XLVIL] 



SECONDARY CONTRACTION. 



241 



(/.) Prepare another limb and adjust it in place of A, ligature the nerve of 

 B. On stimulating the nerve of li, no contraction takes place either in A or 

 B. 



4. Secondaiy Contraction from Nerve. 



(a.) Make a nerve-muscle preparation and place it on a glass 

 plate (B). Dissect out the sciatic nerve of the opposite side (A). 

 Lay i cm. of the isolated sciatic nerve (A) on a similar length of 

 the nerve of the nerve-muscle preparation (B) (tig. 165). 



(/>.) Stimulate A with a single induction shock ; the muscle of 

 B contracts. Stimulate A with an interrupted current ; the muscle 

 of B is thrown into tetanus. 



('.) Ligature A and stimulate again. B does not contract. 

 Therefore its contraction was not due to an escape of the stimulating 

 current. The " secondary contractions " in B are due to the sudden 

 variations of the electro-motivity produced in A when it is stimu- 

 lated. 



FIG. 165. Scheme of Secondary 

 Contraction. 



Fia. 166. Scheme of Paradoxical 

 Contraction. 



5. Paradoxical Contraction. 



(//.) Arrangement. Arrange a Daniell'a cell and key for giving 

 a galvanic current, or use repeated induction shocks. 



(h.) Pith a frog, expose the sciatic nerve down to the knee (fig. 

 1 66, S). Trace the two branches into which it divides. Divide 

 the outer or peroneal branch as near as possible to the knee, and 

 stimulate its central end (P) by a faradic current. A certain 

 strength of current will be found whereby the muscles supplied by 

 the other division of the nerve are thrown into tetanus (T). The 

 tibial nerve to the gastrocnemius is stimulated by escape or spread 

 of " electrotonic " currents from the excited nerve. 



