THE POSTERIOR AORTA. 525 



pheral insertion of the diaphragm. It is recommended to prepare, from before to behind, 

 the various visceral branches of the vessel ; first, the broncho-cesophageal trunk, then 

 the cceliac trunk, next, the great mesenteric artery and the renal arteries, after spreading 

 out the intestinal mass as in figure 271 ; and, lastly, the small mesenteric and testicular 

 arteries, alter arranging the intestines as in figure 272. 



PARIETAL BRANCHES OF THE POSTERIOR AORTA. 



1. Intercostal Arteries. (Fig. 237.) 



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

 of the ribs, number seventeen pairs. 



Origin, Course, and Distribution. The last thirteen emanate from the 

 thoracic aorta only ; the first comes from the cervical artery ; and the next 

 three are furnished by a special branch of the dorsal artery. 



The aortic intercostals escape at a right angle from the superior plane 

 of the trunk, on a level with the bodies of the dorsal vertebrae, and at 

 regular intervals. Their origin is nearer that of the 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 nervous chain 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 proper 

 intercostal ; the other superior, or dorso-spinal. 



The inferior and superior branches of the first four intercostal arteries 

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

 superior cervical artery for the first 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 well as with 

 the circumflex iliac. 



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

 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 

 distributed 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 

 comes to be described. 



Variations in origin. Not ^infrequently the first two pairs of aortic 

 intercostal arteries proceed from a single trunk, which thus gives rise to 

 four branches ; and this trunk is also often the common source of these four 



