Transverse section of the dorsal parietal cortex of a feedlot steer with polioencephalomalacia. Upper panel—macroscopic view under ambient illumination. Affected segments of the cortical gray matter have a yellowish discoloration and irregular contours. Lower panel—same section viewed with ultraviolet (366 nm) illumination. Affected segments and foci of the cortical gray matter have a bright, light blue hue compared with the darker blue of adjacent, normal cortex.