414 THE HORSE. 
described at page 411. About two inches above these it gives off a larga 
branch—the anterior aorta—supplying the anterior extremities, the neck, 
and the head, and then receives the name of the posterior aorta, which is 
destined to afford blood to the walls of the thorax and abdomen, to the 
contents of these cavities, and to the hinder extremities. 
THE ANTERIOR AORTA is about an inch and a half in length before it 
gives off any of its branches. It ascends between the two lamine of the 
anterior mediastinum, lying above the right auricle and below the trachea, 
with the vena cava on its right hand. Opposite the body of the third 
dorsal vertebra it divides into the right and left arteria innominata. The 
former is considerably the larger of the two, being nearly double the 
diameter of the left. This is owing to its supplying both the carotids in 
addition to those which it has previously given off in correspondence with 
the left arteria innominata. These branches common to both are seven 
in number :— 
1.—A. dorsalis branches backwards, and supplies a twig to the superior 
mediastinum, and the four or five first intercostal arteries. 
2.—A. cervicalis superior, distributed to the muscles of the neck lying 
above the spine. 
3.—A. vertebralis, a vessel of considerable size, is given off behind the 
first rib, and passes beneath the transverse process of the seventh cervical 
vertebra to enter the foramen in that of the sixth. From this it pro- 
ceeds through the foramina of all the cervical vertebre in succession, 
and enters the foramen magnum to supply the base of the brain with 
blood. 
4.—A. thoracica interna, given off opposite the last artery, descends at 
once to the upper and inner surface of the sternum, on each side of 
which it lies, supplying the intercostal muscles, and, inosculating with 
the intercostal arteries, terminates by meeting the ascending branches 
from the epigastric artery. 
5.—A. thoracica externa, a small branch which is given off externally to 
the first rib, and descends at once to the inferior surface of the sternum, 
on the muscles covering which it terminates. 
6.—A. cervicalis inferior is a short branch, and supphes the muscles and 
glands at the root of the neck. 
7.—A. axillaris descends at once to the inside of the fore extremity, and 
supplies the scapula, arm, and leg. It is the continuation of the main 
artery after it has given off the above branches, and lies deeply 
imbedded in the cellular membrane which fills up the space between 
the sternum and the shoulder joint. Here it supplies (a) three or 
four thoracic branches; (6) the A. dorsalis scapula; (c) A. subsca- 
pularis; the destinations of which will be explained by their names. It 
then runs along the inner side of the head of the os humeri, where it 
receives the name of A. humeralis, and gives off three or four muscular 
branches, having the ulnar and spiral nerves on its inner side, and in 
front the radial nerve, with the humeral veins behind. Above the 
elbow joint, and in front of the humerus, it splits into three, A. ulnaris, 
spiralis, and radialis; the last again dividing into two, A. plantaris 
externa and A. P. interna, which will again be alluded to in describing 
the anatomy of the foot. 
THE COMMON CAROTID ARTERY, which is the continuation of the right A 
innominata, after it has given off its axillary branches, ascends along the 
