THE GREAT VESSELS 323 



its branches is a valuable diagnostic sign when thrombosis has 

 occurred. 



Tumours affecting viscera which lie in front of the abdominal 

 aorta may present palpable pulsations. The viscera most com- 

 monly involved are (i) The left lobe of the liver, which lies 

 in front of the descending thoracic aorta, and is separated from 

 it only by the diaphragm ; (2) the pylorus, which is separated 

 from the abdominal aorta only by the peritoneal walls of 

 the omental bursa (lesser sac) ; and (3) the pancreas, which 

 crosses in front of the abdominal aorta. The pulsation in 

 these cases consists of a simple, heaving, forward movement, 

 and is not expansile in character. In this way, it may be dis- 

 tinguished from the pulsation of an aneurism of the abdominal 

 aorta, although it is only when the latter is of fairly large size 

 that the expansile nature of its pulsations can be determined 

 in a satisfactory manner. 



Aneurisms of the abdominal aorta usually enlarge in a 

 forward direction, and the tumour which they produce can 

 be palpated through the anterior abdominal wall. In some 

 cases, they enlarge in a backward direction and erode the 

 lumbar vertebrae, ultimately compressing the cauda equina 

 (p. 40) and giving rise to paraplegia. 



The Pulmonary Artery arises from the right ventricle 

 and passes upwards and backwards. At its origin it is placed 

 in front of the ascending aorta, but it inclines to its left side 

 and terminates below the aortic arch by dividing into right and 

 left branches. At its termination it is placed behind the 

 sternal end of the left second intercostal space, and is 

 separated from the surface only by the thin anterior part of the 

 left lung. When this part of the lung becomes retracted in 

 pulmonary tuberculosis, the pulsations of the pulmonary artery 

 are rendered visible in the left second interspace; or, when 

 it becomes consolidated in phthisis or pneumonia, these pulsa- 

 tions may be transmitted to the surface. In either case, light 

 percussion over the left half of the sternum at this level will 

 demonstrate a decrease in the area of lung resonance. 



