to which的疑惑【高分求解】

题目

to which的疑惑【高分求解】
原句是：
Human facial expressions differ from those of animals in the degree to which they can be controlled on purpose.
在上面这句话里,to which的作用我不太清楚,很显然,这是一个定语从句,其分解成两句话便是：
1.Human facial expressions differ from those of animals in the degree.
2.Human facial expressions can be controlled on purpose.
很奇怪的是,上面两句话中,没有了to,这是怎么回事?to which在此句话中应有的作用该是什么?请求赐教!
另外,如果有朋友认为我分解的不对的话,您也可以帮忙分解一下这句话.

答案

分为两句的方法你是错的.要分为三句.Human facial expressions differ from those of animals in the degree to which they can be controlled on purpose.1.Human facial expressions differ from those of animals...