52 



PHYSIOLOGY OF MUSCLE AND NERVE 



beset with ganglion cells. From these plexuses delicate fibrils then 

 pass to the different muscle cells. 



Under ordinary conditions, a muscle contracts only in response 



to impulses generated in the central nervous system and conveyed 



to it through its nerve. Under experimental conditions, however, 



these impulses may be generated anywhere along the course of the 



nerve, and most easily by electrical or mechanical 



stimuli. The natural excitatory agents are usually 



designated as adequate stimuli and the uncommon 



ones, as inadequate stimuli. It is also possible to 



produce a contraction of a muscle by stimulating 



it directly. In the latter case the stimulation is 



said to be direct, while the activation of the muscle 



through its nerve constitutes the method of indirect 



stimulation. 



Independent Irritability of Muscle. — If a muscle 

 is stimulated directly, it may be contended that it 

 reacts in consequence of the excitation of its ner- 

 vous constituents and not on account of the exci- 

 tation of its myoplasmic elements. Obviously, it 

 is quite impossible under ordinary conditions to 

 differentiate between these two factors, unless one 

 of them can be rendered temporarily useless. An 

 experiment^ enabling us to exclude the nervous 

 elements may be performed as follows: Having 

 isolated the sciatic nerve of a frog in the region 

 of the thigh, a ligature is tightly drawn around 

 all the other tissues of this part. The blood 

 A, dorsaTiymph ^^PP^y having thus been cut off from this extremity, 

 sac into which curare a few drops of curare^ are injected into the dorsal 

 is injected; L, liga- lymph-sac. About 20 to 30 minutes later, the op- 



ture upon left thigh. .: ... - - ^ , ^ i ^^ • 



The stimulation of posite sciatic nerve IS isolated, and a small opening 

 the sciatic nerve at 1 made in the skin over each gastrocnemius muscle. 



ineffTctfre'rt^2! ^^ ^^^^ ^^ ^^^ ^^^^^® ^^^ ^^^en effect, the 

 Both gastrocnemius Stimulation of the sciatic nerve fails to evoke a 

 muscles, when stimu- contraction of the gastrocnemius on the side which 

 Ln*d 4, ^vra^fontrac^ ^as not been ligated (at 2), but produces a reaction 

 tion. on the side of the ligature (at 1). If the gastroc- 



nemii muscles are now stimulated directly (at 3 

 and 4), it will be found that both are responsive. By applying a 

 galvanometer to the sciatic nerve of the leg which has not been 

 ligated, it may readily be proved that this nerve has retained its 

 functional power, because every stimulus gives rise to a deflection of 



1 Archiv fiir path. Anat., 1856, or Claude Bernard, Comp. rend., 1856, 825. 



2 Curare, wurare or urare is a poison used by South American Indians upon 

 arrows and other weapons. It is prepared from the roots of the wurare plant, 

 B. concoction being formed with other ingredients to hide the real active principle. 



Fig. 24.— Inde- 

 pendent Irritabil- 

 ity OF Muscle. 



