rachis

noun4

1. •
   An anatomical shaft or axis in a marine invertebrate.

   • In some squid species, the pen rachis flares out into wings on either side, and the tip of the rachis grows into a thick cone. — 2017
2. •
   The central shaft of a feather.
3. •
   The axis of either a compound leaf, inflorescence, or fern frond.
4. •
   (no longer used) The spinal column, or the vertebrae of the spine.