188 NERVE INJURIES 



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 lor 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 m 

 consciousness. But when the nerves of the part 

 are cut, the vasomotor reflex fails, and the local 

 hyperasmia 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 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 summoned by 

 runners. The analogy fails in this particular, that 

 the bacterial invaders of the human frame will not 

 remain constant in numbers till the belated defending 

 forces are at last mustered against them, but will 

 multiply a thousand-fold in the interval and do 



