rachis

• rhachis

From New Latin rachis, from Ancient Greek ῥάχις (rhákhis, “spine, ridge”).

• (UK, US) IPA^(key): /ˈɹeɪkɪs/

rachis (plural rachises or rachides)

1. (obsolete, zoology, anatomy) The spinal column, or the vertebrae of the spine. [17th-19th c.]
2. (zoology) An anatomical shaft or axis in a marine invertebrate. [from 18th c.]
   • 2017, Danna Staaf, Squid Empire, ForeEdge, →ISBN, page 115:

     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.

3. (ornithology) The central shaft of a feather. [from 19th c.]
4. (botany) The main shaft of either a compound leaf, head of grain, or fern frond. [from 19th c.]

• The plural form rachides is based on a mistaken impression of the Ancient Greek stem.

• rachis at OneLook Dictionary Search
• rachis in The Century Dictionary, New York, N.Y.: The Century Co., 1911.

• Charis, Sirach, chairs