[Salaberri 2020] Грамматикализация показателя рефлексива buru < ‘голова’ в баскском - стандартная грамматикализация рефлексивов из частей тела [Schladt 2000]
Semantic change
Stage 1: the BPT is the object of the clause and has only its source meaning.
Stage 2: the BPT is reinterpreted by means of synecdoche and starts to stand for the subject referent, thus acquiring a reflexive function. The expression is, however, still ambiguous (both the source and the target meaning).
Stage 3: the BPT functions only as a reflexive and  develop new uses, such as the reciprocal one.