17. And one leading authority says that these intensely powerful mental events can be not only harnessed but actually brought under conscious control, to help us sleep and feel better.

【分析】复合句。句子主干为one leading authority says that…，that引导宾语从句，其主干为these…mental events can be not only harnessed but... brought under conscious control。其中 not only...but...表示“不仅…而且...”;句末的不定式短语作目的状语。
【译文】一名主要的权威人士说，梦这种异常强烈的精神活动不仅能被驾驭，事实上还可以有意识的加以控制，以帮助我们睡眠更好，感觉更佳。

18. But not all parts of the brain are equally involved;the limbic system (the "emotional brain"）is especially active, while the prefrontal cortex (the center of intellect and reasoning) is relatively quiet.

【分析】并列句。分号连接两个并列的分句，后一分是对前一分句的解释说明，其中又包含由while连接的两个并列分句，表对比关系。
【译文】但并非大脑所有部分的活动都是均等的，脑边缘系统(“情绪大脑”）尤其活跃，而前额皮质（思维和推理的中心部位）则相对安静。