BRAX<'Hi:s r>r THE THORACIC AORTA. 



607 



oesophagus, thoracic duct, sympathetic nerve, and the vena azygos major : the left 

 ng^beneath the superior intercostal vein, the vena azygos minor, and sympa- 

 thetic.^ In the intercostal spaces each artery divides into two branches an ante- 

 rior, or proper intercostal branch : and a posterior, or dorsal branch. 1 



The anterior branch passes outward, at first lying upon the External inter- 



FIG. 367. The abdominal aorta and its branches. 



costal muscle, covered in front by the pleura and a thin fascia. It then passes 

 between the two layers of Intercostal muscles, and, having ascended obliquely to 

 the lower border of the rib above, divides, near the angle of that bone, into two 

 branches : of these the larger runs in the groove on the lower border of the rib 

 above ; the smaller branch along the upper border of the rib below ; passing 



1 Mr. W. J. Walsham describes a small twig as being given off from eacb intercostal close to its 

 origin. He states that they can be traced running between the neck of the rib and the transverse 

 process of the corresponding vertebra ; they anastomose with similar twigs given off from the inter- 

 costal artery next below. In the first and second spaces similar anastomosing twigs are given off from 

 the superior intercostal (Journal of Anatomy and Physiology, vol. xvi. part iii. p. 443). 



