NERVES 273 



494. Rate of transmission of nerve impulses. Ordinary 

 sensations travel about 100 feet per second. This is about the rate of 

 the fastest express trains, but our arms are so short that pain seems to 

 follow an injury instantly. In some diseases the rate is very much 

 retarded, so that if the hand should happen to rest upon a hot stove 

 it would be badly burned before the sensation would travel to the brain 

 and give warning of the danger. 



495. A sensation traveling over a nerve seems to come from its 

 beginning. When the funny bone, or nerve that winds around the 

 back of the elbow, is pinched, the little finger side of the hand, where 

 the nerve ends, feels as if pricked by needles. When an arm or a leg 

 is cut off, and the nerves in the stump are irritated, a pain is felt which 

 seems to be in the lost limb. When a nerve is pressed upon, it may 

 be partly paralyzed for a while. Then the part which it supplies 

 becomes less sensitive and is moved with difficulty. At the same 

 time an impulse caused by the irritation of the pressure produces a 

 sensation which seems to the brain to come from the end of the nerve. 

 Thus when sitting crosslegged the foot often seems asleep and full of 

 needles, while it is itself insensitive when touched. 



A cut nerve will become whole again, but it takes some weeks. In 

 the meantime the parts supplied by the nerve cannot feel or move. 



496. Diseases of the nerves. Nerves may become inflamed, 

 producing the disease called neuritis. Then there will be great pain and 

 tenderness over the entire course of the nerve. In severe cases there will 

 be paralysis and loss of feeling. The disease is very slow in its course. 



Sciatica is a mild but painful form of inflammation in the main nerve 

 of the leg. Inflammation of the nerves may be caused by rheumatism or 

 malaria, but, above all, by alcohol. 



497. Effect of alcohol upon nerves. A little alcohol 

 seems to hasten the rate of transmission of nervous im- 

 pulses by increasing the circulation of the blood, but a 

 few drinks retard their action. A great danger of using 

 alcohol is that it may cause neuritis or inflammation of the 

 nerves. Slow, steady drinking may produce it as well as 

 occasional sprees. It comes without warning, but remains 

 a long while, producing pain and paralysis. Alcohol pro- 

 duces the disease as often as all other causes combined. 



OV. PHYSIOL. 1 8 



