One of a set of small groups of neuron cell bodies located along the trajectory of optic fibers in the mesencephalon. These consist of the posterior nucleus [TA] (nucleus posterior [TA]), medial nucleus [TA] (nucleus medialis [TA]), and lateral nucleus [TA] (nucleus lateralis [TA]), which are also called the posterior, medial, and lateral terminal nuclei. The connections of these nuclei, along with the nucleus of the optic tract, make up the accessory optic system that appears to be concerned with retinal slip in specific directions.