The anatomy of the cilium. Cilia are arrays of nine microtubule doublets that extend from the basal body (a modified centriole) and push out an extension of the plasma membrane (called the ciliary membrane). Some cilia also contain a pair of singlet microtubules in the middle called the central pair that is involved in regulating motility. Motile power is provided by two sets of dynein arms: the inner and outer arms. Motility is regulated by radial spokes that interact with the central pair. Defects in any of these structures can lead to ciliary diseases, although the precise symptoms that arise appear to depend on which structure is altered.