One of two microtubules present in the axonemal central pair. It is distinguishable from the C2 axonemal microtubule (also called C2 tubule) by the presence of differing protein components of the projections. [ GOC:cilia http://www.ncbi.nlm.nih.gov/pubmed/21586547 http://www.ncbi.nlm.nih.gov/pubmed/9295136 ]