136 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
(electroplaxes) arranged at right angles to the axis of the primitive 
muscle, each derived, where the history has been traced (Torpedo, 
Raia), 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 between them, 
the latter in a few cases being weakly developed or absent. Nervous 
stimulation is always by motor roots leading to the nervous layer, the 
connexion corresponding to the nerve-end of a muscle cell. Numbers 
Fic. 143.—Head of Astroscopus y-grecum, after Dahlgren and Silvester. The dotted 
line on right shows extent of electric organ, on theleft 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; olf, olfactory nerve; om, oculomotor; of, optic nerve; re, rif, rint, rs, external, in- 
ferior, internal and superior rectus muscles; rp, palatine nerve; so, superior oblique muscle; 
tf, trigeminal-facial nerve. 
of these electroplaxes are included in a connective-tissue compartment 
with a gelatinous substance between them and all with their nervous 
layer turned in the same direction. 
In Torpedo the organ apparently is derived from part of the jaw 
muscles and the prisms of plates are arranged vertically. In Astro- 
scopus (fig. 143) it is supposed that the tissue comes from one of the eye 
muscles, while in Gymnotus the ventral trunk muscles are concerned 
and the columns of electroplaxes are horizontal. In the same fish the 
discharge is always in the same direction, e.g., in Torpedo from below 
upward. 
