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PHYSIOLOGY 



The results of stimulating motor nerves by means of constant currents 

 were studied by Pfliiger and, embodied in a Table, make up what is known as 

 Pfliiger's law. The result of stimulating varies with the strength of a 

 current. 



LAW OF CONTRACTION 



c = contraction. C = strong contraction. T = tetanus. == no eft'ect. 



With the weakest currents excitation occurs only at make, since a make- 

 stimulus, i.e. the rise of catelectrotonus, is always more effectual than a break- 

 stimulus, i.e. the disappearance of anelectrotonus. With currents of moderate 



FIG. 117. Arrangement of experiment to demonstrate Pfliiger's law of contraction. 



strength excitation occurs both at make and break, being better marked at 

 make, especially in the case of descending currents. With very strong currents 

 we get a contraction at make only when the current is descending, since, when 

 the current is ascending, the excitation started at the cathode cannot pass the 

 block at the anode. For the same reason a break contraction is obtained only 

 with an ascending current, since at the break of a descending current there is 

 a swing-back of the nerve at the cathode to a condition of diminished irrita- 

 bility, which effectually blocks the excitation started higher up the nerve at 

 the anode. 



The arrangement of the experiment for demonstrating Pfliiger's law is 

 shown in Fig. 117. The strength of the current is graduated by means of the 

 rheochord, the current being led into the nerve by means of non-polarisable 

 electrodes. It is extremely important in these experiments to avoid any 

 injury or drying of the nerves at either of the two electrodes, since the excita- 

 tory effect cither at make or break would be abolished by local injury. 



These results, worked out chiefly on motor nerves, have been 

 confirmed as far as possible experimentally on sensory nerves, and on 



