ACTION OF MEDICINES ON MUSCLE 65 



The agents which act on striped muscle tissue may be 

 arranged in the following groups : 



1. Diminishes the excitability of the muscle, as well as 



its capacity for work. This group contains salts of 

 potassium, lithium, and ammonium, the cinchona 

 alkaloids, chloroform, chloral, and alcohol, in large 

 doses. This action is of little practical importance, 

 for it is either insignificant in the living animal or is 

 overshadowed by some other more marked action 

 of the drug. 



2. Diminishes the total amount of work, whilst leaving 



the irritability unaffected, until, with large doses of 

 the drug, the depression ends in death of the muscle. 

 This effect is produced when the drug is brought in 

 contact with the muscle, but not when medicinal 

 doses are given by the mouth. The group contains 

 apomorphine, saponins, salts of copper, zinc and 

 lead, emetine and cocaine. 



3. Alters the form of the muscle curve, as exhibited by 



veratrine and barium, and to a similar, although less 

 extent, by strontium and calcium salts, digitalis, 

 oxycolchicine, and glycerin. This effect is produced 

 only by large or even poisonous doses of most of 

 these agents. 



4. Increases the irritability, as is notably done by physo- 



stigmine. 



5. Increases the capacity for work. The agents belonging 



to this group cause rapid restoration of the muscle 

 after fatigue, and are represented by creatin, hypo- 

 xanthin, caffeine, and glycogen. These substances 

 must hence be regarded, not only as nerve stimulants, 

 but as direct muscle restoratives. 



Voluntary muscles differ from involuntary, not only in 

 structure, but in other particulars. Their contraction and 

 relaxation are more rapid. The nerves in voluntary muscles 

 terminate in end-plates, while the terminal twigs in involun- 

 tary muscles form a plexus round the fibres. 



Involuntary muscle fibres are directly stimulated to con- 

 tract and their tonus increased by barium, and to a less 

 extent by veratrine and lead. That this action is direct 



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