70 GENERAL PROPERTIES OF LIVING TISSUES 



a closed circuit. Make and break the primary 

 current. 



Make and break currents will be induced in 

 the nerve, and the muscle will contract. 



Exclusion of Make or Break Current. Con- 

 nect the dry cell with posts 1 and 2, interposing 

 a key. See that the short-circuiting key, i. e. the 

 thick brass bridge between the posts on the sec- 

 ondary coil, is down. Connect the electrodes 

 with the secondary coil, and place their points 

 against the nerve of a nerve-muscle preparation. 

 Close the primary key. 



The muscle will not contract. 



The resistance to the passage of the induced 

 current through the portion of nerve between 

 the ends of the electrodes is many thousand 

 times greater than the resistance of the brass 

 bridge or short-circuiting key. Practically none 

 of the electricity will pass through the nerve 

 when the short-circuiting key is closed. 



Open the short-circuiting key and then open 

 the primary key. 



The muscle contracts. 



Eepeat the experiment, letting the make cur- 

 rent pass and short-circuiting the break. 



With the primary key and a short-circuiting 

 key either break or make induced currents can 

 be used as stimuli at will. 



