1. The causes of pressure on the neuro-vascular bundle of the upper limb are many and varied. No one cause such as clavicular pressure can explain all cases.

2. Costo-clavicular pressure is not possible when there is a normal first rib and a normal thoracic outlet, but it is certainly a factor when the costo-clavicular interval is narrowed by the presence of a large cervical rib or an abnormal first thoracic rib. Clavicular pressure can act only during retraction and abduction, not in depression of the shoulder.

3. Temporary alterations in the radial pulse on movements of the shoulder in normal individuals are due to causes distal to the clavicle and have no relation to costo-clavicular pressure.

4. While irritation of sympathetic nerve fibres may explain the majority of cases of thrombosis, there are others in which clotting occurs in an aneurismal dilatation produced by pressure between the clavicle and the abnormal costal element. It is likely that the thrombosis occurs in an aneurism which has been present for some length of time. The cause of the aneurismal dilatation may be vaso-motor paralysis of a segment of the artery, ending distally at a point where a fresh intact leash of nerves is relayed to the vessel.

5. The importance of the scalenus anterior syndrome has been over-emphasised. If operative treatment is limited in all cases to anterior scalenotomy the results will be disappointing.

6. If operation is advised it should be performed without rigid and preconceived ideas, through an adequate incision, and with exploration wide enough to allow thorough investigation of the cause of pressure.