This is the man_,the artist said posed as a model for the painting。中间空格为什么填WHO,请教大家

可以把这个句子分解一下来理解, 因为里面包含两个定语从句。
This is the man.
This is the man (who posed as a model for the painting).
This is the man (who [the artist said]posed as a model for the painting).

This is the man_who,(the artist said )posed as a model for the
painting。
who做定语从句的连词。