34 CLINICAL APPLIED ANATOMY. 



the superior mesenteric artery, as a result of which the blood 

 supply of a portion of the small intestine is suddenly arrested, 

 often leading to symptoms simulating mechanical intestinal 

 obstruction. 



Embolism in main arteries with anastomoses most usually 

 occurs at points where the vessels bifurcate : for instance, the 

 common femoral may be blocked at the spot where the deep 

 femoral arises, or the brachial may be occluded where it divides 

 into the radial and ulnar. The embolus may also be arrested by 

 the curvature of an artery opposite the flexure of a joint, as, for 

 instance, in the popliteal trunk behind the knee. The greater 

 the anatomical difficulties in the way of opening up a collateral 

 circulation, the greater is the likelihood of the supervention 

 of gangrene. Main arteries may also be temporarily or per- 

 manently obstructed by external pressure, particularly in the 

 case of unreduced fractures and dislocations. This is well seen 

 in the instance of pressure upon the popliteal artery by the lower 

 posteriorly flexed fragment in transverse fracture above the 

 condyles of the femur. 



Atheroma of an artery may lead to its obstruction, owing to 

 thrombosis being induced by the roughening of its internal coat. 

 This, however, is unlikely except in the case of the medium sized 

 vessels of the lower limb. Arteritis obliterans may further be 

 the cause of the onset of gangrene in both the lower and the 

 upper extremity. 



The line of demarcation in gangrene which has resulted from 

 arterial embolism, or from occluding arterial disease, is deter- 

 mined by the anatomical arrangement of the arterial tree. An 

 embolus, being likely to lodge at a point of bifurcation, will, if 

 the vessel be completely occluded and no collateral circulation 

 be possible, give rise to death of the tissues thus deprived of 

 their blood supply. Therefore the line of demarcation may 

 be expected at the lower limit of the last branches derived 

 from the trunk above the obstruction. For instance, if the 

 popliteal artery be occluded at its point of division, a line of 

 demarcation may form in the upper third of the leg. If an 



