In Emily Dickinson's poem "The Brain Within Its Groove," the speaker seems to be saying that the brain runs smoothly in its groove as long as thoughts run evenly:
The Brain, within its Groove
Runs evenly—and true—
However, if an injury occurs to the brain, such as a splinter which could be a metaphor for mental illness, the brain loses its perfect function.
But let a Splinter swerve—
Such a splinter could occur when one is troubled by mental issues which do not follow the smooth groove of the brain's thoughts. When the splinter causes pain or infection, the brain cannot flow smoothly in its thoughts. It begins to flow haphazardly as in comparison to a flood of water that runs through the hills, taking the path that it so chooses. This can be compared to thoughts that do not take an even path.
The speaker in this poem compares the thoughts that have no rhyme or reason as a flood that "trods" down anything in its path. There is no containing thoughts that run rampant without a direction or natural course. The thoughts that cannot be guided run through the brain like a flood of water runs through the hills, creating a path wherever the flood so desires to flow. The speaker states that it is easier to stop a current from flooding the hills than to organize the thoughts in the brain that have begun to overflow:
'Twere easier for You—
To put a Current back—
When Floods have slit the Hills—
And scooped a Turnpike for Themselves—
And trodden out the Mills—
Oh, how often the brain loses its groove and overflows with thoughts that follow no direction! No doubt, everyone has experienced a time when the brain cannot think upon one thought. Often the brain loses its groove due to the splinters of life. When a splinter breaks up the thoughts in the brain, there is a flood of thoughts that causes one to feel overwhelmed, confused and disoriented.