THE MECHANICAL RESPONSE OF MUSCLE 211 



THE ACTION OF DRUGS 



Of the drugs that have a direct action on muscle, the most remarkable is veratrin, 

 which causes an excessive prolongation of a muscular contraction (produced by a 

 single stimulus). Thus the ' twitch ' of a muscle poisoned with veratrin may last 

 fifty or sixty seconds, instead of the normal one-tenth of a second (Fig. 71). 



Barium salts have a similar, though less marked effect. 



In order to carry out the poisoning with veratrin, very weak solutions (1 in 100,000 

 or 1 in 1,000,000 of normal saline) should be used and the muscle exposed to its action 

 for some time. We get then on a single stimulus a response lasting many seconds 

 and exactly similar in height and form to a tetanus obtained by discontinuous stimu- 

 lation. If stronger solutions be used, the action of the drug is apt to affect the fibres 

 unequally, so that we may have a sharp normal twitch preceding the prolonged con- 

 traction (Fig. 72). If the muscle be excited several times immediately after the pro- 

 longed contraction has passed away, it responds with twitches like those of a normal 

 muscle, but if allowed to rest a few minutes, stimulation is again followed by the peculiar 

 long-drawn-out contraction. 



