‘A fly went by’ is the title of a children’s book by Mike McClintock that I love to read. If you haven’t, Google it! It is funny and it inspired me to write about this fascinating but often overlooked muscle group.

We have all seen horses twitch their skin to get rid of an annoying fly or other insect. They do this with small, quick contractions of the skin, which seem to happen without the horse paying much attention.

But have you noticed the horse can only twitch the skin in some parts of the body and not others?

This is because the twitch is elicited by the panniculus reflex activating a thin layer of muscles that lie just under certain areas of the horse’s skin.

This action, which is triggered without conscious thought by the sensation of the fly landing, is well known and has been described in the literature as the Cutaneus Trunci Muscle (CTM) reflex.

In veterinary medicine, the CTM reflex is used for diagnosing spinal cord and/or neurologic lesions. The reflex can easily be elicited by moving the fingers lateral from the back spine from the iliac crests cranially (towards the head), until a contraction occurs.

But let’s go back to the group of muscles that help your horse get rid of pesky insects.

Budras and his colleagues, as well as Sisson and Grossman have described several muscles of the horse’s skin, including the m. cutaneus omobrachialis, m. cutaneus trunci, m. cutaneus fasciae, and m. cutaneus colli. (Note that m. stands for “musculus” or muscle).

Illustration adapted by Horses and People from an original illustration by Denoix & Pailloux.


Of these cutaneus muscles, colli and fasciae are the smallest. M. cutaneus colli covers the caudoventral part of the neck, starting from the manubrium sterni (the top end of the sternum) and the cervical fascia and ascends in a V-shape on either side (see Figure below). It attaches partly on the m. cleidomastoideus.

The two largest of these muscles are 1, m. cutaneus omobrachialis which covers the shoulder and arms and merges caudally with 2. the m. cutaneus trunci, which covers the abdominal muscles and extends from the area of the olecranon to the withers and caudally to the level of the thigh. The muscle then continues in a large fascia ventral-caudal (see illustration above, adapted by Horses and People from an original illustration by Denoix & Pailloux). The superficial trunk fascia encloses m. cutaneous trunci.

Edge of the m. cutaenous omobrachialis

On some horses you can see the edge of the m. cutaenous omobrachialis and m. cutaneous trunk. Image courtesy of Parnilla Andreazon.

Sometimes the edges of these muscles are visible as a line underneath the skin.

These lines differ from horse to horse, just as our fingerprints are unique to us, and they are not always visible. In any case, you don’t have to worry if you detect them, this is not an indication of inflammation or other problems.

Pärnilla Andreazon kindly shared an image that shows beautifully the m. cutaneous omobrachialis and part of the m. cutaneous trunci.

So there you have it, a fly landing on any of the cutaneus muscles triggers the rapid skin contractions that cause the insect to fly away.

The next question is… how does the horse get rid of flies when they are out of range for these muscles?

In this case, the horse will use his tail, bite or kick at them, or, if the horse has one or two good friends, they will arrange themselves head to tail, so that one’s tail flicks the flies of the other one’s face.

And that is just another of the many reasons why horses need to have friends!

This article is ©Copyright by Hands for Horses Sweden, Dr. Karen Wild and is reproduced here with permission. Sharing excerpts that link to the full article on this platform is welcome but please respect the author’s copyright.