THE GREAT VESSELS 321 



Pressure on the thoracic duct is not easy to recognise, as the 

 communications which it establishes with the right lymphatic 

 trunk (p. 324) may dilate sufficiently to compensate for the 

 obstruction. 



When the aneurism enlarges in an upward and backward 

 direction, it may exert pressure on the upper part of the left 

 sympathetic trunk (Fig. 49). As a result, the left pupil be- 

 comes, at first, dilated and, later, contracted (p. 189). 



Erosion of the vertebral bodies is by no means uncommon, 

 and the condition is accompanied by the characteristic pain of 

 bone affections. Enlargement in a backward direction may 

 bring the aneurism into relation with the intercostal nerves, as 

 they pass forwards and laterally from the intervertebral foramina. 

 The nature of the pain in this case is quite distinctive. It is 

 radiating in character, and is referred to the peripheral distribu- 

 tion of the anterior and lateral cutaneous branches. 



As the aortic arch passes backwards, it lies above the root 

 of the left lung. Each pulsation of an aneurism of this part 

 of the aorta thrusts the left bronchus in a downward direction, 

 and this fact helps to account for the production of " tracheal 

 tugging "(p- 288). 



Enlargement in an upward and forward direction will cause 

 pressure on the left innominate vein with consequent venous 

 engorgement of the left half of the head and neck and of the 

 left upper limb. 



From the convex upper border of the aortic arch, three large 

 branches arise, namely, the innominate, the left common 

 carotid and the left subclavian arteries. 



The Innominate Artery arises in the median plane and 

 passes upwards and to the right, on the anterior surface of the 

 trachea, to terminate behind the right sterno-clavicular articula- 

 tion, where it divides into the right subclavian and common 

 carotid arteries. The relationship of this artery to the 

 jugular (suprasternal) notch is somewhat variable, and, 

 although usually placed at a lower level, it frequently rises 

 so high that its pulsations can easily be felt in that region. 



21 



