THE TISSUES 



49 



changes are apt to be set up and a contraction results. If, 

 on the other hand, a house of cards is built and made extra 

 stable by introducing some additional cards at the founda- 

 tion, if these cards are suddenly withdrawn the chances 

 are that the house falls to bits. So with a muscle. When 

 the current is opened the removal of the state of increased 

 stability at the positive pole may cause disintegration and 

 produce the anodal opening contraction. 



The study of electrotonus thus explains why any sudden 

 change in the flow of electricity through a muscle stimulates 

 it. It further explains why the stimulation and contraction 

 start from the cathode on closing and from the anode on 

 opening; and why the closing contraction is stronger than 

 the opening, since the sudden pro- 

 duction of a condition of actual in- 

 stability must act more powerfully 

 than the simple sudden removal of 

 a condition of increased stability. 



This law of Polar Excitation, while 

 it applies to muscle and nerve, does 

 not apply to all protoplasm. Thus 

 amoeba shows contraction at the 

 anode and expansion at the cathode 

 when a galvanic current is passed 

 through it. 



When muscle is stimulated by 

 induced electricity (Fig. 21) the ques- 

 tion is much easier, for, with each make 

 and break or each sudden alteration 

 in the strength of the primary circuit, 

 there is a sudden appearance and 

 equally sudden disappearance of a 

 flow of electricity in the secondary 

 coil. If, therefore, wires from the 

 secondary coil are led off to a muscle, each change in the 

 primary circuit causes the sudden and practically simul- 

 taneous appearance and disappearance of an electric current 

 in the muscle, and this of course causes a contraction. 

 But here the effects of closing and opening the current are 



4 



FIG. 18. Course of Electric 

 Current in primary circuit 

 (lower line), and in secon- 

 dary circuit (upper line) of 

 an induction coil. Observe 

 that in the secondary the 

 make (upstroke) and break 

 (downstroke) are com- 

 bined, and that a stronger 

 current is developed in 

 the secondary circuit upon 

 breaking than upon mak- 

 ing the primary circuit. 



