why can a chicken run without its head on

A chicken can stagger around without its noggin because the brain stem, often left partially intact after a beheading, controls most of its reflexes. One robust fellow lived a full eighteen months. Likely he was a real birdbrain, however.
因为鸡的神经源部分还活着，脉搏还跳动，所以它头掉了后还能跑，还能活着。
1945年的时候有一只叫Mike的鸡，头被砍掉后活了18个月。

.......but without the head ,how could it eat and drink to make itself alive for 18 months??....

真是无鸡之谈

是要翻译么？
【翻译】：为什么鸡没有头还可以跑？

为什么小鸡可以跑的时候可以不抬头呢？

为什么被砍掉头的鸡还会跑呢？