I’m slowing down, friends, rolling toward a stop for this column. The best way to describe my reason is through an analogy: I want you to think of me as a horse-drawn wagon and driver — not just as the driver, but as the whole shebang.

A long time ago, the whole shebang worked great. The horse was never handsome, but he was sturdy, healthy, had a good gait and wind, and could pull most loads asked of him. These days the poor beast is sway-backed, spavined, short of breath. He is, in sum, “knackered,” as the Brits say: ready for the glue factory.

And the wagon, which in its day was solid and well maintained (axles greased, splits repaired), is now all creaks and squeaks; the wheels spin in erratic circles, making the rig judder, wobble and throw the sore-footed horse out of step and into stumbling.

And the rig’s driver, up on the box? His reflexes have slowed down, his judgment weakened, his perceptions turned untrustworthy. That all means that the poor horse no longer feels a sense of command and direction flowing through the reins. As if things weren’t bad enough for it!

Not a good situation for the whole shebang, as you can see. And so it’s time for the driver, with what prudence is left to him, to slow down. He needs slowly to rein in the footsore horse and to press down on the wheel brake.

In the old days, a mechanical wheel brake was essential to any horse-drawn rig, from fancy coach to farm buckboard. It could spare a team from injury or death if, going down a steep hill, the wheeled burden behind them got rolling faster than they could go and literally overran them, knocking them down. Horrific injuries would follow, with the team and often the wagon’s riders mangled and killed.

That brake was a safety mechanism. When the driver pressed down hard on it, curved lengths of wood pressed just as hard against the turning wheels to quench increasing speed. And, friends, just now I need to press down on my own brake, slowing down my own horse and wagon.

In fact, my whole rig was doing just fine, well into my 60s. Then I took on a passenger. I didn’t know his name as he climbed into the back, and I was a long time finding it out. Turned out to be Parkinson.

He’s been rough company. In short order, the horse was in serious trouble, and the wagon was soon faring no better. At the same time the driver began to feel dazed, unsteady, ready to fall right off the seat. He had to struggle to hold his balance, and he’d get worn out, just trying to keep eyes fixed on the road ahead. Just driving the rig was wearing him out.

And worse: The first thing that Parkinson monkeyed with was the wagon’s brake.

OK, I’ve worn out the analogy, and maybe your patience. So, for a minute, here’s the straight skinny: Parkinson wouldn’t be aboard, wreaking havoc with the whole shebang, if my brain hadn’t diminished production of dopamine, a substance that’s a natural governor for the central nervous system, controlling bodily and mental stop-and-start, fast-and-slow in a myriad of operations.

Long before I knew Parkinson’s was with me in the wagon, my dopamine had been diminishing, for perhaps seven or eight years. But my brain, bless it, had been compensating; and it wasn’t until 2007 that suddenly shaky hands made me realized someone had climbed aboard my wagon and, in fact, was trying to take control away from me.

I don’t know how Parkinson managed to sabotage horse and wagon, but his work on the wheel brake was his worst effect. And, as I suggested, it’s not just the horse and wagon that are affected.

The bloke’s at work in my head, too:

For instance, the rational part of me says, “Jim, all things considered, (including those books you’re trying to finish), it’s time to stop the column.” But, of course, emotion in me says, “What? I’m past nine hundred columns now! Why not rein in at a thousand?”

“Because,” says reason, “you’re running out of energy and maybe time, to finish those books, buddy. You’re already like an old battery that won’t hold a charge.”

Well, I hate admitting it, but reason is right. And that’s why (shifting back to the analogy) I’m gently saying, “Whoa,” gently tugging on the reins, and gently depressing the brake.

We’re coming to a stop, friends. Right here, with this column, at the end of an 18th year of my writing to you.

But don’t think that I’m planning to clip-clop off, into the sunset. Crier Editor Jim Austin, my friend of so many years, has invited me to drop by a column every so often, just to keep in contact with you. And I will. After 18 years of talking with you, going cold turkey would bring withdrawal symptoms.

Thanks, friends, for giving me opportunity to become much more skilled at my craft. You’ve been such a gift to me!