610 THE ARTERIES. 



Relations. — To facilitate the study of its connections, the posterior aorta may 

 be divided into two sections — one thoracic, the other abdominal. 



a. At its origin or arch, the thoracic aorta is crossed to the right by the 

 trachea and oesophagus ; on the opposite side, it is related to the pulmonary 

 artery and the left lung. For the remainder of its extent, it is comprised 

 between the two layers of the posterior mediastinum, and through these is in 

 relation with the pulmonary lobes, which are fissured for its reception ; this 

 fissure is much deeper in the left than the right lung. Above, it is in contact 

 with the bodies of the last twelve dorsal vertebras, and is accompanied on the 

 right by the large vena azygos and the thoracic duct ; the latter is often carried 

 to the left for the whole or a portion of its extent. 



(Remak observed muscular fibres on the external face of the aortic arch and 

 thoracic aorta in the Horse, Sheep, and Pig ; the fasciculi they form are so large 

 as to be visible to the naked eye.) 



b. The posterior or abdominal aorta, enlaced by the abdominal nerves of the 

 great sympathetic, is in relation, above, with the bodies of the lumbar vertebra, 

 the originating tendon of the diaphragmatic pillars, Pecquet's reservoir {recepta- 

 culum chyli), and the common inferior vertebral ligament ; it passes above the 

 pancreas and the peritoneum, the latter by its sublumbar layer covering the 

 posterior two-thirds of the vessel. On the right, it is accompanied by the posterior 

 vena cava, which perhaps it slightly pushes to the left of the median plane. 



Collateral branches. — The arteries emanating from the posterior aorta during 

 its long course, very naturally form two classes ; some are designated pariptal, 

 because they are distributed to the parietes of the great splanchnic cavities ; the 

 others are the visceral branches, destined for the organs lodged in these cavities. 



Among the parietal branches, may be noticed : — 



1. The intercostal arteries, furnished by the thoracic aorta. 



2. The diaphragmatic (or phrenic) arteries, the origin of which is placed on 

 the limits of the two portions of the vessel. 



3. The lumbar arteries, and the middle sacral artery, arising from the abdo- 

 minal aorta. 



The visceral branches are : — 



1. The broncho-ORSophageal trunk, emitted by the thoracic portion of the 

 aorta. 



2. The co&liac axis, great (or anterior) mesenteric artery, small (or posterior') 

 mesenteric artery, renal arteries, spermatic arteries, and small testindar (or artery 

 of the cord) or uterine art&ries, which emerge from the abdominal portion. 



Preparation of the posterior aorta and its collateral branches. — Immediately after injecting 

 according to one of the modes recommended at p. 606, place the subject in the first position, 

 the two posterior limbs being well extended backwards. Open the abdominal cavity, and 

 remove from it the intestines in the manner already indicated. Tlie tallow having become 

 perfectly solidified during these necessary manipulations, dissection may be proceeded with 

 at once. It is requisite, however, to remove the right and left walls of the thoracic cavity 

 lieforehand, by sawing through the last fourteen or fifteen ribs at six or seven inches from their 

 superior extremity, and then separating them from the sternum by the saw, taking the pre- 

 caution of detaching the periph( ral insertion of the diaphragm. It is recommended to prepare, 

 from before to behind, the various visceral branches of the vessel ; first, the broncho-oesophageal 

 artery ; then the coeliac axis ; next, the anterior mesenteric artery and the renal arteries, after 

 spreadmg out the intestinal mass as in Fii,'. 283 ; and, lastly, the small mesenteric and testicular 

 wteries, after arranging the intestinea as in Fig. 366. 



