NERVE INJURIES 127 



limb all but amputated left connected with the 

 body only by its main artery and vein will show 

 active hyperaemia if its blood-supply has been 

 stopped for a minute, and then released. The 

 chemical substances liberated in starved, fatigued, or 

 damaged tissues exert a local action on the small 

 arteries supplying them, causing them to dilate. 

 But there is also a vasomotor reflex, whereby a message 

 is sent to the spinal cord and vasomotor centre in 

 the medulla asking for more blood, and in conse- 

 quence vasodilator impulses are sent to that part, 

 and vasoconstrictor impulses to the rest of the body. 

 Normally, these occurrences are the inevitable 

 result of every insult or injury, of every invasion by 

 a few bacteria, and we know nothing of them in 

 consciousness. But when the nerves of the part 

 are cut, the vasomotor reflex fails, and the local 

 hyperaemia takes place too late to check the 

 mischief. 



One may illustrate the circumstances by the 

 analogy of a guarded frontier. An armed raid is 

 made by an enemy ; the nearest garrison is too 

 weak to repel it, and telegraphs to the base to urge 

 a hasty concentration of the troops. The message 

 goes astray because the wire has been cut. The 

 garrison must make what resistance they can with 

 the aid of local volunteers and small levies sum- 

 moned by runners. The analogy fails in this par- 

 ticular, that the bacterial invaders of the human 

 frame wih 1 not remain constant in numbers till the 

 belated defending forces are at last mustered against 

 them, but will multiply a thousand-fold in the 



