. Prehumeral, or Anterior Circumflex. 1 3. Ulnar. 



2. Deep Humeral, or Humeralis Profunda. | 4. Artery of the Biceps. 



On nearing the upper extremity of the radius or forearm the humeral 

 artery divides into two unequal branches, the anterior and posterior radials : 

 one, the smaller anterior radial, passes in a forward direction, and after 

 distributing several small vessels to the extensor muscles, courses its way 

 down the front of the leg as far as the knee, to which it furnishes 

 numerous small branches. 



The Posterior Radial, much more voluminous than the anterior, 

 descends on the inner side of the radius or forearm, whei-e it is covered 

 by the flexor metacarpi internus muscle. In its course along the limb 

 the 2^oste7nor radial gives off branches to the elbow-joint, the flexor 

 and extensor muscles, the skin, and also to the knee-joint. On reaching 



