64 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



of the anode of the nerve with a make shock possessing, as has just been 

 determined, a threshold value. The anelectrotonic diminution in the 

 excitability of the nerve will, in all probability, now render this stimulus 

 ineffective. 



If not sufficiently decisive the depression may be increased by the 

 addition of another dry cell or two. It eventually becomes so strong 

 that it prevents the passage of the nerve impulse developed at the 

 cathode. In order to show this blocking effect place the anode near 

 the muscle, i. e., render the current ascending. On the make of this 

 current the excitation is developed at the cathode. The resulting nerve 

 impulse travels toward the muscle, but never reaches it, because it is 

 blocked by the area of depression in the region of the anode. 



This point may also be proved by placing a drop of a solution of 

 sodium chlorid upon the nerve in the region of the anode, the anode 



FIG. 41. METHOD USED TO SHOW ELECTROTONIC CHANGES ON MAKING AND BREAK- 

 ING OF GALVANIC CURRENT. 



K, Key for making and breaking of current; P, pole changer for making either end 

 of muscle (M) anodic or cathodic; D, clamp applied to muscle to destroy contraction 

 wave, but not wave of excitation; W, weight sattached to ends of muscle. These may 

 be displaced by writing levers. 



being in this case situated near the muscles. Presently the muscles of 

 this leg will begin to twitch and finally become tetanically contracted. 

 Close the key in the circuit of the constant current. The excitability of 

 this region now having been diminished, the muscles will relax or at 

 least show a slighter degree of tetanus. Quickly change the wires so 

 that the cathode comes to lie near the muscle. The limb again becomes 

 tetanic. Repeat this test. 



Knowing these facts, explain the third phase of the law of Pfliiger, 

 pertaining to the effect of the constant current upon normal excised 

 muscle and nerve. 



2. Cathodic and Anodic Excitation. When the constant current is 

 made, the stimulation arises at the cathode, and when broken, at the 

 anode. Suspend a sartorius muscle of a curarized frog vertically from 



