__apple fell from the tree and hit him on ___head为什么用THE

the特指那个苹果,后面是固定结构：hit sb on the +身体部位（硬的,如head);hit sb in the +身体部位（软的,如face）；hit sb by the +身体部位（特指抓住什么,如arm).
注意：这个结构的介词也要懂得怎么用,并且一定要用定冠词the,不能用his/her/its等