6 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, 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 ot 

 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. 



