Rhachis

Rhachis, often written as rachis, is a term of Greek origin (from rhakhēs, meaning spine) used to denote a central axis or spine in various biological structures. The spelling with an initial h is less common in modern usage and tends to appear mainly in historical or linguistic contexts.

In anatomy, rhachis can be found in older texts as a reference to the backbone or spinal column. Contemporary medical language typically uses spine or vertebral column instead, with rhachis appearing primarily for historical or descriptive purposes.

In botany, rachis is a standard term for the main axis of a compound leaf or frond. It runs from the petiole and bears leaflets (pinnae) along its length. The concept is central to the organization of many plants, including ferns and palms, where the rachis provides structural support for the distributed leaflets or segments.

In zoology and especially in ornithology, the central shaft of a feather is called the rachis. This shaft supports the vane of the feather and from it emerge the barbs and barbules that form the feather’s structure. The spelling rhachis may appear in some older texts describing feathers or in linguistic discussions of the term.

Overall, rhachis/rachis denotes a central, supporting axis across multiple disciplines, reflecting its Greek root meaning spine. The modern standard form is rachis, with rhachis used primarily in historical or etymological contexts.