Considering the size of a horse’s head, you’d think that it’s brain would be pretty large. And the brain in total is a reasonable size. Picture a small grapefruit. The problem is that the thinking part, the cerebrum, is only slightly larger than a walnut or slightly smaller than a lemon. The rest is cerebellum, the part of the brain that controls gross muscle coordination, balance and body functions.
“Medical folks actually talk about the horse brain as having three sections:
1. The Hind Brain, which allows the horse to coordinate its balance and movements without having to “think” about them. Otherwise, a horse might constantly be tripping over its own feet.
2. The Mid Brain, which consists of a variety of nerve bundles that busily convey signals to and from the cerebellum.
3. The Forebrain — which contains the part that I’m talking about when I use the term “brain.” Perhaps “gray matter” would be a more helpful term, although there are bits and pieces of gray matter scattered elsewhere which aren’t connected with a horse’s thought patterns. (In the photo, I’ve faded all but the cerebellum so it will be easier to see.)
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 cerebellum 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 cerebellum into the space in the middle, and you’ll see how it comes closer to the size of the walnut I’m holding.”
Interestingly, the modern horse has a much smaller brain than its ancestors. It is apparently a common result of domestication: domestic mammals have brains that are 10-30% smaller than their wild ancestors. Horse’s brains have shrunk approximately 16%.