There is a tiny little muscle under the neck. It doesn’t get mentioned in many anatomy charts and is mostly ignored. It’s those little details that often prove to be important.

Called the cutaneous coli it is a ‘skin’ muscle. Cutaneous means skin, coli gives the location. I guess These skin muscles are held within the fascia directly under the skin and their main purpose is to move the skin to get rid of flies. I’m not sure I’ve seen horses twitch the skin under their necks though. Or under the belly where another of these mostly ignored cutaneous muscles lay.

Not seemingly useful in the chasing of flies this skin muscle under the neck does assist the thoracic sling when needed. Which is where whorls come in. The cutaneous coli starts with thick V shaped muscling at the base of the neck that is connected to other muscles, not held within the fascia, then extends upwards much thinner in more typical skin muscle form around the base of the neck where it blends with the other neck muscles.

In theory it is all very simple, in reality the inside shape and function of every horse is very different. It is perfectly possible that the different shapes this muscle comes in are related to the different whorls that develop over the top of it. When the whorls trail off to one side is the coli muscle running off in one direction? When there is large open feathering, is the coli muscle weaker there and unable to assist as strongly in supporting the thoracic sling? In which case the opposite would be true and a tight whorl, a zipper, would show a very strong coli muscle able to assist powerfully in supporting the thoracic sling.

This doesn’t mean that the large muscle groups underneath the skin muscles aren’t involved. Whorls seem to show what is going on in the fascia. The fascia holds the skin muscles within, but it also wraps around the other muscles in the area. Nothing works alone.

In this picture we can see how the large open whorl lies well within the area that can be covered by the cutaneous coli.