322 THE VASCULAR SYSTEM 



Aneurisms of the innominate artery, and aneurisms of the 

 aortic arch which bulge upwards, also produce pulsations 

 which are palpable and sometimes visible at the upper border 

 of the manubrium stemi. 



The Descending Thoracic Aorta begins at the left side 

 of the body of the fourth thoracic vertebra and inclines 

 forwards to reach the median plane, so that, as it passes 

 through the diaphragm, it lies in front of the vertebral column. 

 This part of the aorta is closely related to the oesophagus. 

 Above, the oesophagus lies to the right of the aorta and on a 

 slightly anterior plane. As the vessel descends, it inclines 

 medially, and it is crossed by the oesophagus just before it 

 pierces the diaphragm. It is the latter situation which is 

 the usual site of aneurisms of the descending thoracic aorta, 

 and consequently these aneurisms are usually associated with 

 marked difficulty in swallowing. 



The Abdominal Aorta begins at the aortic opening in the 

 diaphragm and descends in front of the vertebral column. Its 

 terminal bifurcation into the two common iliac arteries occurs 

 opposite the left side of the fourth lumbar vertebra and corre- 

 sponds, on the anterior abdominal wall, to a point a little 

 below and a little to the left of the umbilicus. At first the 

 abdominal aorta lies behind the posterior wall of the omental 

 bursa, and then it descends behind the body of the pancreas, 

 which crosses the vessel at the level of the second lumbar 

 vertebra. Below the pancreas, the aorta is crossed by the 

 third or horizontal portion of the duodenum, and, at a still 

 lower level, it lies immediately behind the peritoneum on the 

 posterior wall of the infra-colic compartment of the abdomen. 



In neurasthenic patients with flaccid abdominal walls, the 

 pulsations of the abdominal aorta can be felt with extra- 

 ordinary distinctness^ but under normal conditions very firm 

 deep palpation must be employed, and, even then, it is 

 impossible to determine the character of the pulse. In 

 aneurisms affecting the descending thoracic or the abdominal 

 aorta, complete absence of pulsation in the femoral artery and 



