EVOLUTION OF ELECTRICITY. 637 



electromotive power ; a fact which seems to harmonize well with the general 

 views formerly adverted to in regard to -the "correlation of forces;" the changes 

 which operate to produce disturbance of electric equilibrium whilst the muscle 

 is at rest, being concerned in the development of motor power when it is thrown 

 into contraction. This alteration has been demonstrated by M. du Bois-Reymond 

 in the living animal, after the following manner. The two feet of a live frog 

 were immersed in the two connecting vessels, but one of the legs was paralyzed 

 by division of its sciatic plexus; the muscular currents of the muscles of the 

 two limbs neutralized each other, so long as they remained at rest ; but upon 

 the frog being poisoned with strychnia, so that tetanic convulsions occurred in 

 one limb whilst the other remained motionless, the current in the former limb 

 was weakened, whilst that of the other remained unaffected, and a deflection of 

 the needle took place, indicating an upward current in the paralyzed limb and 

 a downward current in the tetanized one. The same thing may be shown in 

 the Human subject, by dipping the forefingers of the two hands into the two 

 conducting vessels connected with the galvanometer, so that the two arms are 

 included in opposite directions in the circuit ; when if, after the needle (which 

 usually undergoes a temporary disturbance on their first immersion) has come 

 to a state of rest, all the muscles of one of the arms be strongly and permanently 

 contracted, so as to give them the greatest possible tension without changing 

 the position of the arm, the needle is instantly deflected, always indicating a 

 current from the hand to the shoulder, that is, an upward current in the con- 

 tracted arm. Hence, according to the explanation just given, the contracted 

 arm plays the part of the negative metal in the circuit, in regard to the arm 

 whose muscles remain in the state of relaxation, showing that the normal current 

 will be a downward one. This change, however, is so extremely slight, that a 

 very delicate galvanometer is requisite to render it perceptible. Its intensity 

 depends very much on the muscular energy of the experimenter ; and even the 

 greater power which the right arm usually possesses becomes perceptible in the 

 greater deflection of the needle when it is put in action. 1 



671. The discovery that an electric current exists in nerves, the conditions of 

 which are in most respects similar to that of thfe Muscular current, is entirely 

 due to M. du Bois-Reymond. When a small piece of a nerve-trunk is cut out 

 from the recently killed body, and is so placed upon the electrodes that it touches 

 one of them with its surface (or natural longitudinal section), and the other 

 with its cut extremity (or artificial transverse section), a considerable deflection 

 of the index is produced, the direction of which always indicates the passage of 

 a current from the interior to the exterior of the nerve-trunk. It is indifferent 

 in regard to the direction of the current, whether the central or the peripheral 

 cut extremity be applied to the electrode ; and in fact the most powerful effect 

 is obtained by doubling the nerve in the middle, and applying both transverse 

 sections to one electrode, whilst the loop is applied to the other. On the other 

 hand, if the two cut extremities be applied to the two electrodes respectively, no 

 decided effect is produced ; and the same neutrality exists between any two 

 points of the surface of the trunk, equidistant from the middle of its length ; 



1 Of this very remarkable experiment, which was first made by M. du Bois-Reymond, 

 the Author has himself (through that gentleman's kindness) been a witness; and he gladly 

 bears his testimony to its highly satisfactory character, and withdraws the doubt previously 

 expressed (on the authority of Prof. Matteucci's negative statements) in regard to the 

 reality of this phenomenon ($ 330, note). The success of M. du Bois-Reymond in these 

 and similar investigations, is doubtless due in great part to the marvellous sensitiveness 

 of the galvanometer he employs, the coils of which consist of three miles of wire, as well 

 as to the perfection of the various arrangements by which he is enabled to avoid or 

 eliminate sources of error; but it must be attributed in great part also to the philosophic 

 method on which his inquiries are planned, and to the skill and perseverance with which 

 they are carried out. 



