INHIBITION OF REFLEXES 217 



applied, say, to one part of a limb, a fixed response is obtained ; but if 

 the strength of the stimulus be increased, the movement may involve 

 parts on the other side. This is termed irradiation. In studying the 

 question of response with regard to strength of stimulus employed it is 

 found that as the strength increases the stimulus tends to spread first 

 to the same level on the opposite side, e.g. from one leg to the other ; 

 and that only when the stimulus is still further increased does it tend to 

 spread upwards and downwards to fresh levels. There are conditions 

 in which the extension of a stimulus to other parts is greatly facilitated ; 

 as, for instance, in strychnine poisoning. 



Take a frog with its brain destroyed and inject 2 drops of a 0*5 per cent, 

 solution of strychnine sulphate. In a few minutes stimulation of the skin in 

 any part of the body excites a general convulsion of the whole body. All 

 the muscles are thrown into violent tetanic spasms and the limbs become 

 extended and rigid. The tetanic spasm passes off to be at once repeated on 

 even the slightest stimulation, such as a tap on the table. Note that these 

 contractions are not co-ordinated muscular movements, but are general 

 tetanic contractions. 



Destroy the cord by pithing. At once the contractions cease, showing 

 that the effect of strychnine is one acting directly upon the cord, not upon the 

 nerves and muscles. 



4. Inhibition of reflexes. — Employing the frog at first taken, expose 

 the upper end of the cord and place a crystal of sodium chloride upon 

 it. After a minute try to obtain any of the reflexes previously 

 obtained easily. They will now be found not to occur, and may not 

 be produced even though the strength of the stimulus be considerably 

 increased. 



