How is this perfect passive participle being used?

Question

From Metamorphoses book II:

nec minus Heliades fletus et, inania morti
munera, dant lacrimas, et caesae pectora palmis
non auditurum miseras Phaethonta querellas
nocte dieque vocant adsternunturque sepulcro.

Loeb translates "caesae pectora palmis" as "With bruising hands beating their naked breasts...". But the thing I don't understand is that caesus is "having been beaten", not "having beaten". I would have expected the inflection of caesus to agree with pectora, not Heliades (or implicit illae). But pectora is nom./acc. neuter pl.

There's probably some very simple explanation here, but having mulled it over for an hour or so I just don't understand what's going on.

Answer 1

This is sometimes known as the "accusative of body parts", where an accusative noun indicates the body part (or more generally, the part of something) where an action takes place.

They (nom) were struck (nom) by palms (abl) on their breasts (acc).