Here's why:
* Metaphor: A figure of speech that compares two unlike things without using words like "like" or "as."
* In this case: The phrase directly compares the deterioration of a brain to the dry rot of timbers in a building. This comparison is not literal, but it creates a vivid image in the reader's mind of the brain's decay.
Let me know if you'd like more examples or have any other literary terms you'd like to explore!