282 INTERCOSTAL ARTERIES. 



and sometimes are five or six in number. They also send twigs 

 to the contiguous parts ; the lowermost descend to the stomach. 

 to anastomose with branches from the coronary artery of that 

 organ ; the upper ones communicate with branches of the infe- 

 rior thyroid. 



The Inferior Intercostals 







Are the arteries which proceed directly from the aorta to the 

 parietes of the thorax. Their name is derived from their posi- 

 tion between the ribs. They are ramified on the intercostal 

 muscles and ribs, and on the pleura and some of the contigu- 

 ous parts. They are called Inferior or Aortic Intercostals, to 

 distinguish them from the superior intercostals, which are 

 derived from the subclavian artery. Their number varies from 

 ten to eight, according as the superior intercostals are more or 

 less numerous. 



There are usually eight on the left side and ten on the 

 right. 



They originate in pairs on the posterior surface of the aorta. 

 The uppermost of them pass obliquely upwards, and the lower- 

 most nearly in a horizontal direction, to the lower edges of 

 those ribs to which they are appropriated. They meet the 

 rib near its tubercles, or place of junction with the transverse 

 process of the vertebra, and then proceed forward, between 

 the internal and external intercostal muscles, in a superficial 

 but large groove, which is generally to be found on the interior 

 margin of the lower surface of the rib.* There is necessarily 

 a difference in the length of the right and left intercostals, 

 owing to the position of the aorta, which is rather on the left 

 of the spine. In consequence of this circumstance, the oesoph- 

 agus is anterior to, and also in contact with those of the right 

 side. 



They generally send off an important branch, called the 

 Dorsal, which arises near their origin, and, passing backwards, 

 sends ramifications to the muscles of the back. From this 



* See Vol. i. page 155. 



