这个句子中的定语从句较为特殊一些，相对少见。因此我根据《剑桥语法》的例句，对此再稍作说明。我们先看下面例句：

I said that it might be more efficient to hold the meeting on Saturday morning, which suggestion they all enthusiastically endorsed.

在这个定语从句中，which为关系限定词，与suggestion构成关系短语 which suggestion，指代先行项 it might be more efficient to hold the meeting on Saturday morning （指代先行项的不仅仅是which，而是which suggestion）。which suggestion在定语从句中作宾语。如果将这个句子改为并列句，则为：

I said that it might be more efficient to hold the meeting on Saturday morning, and they all enthusiastically endorsed this suggestion.

可以清楚看出，后一分句的宾语this suggestion 指的就是 it might be more efficient to hold the meeting on Saturday morning这个分句的内容。

现在我们反过来，将这个并列句改为主从复合句，由于前一分句中的that从句与后一分句的this suggestion 共指同一个内容，我们需要做的就是就是用关系限定词which取代指示限定词this，将which suggestion移至该分句之首，去掉并列标记and, 就得到一个非限制性关系从句（传统语法称为非限制性定语从句）。

Many of the measures of the government were determined by the action of great assemblies of the free citizens, which action was itself, in a great measure, controlled by the harangues of orators...

网友的这个句子与《剑桥语法》的例句类似。主句中的the action of great assemblies of the free citizens，of介词短语表示同位关系，即great assemblies of the free citizens是一种行动。which action中which为关系限定词，与action构成关系短语，指代先行词great assemblies of the free citizens，并在定语从句中作主语。如果将定语从句改为并列分句，则为：

Many of the measures of the government were determined by the action of great assemblies of the free citizens, and this action was itself, in a great measure, controlled by the harangues of orators...

itself为反身代词，作主语的同位语，起强调作用，去掉后不影响句义。其位置与中位状语相同，即位于主语后，实意动词之前，如果谓语含有操作词，则位于操作词之后。