524 THE ABTERIES. 



the vertebral bodies, though a little to the left at first ; it gradually inclines 

 to the right, however, and reaches the median plane at the pillars of the 

 diaphragm. ' Here it passes through the opening circumscribed by these 

 two pillars, enters the abdominal cavity, and extends to the entrance of the 

 pelvis, under the spine, still preserving its median position. On reaching 

 the last intervertebral articulation, the posterior aorta terminates by a 

 double bifurcation, from which arises the external and internal iliac arteries. 



Selations. — 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 responds 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 vertebrse, 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. 



(Eemak 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.) 



h. The abdominal aorta, enlaced by the abdominal nerves of the great 

 sympathetic, corresponds, above, with the bodies of the lumbar vertebrsB, 

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

 nated parietal, 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 diaghragmatic arteries, whose origin is placed on the limits of the 

 two portions of the vessel. 



3. The lumbar arteries, and the middle sacral artery, springing from the 

 abdominal aorta. 



The visceral branches are : 



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

 aorta. 



2. The coeliac trunk, great mesenteric artery, small mesenteric artery, renal 

 arteries, spermatic arteries, and small testicular or uterine arteries, which 

 emerge from the abdominal portion. 



^ _ Preparation of the posterior aorta and Hs collateral imne^es.— Immediately after 

 mjecting according to one of the modes recommended at p. 519, place the subject in 

 the first position, the two posterior members being well extended backwards. Open the 

 abdominal cavity, and remove from it the intestines in tl.e manner already indicated. 

 The tallow having become perfectly solidified during these necessary manipulations, 

 dissection may be proceeded with at once. It is requisite, however, to remove the li-ht 

 and left walls of the thoracic cavity beforehand, by sawing through the last fourteen or 

 fifteen ribs aUix or seven inches from their superior extremity, and afterwards sepa- 

 rating them from the sternmn by the saw, taking the preoaulion of detaching the peri- 



