“身体部位”前使用定冠词、而且使用单数这种用法，属于习惯表达。因为它不是具体指谁的，也不是泛指一个或一些，而是表达“类别”概念。受事者的单复数对它不产生影响，仍用单数形式：in the head。

例句：

1. We’d been hit in the head with a rock; like a neighborhood bully, we grabbed the first person we could get our hands on and beat him senseless. 我们被石头砸伤了头，我们就像邻居打手那样，抓住另一个我们能对付的人，把他痛打了一顿。

2. People fall from great heights, get hit in the head by solid iron balls, and do all sorts of things which should cause serious harm. 打球的人从高空掉下来，脑袋撞在硬梆梆的铁球上，还有各种各样能让他们严重挂彩的伤害。

3. Employees are like nails, as nails must be hit in the head in order to make them work. 雇员像钉子，因为敲打钉子的头部才能让他起到固定东西的作用。

4. She calmly explained to me that you hit the frogs in the head with a club to knock them out. 她平静的向我解释说就是拿棍子打青蛙的头，把它们打晕。