Derivational etymology relying on an anatomical description, the symmetrical morphology of the body on each side of the spine. Formally, it relies on the presence of forms with ᾰ in the paradigm of ῥήγνυμι, like the aorist ἐρράγην. Therefore the different vocalism between the lemma ῥάχις  and the etymon ῥῆξις could be explained by the alternation ᾰ ~ η. The semantic relationship is rather loose, since ῥῆξις and ῥῆγνυμι mean "breaking", "to break", not "to divide", but division can be a consequence of breaking. The presence of this etymology in Meletius and in the Byzantine Etymologica makes it likely that the source is Orion, although the etymology did not survive in the extant versions of Orion's Etymologicum