rachis

 In biology, a rachis /ˈreɪkɪs/ (from the Ancient Greek: ῥάχις [rhákhis], "backbone, spine") is a main axis or "shaft". 

In vertebrates, rachis can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the rachis usually forms the supporting axis of the body and is then called the spine or vertebral column. Rachis can also mean the central shaft of pennaceous feathers.