A 12-year-old castrated Weimaraner was referred to our hospital with an acute onset of tetraparesis. Intervertebral disk extrusion at the level of C5-6 was diagnosed based on diagnostic imaging, and a left-sided hemilaminectomy at the lesion was performed. During the operation, the extruded disk material was turned out to be covered with the membranous tissue, and stripping of the membranous tissue resulted in severe bleeding. Observation of the surgical field after removal of the intervertebral material revealed that the membranous tissue was the internal vertebral venous plexus. In this case, it was difficult to confirm the internal vertebral venous plexus because the venous plexus was compressed by the laterally prolapsed disk material and there was no blood flow.