GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 157 



of a part of the current by the lymph, etc., the currents which may be obtained 

 are usually small. Under especially favorable conditions the current may 

 be twice as great as the current of rest, and Gotch and Burch state that 

 they have found it in the sciatic nerve of the frog to have the excitatory effect 

 of 0.033 volt. 1 Hering 2 has shown that it may suffice to excite another nerve 

 in close contact with it, the experiment being of the same type as that de- 

 scribed as secondary tetanus, p. 150. 



The fact that a negative variation of the demarcation current of a bundle 

 of nerve-fibres may excite other fibres lying beside them in the same nerve, 

 is the explanation of the so-called " paradoxical contraction." This may be 

 seen under the following conditions : Take a frog that has been kept in the 

 cold so that its nerves are very irritable; dissect out the branches of the 

 sciatic nerve at the knee, ligature, divide below the ligatures, and then dissect 

 up the nerve as far as the branches given off to the muscles of the thigh ; 

 expose the nervous plexus at the back of the abdominal cavity, and divide 

 close to the spinal cord ; remove all extra fluid, and avoid wetting the nerves 

 with salt solution. Raise up the knee end of the nerve, and place on elec- 

 trodes connected with the secondary coil of an induction apparatus : excite 

 with weak tetanizing current close to the end of the nerve, and see the thigh 

 muscles contract. As the fibres excited at the knee have no anatomical or 

 phvsiological connection with the fibres of the branches of the nerve dis- 

 tributed to the thigh muscles, the nerve impulse which passes up the nerve 

 could not reach these fibres, and the contraction could not occur were it not 

 for the spread of the current of action under these abnormal conditions. To 

 make sure that the contraction is not due to a spread of the exciting current 

 or to unipolar stimulation effects, one can ligature the nerve above the excited 

 point. This would stop the progress of the nerve impulse and the accom- 

 panying current of action, and prevent the " paradoxical contraction." There 

 is still another possible source of error that has to be guarded against : the 

 exciting current, through electrolytic effects, may cause electrotonic currents 

 (see p. 62) to be developed in the nerve, and these may spread sufficiently 

 to excite branches of the thigh muscles. A ligature will stop the spread of 

 these currents, but their presence can be recognized from the fact that the 

 contractions will be the stronger the nearer the electrodes arc brought to the 

 part of the nerve from which the branches to the thigh muscles arc given 

 off. A contraction called out in this way is called the - electrotonic twitch." 

 A spread of the current of action to neighboring fibres never occurs when 

 the nerve is intact, the rule of isolated conduction by nerve-fibres holding 

 good. 



Relation of the Electrical Phenomena of the Nerve to Physiological 

 Processes. — Certain writers of the extreme mechanical school would explain 

 all the forms of functional activity of the nerve as purely physical processes, 

 which result from chemical change occurring within it. This point of view 



1 Proceedings of the Royal Society, 1900, rol. Ixv. p. I H. 



* Hering: Sitzungsberichte der Wiener Akadamie, 1882, Ixxxv. 3, S. 237. 



