166 ELEMENTARY BIOLOGY. [CHAP. 



a. Remove the skin from one leg, so as to lay bare 

 the muscles: send an interrupted electric cur- 

 rent through any one of them (or tap the muscle 

 sharply with the back of a scalpel) : it will im- 

 mediately contract, or alter its form in a definite 

 way ; it becomes shorter and thicker p , and in so 

 doing moves the bones to which it is attached. 



b. Very carefully lay bare the sciatic nerve, taking 

 care not to crush or drag it : divide it as high 

 up as possible and, seizing it with a pair of for- 

 ceps close to its cut end, lay it over the elec- 

 trodes of an induction-coil. Probably when the 

 nerve is cut the muscles of the limb will con- 

 tract : whether or not, however, they will con- 

 tract violently while the interrupted current is 

 going through the nerve. 



[If an induction-coil is not at hand a bit of 

 clean copper wire twisted round a strip of zinc, 

 with the points of contact moistened with dilute 

 acetic acid, may be used to stimulate the nerve; 

 smart tapping or pinching with a pair of forceps 

 will also excite it, but by such means the nerve is 

 soon killed.] 



The above experiments shew: 



c. That the muscle is irritable and contractile: 

 certain external agencies (stimuli) excite some 

 change in it, the result of which is a muscular 

 contraction. 



d. The nerve is irritable: certain external agencies 

 excite some change in it, which in this par- 

 ticular case manifests itself by a contraction of 

 the muscles connected with the nerve. 



