620 MANUAL OF PHYSIOLOGY. 



ing of the impulse from one group of nerve cells in the cord to 

 another by a kind of radiation from the focus of excitation. 



Very slight stimulation, though not sufficient to produce im- 

 mediate response, may, after a time, give rise to definite reflex 

 action, as if the weak impulses arriving at the nerve cells in the 

 cord were stored up until their sum sufficed to produce a definite 

 reflex movement. This may also indeed, much better be seen 

 in animals whose nerve centres are intact, for the cells of more 

 remote parts exercise a kind of checking influence on those in 

 the region receiving the stimulus, and thus the accumulative 

 action (summation) comes more commonly and more effectively 

 into play. This is seen in the human subject where slight visceral 

 stimulations exist for a long time. In some of these cases, even 

 without any really sensory appreciation of any local excitation, 

 an amount of energy may be accumulated along the gray tract 

 of the cord from the prolonged income of impulses, that will 

 bring on the most extensive forms of reflex muscular movement, 

 and give rise to serious results. These movements are generally 

 different from the regular coordinated motion resulting at once 

 from an adequate skin stimulation, and have usually a tendency 

 to assume a convulsive form. As an example of this may be 

 named the convulsions that commonly occur in young children, 

 from the prolonged irritation of intestinal worms, or during the 

 painful period of dentition. Epilepsy and hydrophobia may 

 possibly be explained in the same way. 



In certain conditions of the nervous system these irregular 

 movements or spasms (convulsions) can be excited much more 

 readily than is normally the case. As most striking among these 

 may be named poisoning with the alkaloid of nux vomica (strych- 

 nia) and the state of the blood which is produced by cessation of 

 the respiratory function (asphyxia). These toxic conditions of 

 the blood bring about a peculiar excitable condition of the cells 

 or conducting fibres of the spinal cord, in which impulses seem 

 to pass with unwonted facility from one part to another, and give 

 rise to an excessive degree of action even in response to normal 

 stimulations. A frog poisoned with strychnia is thrown into 

 general spasm of the entire body by even the least touch, 



