THE POSTERIOR AORTA. 611 



Parietal Branches of the Posterior Aorta. 



1. Intercostal Arteries (Fig. 375). 



The intercostal arteries, placed — as their name indicates — in the intervals of 

 the ribs, number seventeen pairs. 



Oriijin, Course, and Distribution. — The last thirteen arise from the tlioracic 

 aorta only ; the tlrst comes from the cervical artery ; and the next three are 

 furnished by a special branch of the dorsal artery. 



The aortic intercostals emerge at a right angle from the superior plane of 

 the trunk, on a level with the bodies of the dorsal vertel)ra3, and at regular 

 intervals. Their origin is nearer that of t;he arteries on the opposite side as 

 they are more anterior, the first two or three arising in pairs from a common 

 trunk. 



These aortic intercostals ascend to the vertebral bodies, beneath the pleura, 

 in crossing the direction of the sympathetic nerve-trunk and — the arteries of the 

 right side only — in addition, that of the vena azygos and the thoracic duct, to 

 the superior extremity of the intercostal spaces, where those of both sides divide 

 into two branches — the one inferior, or lyrojper intercostal ; the other superior, or 

 dorso-spinal. 



The inferior and superior branches of the first four intercostal arteries 

 emanate solely from the trunk that furnishes them, and which is the superior 

 cervical artery for the lirst intercostal, and the subcostal branch of the dorsal 

 artery for the succeeding three. 



Inferior or intercostal branch. — This branch, the most considerable of the 

 two, placed at first beneath the pleura, then between the two intercostal muscles, 

 is lodged, along with a satellite vein and nerve, in the furrow on the posterior 

 face of the rib, and descends to the inferior extremity of the intercostal space, 

 where it terminates in the following manner : the first twelve or thirteen 

 branches anastomose with the intercostal ramifications of the internal thoracic 

 artery and its asternal branch ; the others are prolonged into the abdominal 

 muscles, where their divisions communicate with those of the anterior and 

 posterior abdominal arteries, as w-ell as with the circumflex iliac. 



In their course, these intercostal branches give arterioles to the pleurte, the 

 ribs, and the thoracic muscles, with the perforating ramuscules which cross these 

 muscles to ramify in the skin and the panniculus carnosus, but which, of course, 

 are absent where the pectoral wall is covered by the thoracic limb. 



Superior or dorso-spinal branch. — This passes directly upwards to be dis- 

 tributed to the spinal muscles of the dorsal region and the integument covering 

 them, after giving off, when passing the intervertebral foramen, a branch which 

 enters the spinal canal by that opening, and is destined for the spinal cord and 

 its envelopes. An auxiliary of the middle spinal artery, this branch will be 

 studied at greater length when the cerebro-spinal artery is described. 



Variations in origin. — Not unfrequently the first two pairs of aortic inter- 

 costal arteries proceed from a single trunk, thus giving rise to four branches ; 

 and this trunk is also often the source of these four intercostals and the bronchial 

 and oesophageal arteries, when its volume is very considerable. It is much 

 smaller when it only gives off the second pair of intercostals, which is sometimes 

 the case. 



