Medical Definition of annulus fibrosus
: a ring of fibrous tissue (such as that surrounding a heart valve) especially : the outer portion of an intervertebral disk that surrounds the nucleus pulposus and is composed chiefly of layers of collagen fibers Disks between the vertebrae have a soft center—a nucleus pulposus—and a tougher exterior, annulus fibrosus. — Matthew Sturdevant, The Hartford (Connecticut) Courant, 30 May 2010 The nucleus pulposus can bulge out or herniate the anulus fibrosus, but the disk itself does not "slip." — John Palo, Newsday, 21 Feb. 1990
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