MUSCULAR SYSTEM 



141 



cranial nerves are concerned, while in Gymnotus and the skates the 

 supply is from the spinal nerves. Malapterurus is peculiar in that the 

 organ is in the integument and has been supposed by some to arise 

 from modified glands. It is more probable that here as elsewhere it 

 is derived from the muscles, as the organ is under control of the will; 

 the development has yet to be studied. This diversity of origin 

 clearly shows that the electrical functions have been separately ac- 

 quired in the different species. 



The organs are composed of a large number of electrical plates 

 (electroplaxes) arranged at right angles to the axis of the primitive 



Fig. 150. — Head of Astroscopus y-grcecutn, after Dahlgren and Silvester. The 

 dotted line on right shows extent of electric organ, on the left the eye muscles and 

 nerves as forced out of place by the electric organ, ab, abducens; cil, ciliary nerve; e, 

 eye; en, electric nerve; n, naris; olj, olfactory nerve; om, oculomotor; op, optic nerve; 

 re, rif, rint, rs, external, inferior, internal and superior rectus muscles; rp, palatine 

 nerve; so, superior oblique muscle; tf, trigeminal-facial nerve; tr, trochlearis nerve. 



muscle, each derived, where the history has been traced (Torpedo, 

 Rata), from a primitive muscle cell. In the typical condition each 

 plate consists of an outer electric layer, differentiated into a nervous 

 side and a so-called nutritive side, with a middle striated layer be- 

 tween them, the latter in a few cases being weakly developed or ab- 

 sent. Nervous stimulation is always by motor roots leading to the 

 nervous layer, the connexion corresponding to the nerve-end of a 

 muscle cell. Numbers of these electroplaxes are included in a 



