"it made me a limbic system on two legs with waterfalls pouring out from my armpits"
Source : http://yourbrainonporn.com/what-are-the-symptoms-of-excessive-porn-use
lymbic system on LEGS? pouring from armpit? Is this sentence doesn't make sense or i just don't understand it?
This might mean "My entire being has been transformed into a sweaty beast that is ruled by reward, fear, and mating."
The phrase demonstrates both synecdoche and metaphor.
The use of synecdoche is having a part represent the whole. The part is the limbic system, which is the emotional center of the brain that includes the amygdala, which in turn governs reward, fear, and mating. The whole is the person. Here, the part has hijacked the whole. (And my apologies if this is really metonymy; I often confuse the two.)
In metaphor, the writer uses something to evoke something quite different. The author uses a waterfall to represent copious perspiration from the armpits.
The limbic system is, for lack of better wording, the cognitive "center" of the brain. As such the first part could imply that as a limbic system on two legs he was likely overstimulated.
Waterfalls pouring out of his armpits is likely a reference to perspiration - either due to arousal or nerves.
Therefore, my take on it would be that the author implies that "it" overstimulated him and made him either aroused or nervous for extended periods of time.
The limbic system is not the cognitive center really, unlike another answer suggests. That role is absolutely more accurately given to the prefrontal cortex, whose large size in homo sapiens is what gives us the power to process logic at high levels, and thus differentiate ourselves from primarily instinct-driven "animals". What the limbic system is primarily involved with, is emotional processing, motivation, behavior, but primarily — emotion and the general "emotional coloring" our individual perceptions have (if this doesn't make sense, please ask me to clarify).
So he is saying, by being a limbic system on legs, that he sees his (current) existence defined by his emotional overwhelmed-ness. He is not a creature of logic, for the present, but rather a creature defined by the deluge of emotion he is currently experiencing. As for the waterfalls — the physical expression of nervousness or whatever — just provides a corporeal example of his emotional state, while the limbic system mention refers to the more ethereal/intangible element of his emotional state.
