358 ANATOMY OF VERTEBRATES. 



siistaiiiing the terminal meshes of the nerves and vessels are hori- 

 zontal in the Torpedo ; the course of the electric current is 

 from above downwards. The corresponding plates in the Gym- 

 notus are vertical ; the direction of the electric current is from 

 before backwards : i. e. it is vertical to the planes of the pjlates in 

 both cases. 



The row of compressed cells constituting the electric prism of 

 the Torpedo offers some analogy to the row of microscopic discs 

 of which the elementary muscular filament appears to consist, 

 fig. 128, B. The looped termination of the exciting nerve is 

 common to muscular tissue and that of the electric organ. The 

 electric, like the motory nerves, rise from the anterior myelonal 

 tracts ; and, though they have a special lobe at their origin, beyond 

 that origin, in the Torpedo, they have no ganglion. An impression 

 on any jiart of the body of the Torpedo is carried by the sensory 

 nerves either directly, or through the posterior myelonal tracts, to 

 the brain, excites there the act of volition, which is convej'ed 

 along the electric nerves to the organs, and ])roduces the shock : 

 in muscular contraction, the impression and volition take' the same 

 course to the muscular fibres. If the electric nerves are divided 

 at their origin from the brain, the course of the stimulus is inter- 

 ruj)ted, and no irritant to the body has any efl^ect on the electric 

 organs any more than it would have under the like circumstances 

 on the muscles. But, if the ends- of the nerves in connection 

 witli the organ be irritated, the discharge of electricity takes pjlace, 

 just as irritating the end of the divided motor nerve in connection 

 with the muscle would induce its contraction. If part of the 

 electric nerves be left in connection with the brain, the stimulus 

 of volition cannot, through these, excite the discharge of the 

 whole organ, but only of that part of the organ to which the 

 undivided nerves are distributed. So, likewise, the irritation of 

 the end of a divided nerve in connection with the electric appa- 

 ratus, excites the discharge of only that part to which such nerve 

 is distributed. We have seen that the power of exciting the 

 electric action, like that of exciting the muscular contraction, is 

 exhausted by exercise and recovered by repose : it is also augmented 

 by energetic circulation and respiration ; and what is more signi- 

 ficative of their close analiigy, both powers are exalted by the 

 direct action, on the nervous centres, of the drug ' strychnine : ' 

 its application causes simultaneously a tetanic state of the muscles 

 of the fish, and a rapid succession of involuntary electric dis- 

 charges.' 



' I.X.WH. \i. U'.2. 



