424 MOVEMENTS 



When the motor nerves are divided, the nutrition of the muscles to 

 which they are distributed is disturbed ; and although muscular con- 

 tractility may persist for some time after nervous excitability has disap- 

 peared, it is much diminished at the end of six weeks. Some varieties 

 of curare paralyze the end-plates of the motor nerves, leaving the sen- 

 sory nerves intact. If a frog is poisoned by introducing a little of this 

 agent under the skin, stimulation, electric or mechanical, applied to an 

 exposed nerve, fails to produce muscular contraction ; but if the stimu- 

 lus is applied directly to the muscles, they will contract vigorously. 1 

 If a frog is poisoned with potassium sulphocyanate, however, the con- 

 trary effect is observed ; that is, the muscles become insensible to exci- 

 tation, while the nervous system is unaffected. This may be demonstrated 

 by applying a tight ligature around the body in the lumbar region, 

 involving all the parts except the lumbar nerves. If the poison is now 

 introduced beneath the skin above the ligature, only the anterior parts 

 are affected, because the vascular communication with the posterior 

 extremities is cut off. If the exposed nerves are now stimulated, the 

 muscles of the legs are thrown into contraction, showing that the ner- 

 vous conductivity and excitability remain. Reflex movements in the 

 posterior extremities also may be produced by irritation of the parts 

 above the ligature. These experiments leave no doubt as to the exist- 

 ence of an independent excitability in the muscular tissue. Contractions 

 of muscles, it is true, are excited normally through the nervous system, 

 and artificial stimulation of a motor nerve is the most efficient method 

 of producing the simultaneous action of all the fibres of a muscle or of 

 a set of muscles ; but direct electric, mechanical or chemical irritation 

 of the muscles themselves will produce contraction after the nervous 

 connections have been destroyed. 



MUSCULAR CONTRACTION 



The stimulus of the will, conveyed through the conductors of motor 

 impulses from the brain to a muscle or set of muscles, excites the mus- 

 cular fibres and causes them to contract. In muscles that have been 

 exercised and educated, this action is regulated with great nicety, so 

 that the most delicate and rapid as well as powerful contractions may 

 be produced. Certain movements not under the control of the will are 

 produced as the result of unconscious reflex action of nervous centres. 



1 It has been shown, by a very simple experiment, that curare paralyzes the end-plates only 

 of the motor nerves. -If a frog is poisoned with curare and one leg is ligated above the knee, 

 leaving the nerve out of the ligature, reflex movements may be excited in the ligated leg, but 

 not in other parts. This shows that the motor nerves still retain their conductivity. 



