The eel has some specialized respiratory aspects as skin respiration, rest of opercular
movement and unilateral branchial respiration. As for this unilateral branchial respiration,
some observations have been reported. Eels can rest an one-sided opercular movement and its
gill slit is induced to shut. Often at that time, the resting operculum (opercular area) becomes
hollow contrary to the other side. Then, respiration is continued by the other side
movement02'. Respected muscles relating with such a process would be firstly adductors of
operculum. They are the Maope. and Mar. . The Mar. seems to be more effective for resting
the opercular movement, especially to shut the gill slit due to its anatomical situation. To these
muscles the r.facialis and r.hyoideus are sent both from the ventral of the m.oblongata. At this
ventral area of the m.oblongata, there distribute such nulei of the motor cranial nerves as the fifth,
seventh, ninth and tenth and a primary motor center of cerebrum is considered to be formed3)4).
As for the sensory route in eels, the vagus nerve which is the tenth cranial nerve sends out a
branchial branch to the gill arches. This nerve connects directly the brain and the gill and so
that it may become to be a transmission route to the respiratory center which is considered to be
situated in the m.oblongata. In the m.oblongata, there exists also a primary sensory center
connected with such sensory cranial nerves as the fifth, seventh, eighth, ninth and tenth3)4>. If
considered concisely, it would be possible that these centers cause the above mentioned unilateral
opercular movement, accompanying with supposed irregular nervous activities.