462 THE ANATOMY OF THE HOUSE 



arteries of the body ; but being on so small a scale, it can only convey a 

 general idea of their numbers and the situation at which they each leave 

 the parent trunk. 



The aorta, or great artery of the bod}', as it emerges from the sub- 

 stance of the heart and rises towards the spine, describes a curve whose 

 convexity looks upwards and forwards. Immediately above the valves at 

 its root are the origins of the two coronary arteries, supplying the heart as 

 described at page 460. About two inches above these it gives off a large 

 branch — the anterior aorta — supplying the anterior extremities, the neck, 

 and the head ; the hinder portion is called the posterior aorta, and is destined 

 to afford blood to the walls of the thoi-ax 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 laminse 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 vertebi-a it divides into the I'ight 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 previousl}'- 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. servicalis sivperior, 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 ti^ansverse process of the seventh 

 cervical vertebra to enter the foramen in that of the sixth. From 

 this it proceeds through the foramina of all the cervical vertebra 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 mus^Jcs, and, inosculating with the inter- 

 costal arteries, terminates by meeting the ascending branches from tlic 

 epigastric artery. 



5. — A. thoracica externa, a small Ijranch 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 supplies 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 scapulae ; (c) A. subsca- 

 pularis ; the destinaLions of whicli will hv explained by their names. It 



