定语从句中表语中by+which的问题？

Question

the mechanism by which brain cells store memories is not clearly understood!中的by which是什么意思啊？by修饰什么的？还原成移动的句子，by放在哪里啊？我怎么感觉by是多余的，哪位高手帮忙解决一下？

Answer 1

这里的by是使用、依靠的意思。
把句子拆分成两个句子你就理解了。
1.the mechanism is not clearly understood.
2.brain cells store memories by mechanism.
然后合成一个句子当然就要对mechanism使用定语从句来修饰。
the mechanism （by which brain cells store memories） is not clearly understood
理解了吧。

Answer 2

by表示被动。 因为后面出现is not clearly understood
还原的话它是放在the mechanism的后面。

Answer 3

The mechanism( by which brain cells store memories) is not clearly understood!
也可以这样写
the mechanism ( where brain cells store memories ) is not clearly understood
也等于
the mechanism (which brain cells store memories by) is not clearly understood

by which = where