512 APPENDIX 



It was the great Helmholtz who first noticed that nervous impulses 

 require time for their transmission, and he demonstrated it on the sciatic 

 of the frog and on man. Time is required because the setting-up of 

 chemical (or electrical or molecular) change in the millions of complex 

 molecules along the progress of the impulse takes time. The actual rate 

 varies widely in the different nerves of different animals, but along the 

 frog's sciatic (poikilothermous mixed nerve) the rate in the winter-season 

 is usually 20 meters per second. It is often much less. In sensory nerves 

 it is apparently about the same. In man the average rate is not far 

 from 40 meters per second. 



Some of the conditions which decrease the speed are the dying degenera- 

 tion of the nerve, cold, pressure, stretching, fatigue, alcohol, ether, and 

 strong electricity. Heat increases the rate. 



Expt. 80. Electrotonus. (Apparatus: Dry-cell, key, commutator 

 with cross-wires, myograph, glass nerve-plate, 15 per cent, sodium- 

 chloride solution, fine copper wires.) Twist two copper wires 4 cm. apart 

 about the glass nerve-plate and connect their ends with two adjacent 

 posts of the commutator with cross-wires. Connect the rocker-posts 

 with a dry-cell through the key. Adjust the nerve-plate close to top of 

 gastrocnemius in the myograph so that its long and normal nerve will lie 

 over the electrodes. 



(A) Anelectrotonus. Between the electrodes and 1 cm. from that nearer 

 the muscle, place on the nerve a single drop of the sodium-chloride 

 solution. Rock the commutator so that the nearer electrode shall be the 

 anode. Observe the irregular contractions due to the chemical stimula- 

 tion by the chlorine. Now close the key. The contractions lessen or 

 cease. Anelectrotonus is in this case a condition of lessened irritability. 

 Sometimes it is not so. 



(B) Catelectrotonus. Now shift the rocker so that it is the cathode 

 that is nearer the strong salt solution, and close the key. The irregular 

 contractions of the muscle occasioned by the salt are much increased. 

 Catelectrotonus is a condition of increased irritability. 



Expts. 59, 60, and 70 suggest the same fact that this experiment 

 demonstrates. 



Gotch thus summarizes Pfliiger's general conclusions as to electrotonus, 

 which, as its name implies, is only a condition of irritability or tone 

 caused in a nerve by a stimulus: "Under the influence of a constant 

 current flowing through a nerve there is an increase in the nerve-excita- 

 bility, at or near the negative pole (cathode), a decrease at or near the 

 positive pole (anode). On the cessation of the current these changes 

 are reversed, the cathode being the seat of the fall, and the anode that of 

 a rise in excitability. The alterations in excitability are most intense 

 at the poles, but spread, diminishing with the distance, into the intra- 

 polar and extrapolar regions. At some point in the intrapolar region 

 the boundary between the two polar extensions is reached; this point is 

 therefore unaffected, and is termed the indifference-point. The excita- 

 bility changes are true for all forms of stimulation, electrical, mechanical, 



