Sagittal and horizontal sections of 257 intervertebral discs obtained at autopsy and material obtained from 441 operations for herniation of a disc were examined histologically. In the material that was taken at autopsy, myxomatous degeneration of the annulus fibrosus increased in proportion to the age of the subject. The bundles in the internal layer of the annulus fibrosus reversed their usual direction and showed myxomatous degeneration, sometimes resulting in posterior and anterior convex bulging in the internal layer of the anterior and posterior parts of the annulus fibrosus, respectively. When material from a disc was surgically removed as a single free fragment (as in a complete extrusion or a sequestration type of herniation), annulus fibrosus with myxomatous degeneration was found in most material, while the nucleus pulposus rarely was. These results suggest that, from the standpoint of pathomechanism, a protrusion type of herniation of the annulus fibrosus exists in which only the annulus fibrosus is protruded due to reversal of the bundles of the annulus fibrosus, without involvement of the nucleus pulposus. This type of herniation would be a separate entity from the protrusion type of herniation of the nucleus pulposus that occurs when the nucleus pulposus is protruded through a fissure in the annulus fibrosus.