404 MUSCULAR MOTION. 



only found to succeed, when the frogs were in full vital activity, espe- 

 cially in spring, after pairing ; when the animal was of sufficient size, 

 and its preparation for the experiment had been rapidly effected. 



From all these experiments it might be inferred, that parts of an ani- 

 mal may form galvanic chains, and produce a galvanic effect, which, 

 independently of any mechanical excitation, may give rise to the con- 

 traction of muscles. This excitation of electricity in chains of animal 

 parts, M. Tiedemann thinks, ought not to be esteemed a vital act. Its 

 effects only the contractions excited in the muscles are dependent on 

 the vital condition of the muscles and nerves. He considers, that elec- 

 tricity, excited in chains of heterogeneous animal parts, may be modi- 

 fied and augmented by the organic or living forces ; and that, more- 

 over, in certain animals, organs exist, the arrangement of which is such 

 as to excite electricity during their vital action as in the different kinds 

 of electrical fishes ; but in some experiments, instituted by M. Edwards, 1 

 the effects above referred to were produced by touching a denuded 

 nerve with a slender rod of silver, copper, zinc, lead, iron, gold, tin, or 

 platinum, and drawing it along the nerve for the space of from a quar- 

 ter to a third of an inch. He took care to employ metals of the great- 

 est purity, as they were furnished him by the assayers of the mint. 

 But it was not even necessary that the rod should be metallic : he suc- 

 ceeded with glass or horn. All metals, however, did not produce 

 equally vigorous contractions. Iron and zinc were far less effective 

 than the others ; but no accurate scale could be formed of their respect- 

 ive powers. 



Much difference is found to exist, when electricity is employed, ac- 

 cording as the nerve is insulated or not ; for as the muscular fibre is a 

 good conductor of electricity, if the nerve be not insulated, the electri- 

 city is communicated to both nerve and muscle, and its effect is conse- 

 quently diminished. It became, therefore, interesting to M. Edwards 

 to discover, whether any difference would be observable, when one metal 

 only was used, whether the nerve was insulated or not. In the expe- 

 riments above referred to, the nerve was insulated by passing a strip of 

 oiled silk beneath it. A comparison was now instituted between an 

 animal thus prepared, and another whose nerves instead of being insu- 

 lated, rested on the subjacent flesh. He made use of small rods, with 

 which he easily excited contractions when he drew them from above to 

 below along the denuded portion of nerve that was supported by the 

 oiled silk ; but he was unable to cause them when he passed the rod 

 along the nerve of the other animal which was not insulated. His 

 experiments were then made on two nerves of the same animal ; an.d he 

 found that after having vainly attempted to produce contractions by 

 the contact of a nerve resting upon muscle, they could still be induced 

 if the oiled silk were had recourse to; and he was able to command their 

 alternate appearance and disappearance by using a non-conductor or a 

 conductor for the support of the nerve. Somewhat surprised at these 

 results, M. Edwards was stimulated to the investigation, whether 



1 Appendix to Edwards on the Influence of Physical Agents on Life, Hodgkin and 

 Fishers translation, Lond., 1832. 



