GANGKENE. 35 



embolus be small enough to pass into one of the tibial vessels 

 the collateral circulation is usually sufficient to preserve the 

 vitality of the leg. When arterial disease is present the smaller 

 branches are likely to be occluded first, hence in the lower limb 

 gangrene of toes may occur, or the line of demarcation may 

 form just below the ankle joint, or, if the disease be more exten- 

 sive, both tibial arteries may be occluded and the popliteal still 

 patent owing to its greater calibre. The conditions will then 

 approximate to those present in embolism mentioned above. 



Besides the toes and the feet the skin of the middle third of 

 the front of the leg, being exposed to slight injuries, is liable to 

 become gangrenous in old people in whom the circulation is 

 feeble and the arteries are narrowed. 



Obstruction to both Veins and Arteries. This is by no means 

 an infrequent cause of death in the periphery. It is perhaps most 

 typically seen in the instance of gangrene following upon liga- 

 ture of the superficial femoral artery for the cure of a popliteal 

 aneurysm. The aneurysmal sac itself has pressed upon the accom- 

 panying vein, and has thus given rise to more or less congestion 

 of the parts below. The blood flow through the main artery is 

 then suddenly arrested, and the vis a tergo, which before the 

 ligation of the artery was only barely sufficient to return the 

 blood past the obstruction in the vein, now completely fails to do 

 so. In consequence, the distal portion of the limb becomes 

 greatly congested and nutrition interfered with, and moist 

 gangrene may result. It is possible also for the aneurysmal sac 

 to so press upon the surrounding articular arteries as to interfere 

 very seriously with the anastomotic circulation. 



Obstruction to Capillaries. The cessation of the flow of blood 

 in the capillaries is most commonly brought about as a result of 

 certain anatomical dispositions. 



The first of these is the distance of the capillaries from the 

 central pump, the heart. The flow of blood through the minute 

 vessels in a weakened state of the force of the heart's systole may 

 become slower and slower till at last stasis ensues, and if this is 

 prolonged, gangrene sets in. Such a condition is seen in certain 



32 



