The Statocyst-Oculomotor System of Octopus vulgaris: Extraocular Eye Muscles, Eye Muscle Nerves, Statocyst Nerves and the Oculomotor Centre in the Central Nervous System

Seven extraocular eye muscles are described in Octopus vulgaris. There are three powerful recti muscles that produce linear movements and four oblique muscles producing rotation. Some of these oblique muscles are very thin sheets passing halfway round the eyeball. The eye muscles are controlled by seven nerves, but several of these innervate more than one muscle. Stimulation of the individual nerves produces the linear and rotatory movements, or both, to be expected from the morphological organization of the muscles they innervate. Two of the nerves run only to extraocular eye muscles, the other five contain additional fibres for the iris, chromatophores or skin. Cobalt filling of the central ends of the eye muscle nerves showed that all have fibres originating in the ipsilateral anterior lateral pedal lobe which is the oculomotor centre. The two nerves whose stimulation gave expansion of the chromatophores of the iris were shown to contain fibres with somata in the ipsilateral anterior chromatophore lobe. Two nerves gave constriction of the pupil and proved to contain fibres with somata in an area between the posterior pedal and magnocellular lobes, demonstrating the position of a pupillary control centre. Stimulation of one nerve gave dilation of the pupil but the origin of the relevant cells remains unclear. Cobalt filling of the central ends of the macula and crista nerves of the statocyst showed the destinations of their afferent fibres in many parts of the brain, including the oculomotor centre and higher motor centres of the basal and peduncle lobes. In addition, many somata of efferent fibres to the statocyst were filled in the oculomotor centre, in the posterior lateral pedal lobe, and in the posterior pedal and magnocellular lobes. The statocyst-oculomotor system of Octopus thus includes two pathways from the statocyst equilibrium receptor organs to the motoneurons of the eyes: one direct pathway, and another indirect path via higher integrative centres where visual information about movement is combined with that coming from the statocysts. This situation points to a remarkable convergence between the Octopus statocyst-oculomotor system and the vestibulo-ocular system of vertebrates.