Annulus fibrosus disci intervertebralis

Each intervertebral fibrocartilage is composed, at its circumference, of laminæ of fibrous tissue and fibrocartilage, forming the annulus fibrosus.

Spelling notes
Either "anulus" or "annulus" are correct spellings. Gray's Anatomy uses "annulus fibrosus," Terminologia Anatomica uses "anulus fibrosus," and Nomina Anatomica uses both forms. A Google search indicates that "annulus fibrosus" is the most common usage.

"Fibrosus" has no correct alternative spelling; "fibrosis" has a different meaning and is incorrect in this context.