LABORATORY MANUAL OF PHYSIOLOGY. 



changes likewise, but not necessarily to the same degree. If the 

 current is ascending and the increase in irritability is sufficient, the 

 muscle will respond with a contraction. If the current is descend- 

 ing, and the fall in conductivity at the kathode is sufficient, this 

 will act as a block between the anode and the muscle and no con- 

 traction will be obtained. The relative increase and decrease of 

 irritability and conductivity at the anode and kathode vary with 

 the strength of the contsant current employed. Other things being 

 equal, the make of the constant current is more efficacious as a 

 stimulus than the break. 



These facts have been formulated as Pfluger's laws. Briefly, 

 they are as follows: 



The words "yes" and "no" in the above table indicate the oc- 

 currence or absence of a muscle contraction under the circum- 

 stances noted. The strength of the current is regulated by increas- 

 ing or decreasing the resistance by means of a rheocord or resist- 

 ance box. 



With a fresh muscle-nerve preparation, make and break a con- 

 stant current through the nerve as indicated by the above table. 

 In the light of the explanations which have been given and the 

 previous experiments performed, how are these results to be ex- 

 plained ? 



XXIII. STIMULATION OF HUMAN NERVES. 



In human nerves, in the body during life, it is obviously imprac- 

 ticable to bring the electrodes into direct contact with the nerve. 

 There is more or less insulation from the intervening integument 



[4*] 



