ˈrīsnə(r)z- noun
Usage: usually capitalized R
Etymology: after Ernst Reissner died 1878 German anatomist
: a band of fibers arising from the roof of the midbrain in many vertebrates, passing along the aqueduct and fourth ventricle to enter the central canal of the spinal cord, ending in the regions of the spinal nerves, being especially large in fishes, and probably taking part in visual reflexes or regulating flexion of the body