Part of the 9+2 axoneme, that occurs in most motile cilia, consisting of the pair of two single central microtubules and their associated structures which include the central pair projections, the central pair bridges linking the two tubules, and the central pair caps which are attached to the distal or plus ends of the microtubules. [ http://www.ncbi.nlm.nih.gov/pubmed/9295136 http://www.ncbi.nlm.nih.gov/pubmed/21586547 ]