10. ELECTRIC ORGANS
405
and rapidly propagates into the main part of the cell. The external
record is initially positive on the side of the uninnervated face, which
indicates that the spike in the innervated (stalk) face rises more rapidly,
although this is not clear in the figure. The impulse in the uninnervated
face then becomes larger and a phase of head negativity appears in the
external record. This latter phase is considerably larger than the initial,
head-positive phase.
Steatogenys has an accessory electric organ in the chin region
(Lowrey, 1913) which was called the submental filament by taxonomists
( Fig. 32). In Steatogenys elegans ( a relatively large species reaching
15-20 cm long) the submental organ consists of a single column of cells
innervated on their anterior faces. The posterior part of the organ can be
seen through the overlying epidermis; the anterior part lies more deeply
in a fold of dermis and is not visible in the intact animal. There is an
, PO0
Fig. 32. Rostra1 accessory organs of Steatogenys elegans. ( a ) Appearance in
intact fish; both postopercular organ (POO) and submental organ (SMO) are
visible. ( b ) Overlying tissue is removed to show the full extent of the organs and
the nerve ( N ) coming from spinal nerves to innervate them. The most rostra1
electrocytes of the main organ are also shown. Spinal cord, C; main organ, MO;
longitudinal nerve plexus, P; and rib, R.