Merriam-Webster Collegiate Dictionary - rachis

 noun  (plural ~es; also rachides)  Etymology: New Latin rachid-, ~, from Greek rhachis; akin to Greek rhachos thorn, Lithuanian razas dry twig, tine  Date: 1785  1. an axial structure: as  a.  (1) the elongated axis of an inflorescence  (2) an extension of the petiole of a compound leaf that bears the leaflets  b. the distal part of the shaft of a feather that bears the web  2. spinal column