142 GENERAL PROPERTIES OF LIVING TISSUES 



pass through the muscle until the closure tonic 

 contraction has disappeared, then open the key. 

 Neglect the opening twitch. 



The muscle will not return to its original 

 length, but will remain contracted for a time 

 (opening tonic contraction). 



Close the key again. 



The tonic contraction will disappear. 



The galvanic current in this case checks (in- 

 hibits) a contraction. This new action is dis- 

 cussed on page 153. 



6. Rhythmic Contraction. That the galvanic 

 current acts as a stimulus so long as it continues 

 to flow is shown also by the fact that its passage 

 through contractile tissue may cause the muscle 

 to fall into rhythmic contractions. These are 

 easy to produce in muscles which normally con- 

 tract in rhythms, for example, the heart ; but 

 they may under some circumstances be observed 

 also in smooth muscle, and even in skeletal 

 muscles. 



Connect a dry cell through a simple key with 

 the metre posts of the rheochord. Join the non- 

 polarizable electrodes to the positive post and the 

 slider. Bring the slider against the positive post, 

 so that no current shall flow through the elec- 

 trodes when they are joined by the tissue. 



Expose the heart. With a sharp knife bisect 



