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Brain the size of a large walnut?!

At the local Horse Expo, there was a cut-away horse skull. About ten walnuts would've filled the cavity, and that apparently was the cerebral part -- the cerebellum was not exposed to view. I'd say it's more the size of a good grapefruit.

J. T.


A miniature horse standing next to a large draft horse in 1909

I guess this is what I get for trying to over-simplify a rather complex subject! Let me try to clarify things a bit....

In the first place, horse brains come in a variety of sizes; from new-born to adult, from miniature breeds to huge draft horses.

You can see some of that range of size in this 1909 photo of Baby Ruth, a small horse owned by C. W. Gillet, standing in front of Pink Parader, a large draft horse owned by Joseph Medill Patterson. (Photo: DN-0007577, Chicago Daily News negatives collection, Chicago Historical Society.)

The diameter of a horse's brain ranges from a little over an inch to nearly four inches in some breeds. An average saddle horse has a brain diameter of about three inches at full maturity. From birth to adulthood, I'm guessing that the overall average is around two to two-and-a-half inches. In the photo below, I've inserted a full-sized photo of a horse's brain (sectioned in half) into the actual skull of a typical adult horse. (The horse skull is courtesy of my good friend, Howard Pitzen, of the North Star Stampede Rodeo in Effie, Minnesota.) The brain in this case is a bit larger than the overall average.

Photo of a horse skull showing the relative sizes of the cerebellum and a large walnut Second, the brain cavity of a horse is filled with a lot more than what we usually think of as the "brain." Although the space would, in fact, hold a small grapefruit, the cerebral hemisphere -- or "thinking" portion of the brain cavity is a lot smaller. Medical folks actually talk about the horse brain as having three sections:

In addition to those main parts, the brain cavity holds several other items, including the pituitary and pineal glands, the auditory and olfactory bulbs, and a bunch of fluid and other material that acts as a sort of shock absorber for the brain. In short, the "thinking" portion of brain is a whole lot smaller than the brain cavity.

Finally, the cerebral hemisphere is spread out over the mid brain, so you need to kind of "roll it up" if you want to compare it to a spherical object like a walnut. In looking at the photo, try to mentally wrap the right one-third of the cerebral hemisphere into the space in the middle, and you'll see how it comes closer to the size of the walnut I'm holding.

While a horse's brain may actually be slightly larger than a large walnut, I also like that comparison because the convoluted surface of a walnut mimics the brain's convolutions.

So, a walnut may not be a perfect comparison, but a tennis ball -- let alone a grapefruit -- is much too large, a golf ball is too small, and I can't think of any other common object that is as close in size.

Cowboy Bob does his version of Hamlet -- with a horse skull!

"Alas, poor Yorick! I knew him, Horatio: a fellow of infinite jest, of most excellent fancy...."

(Cowboy Bob does his version of Hamlet -- with a horse skull!) (A special thanks to Jay Johnson for his help in cutting the skull!)

Cowboy Bob holds a walnut next to Thumbelina's head

In the case of Thumbelina -- at one time The World's Smallest Horse -- a large walnut is significantly larger than her brain!

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