Itty Bitty Muscles^[1]

by
James Rooney, D.V.M.

Being an anatomist at heart I became interested in small muscles of the fore and hind legs: the mm. interossei and the mm. lumbricales. Actually, I was looking into the innervation of the musculature of the m. interosseus, the misnamed: suspensory ligament. Doing so I paid attention to the small interosseous muscles which lie on either side of the proximal end of the suspensory. That twigged me to the lumbricales muscles farther down the leg on the medial and lateral sides. The lumbricales are marginally larger in the hind legs than in the fore.

The usual statement in the anatomy texts is that these are rudimentary muscles with no mechanical function. It is certainly the case that the muscles are small and have no direct mechanical function. That they are rudimentary or vestigial and have absolutely no function may, however, be questioned and, of course, that is precisely what I intend to do! One may think, for example, of the human appendix, long thought to be rudimentary until investigation revealed its immunological importance.

While one tends to think of these small muscles as underdeveloped, it seems equally plausible that they are just the size they ought to be if the horse were still the size of Hyracotherium, 50 million years ago. That is to say that while the cat-size Hyracotherium became larger and larger over the centuries, the lumbricales and interossei simply remained the original size. The digits with which they were associated disappeared, but the muscles remained as they were.

Topography

The mm. lumbricales are paired, thin, slender muscles about 3-4 cm. long which lie in the deep fascia between the superficial and deep flexor tendons about 3-5 cm. proximal to the fetlock joint. Their tendons attach to the deep fascia in the general area of the proximal sesamoid bones, Figure 1.

The mm. interossei are paired, slender muscles about 3 cm. long lying on either side of the m. interosseus between the interosseus and the splint bones. Their long, slender tendons insert on an indistinct condensation of deep fascia between the distal end of the respective splint bones and the abaxial aspects of the proximal sesamoid bones, Figure 1.

Figure 1

Figure 1: Lateral view of fetlock region. Modified after Ellenberger and Baum 1927.

Innervation

The mm. interossei receive nerve branches from the branch of the metacarpal/metatarsal nerve which also innervates the m. interosseus. As will be noted below the branches to the lumbricalis and interossei contain both sensory and motor components. In the foreleg, the nerve branch to the mm. interossei and the m. interosseus arises from the ulnar nerve component of the lateral palmar nerve. These muscles in the hind leg are innervated by a deep branch arising from the lateral plantar nerve.

Gross dissection of the lumbricalis of the foreleg with the assistance of basic fuchsin staining (Coleman’s Schiff reagent) showed the nerve pattern sketched in Figure 2. The proximal branch of the nerve connected with the abundant motor end plates; the middle branch connected to the muscle spindles concentrated along the equator of the muscle; and the distal branch was connected to the tendon end organs at the musculotendinous junction.

Figure 2

Figure 2: Schematic of lumbricales muscle. The red nerve fibers are motor and the blue sensory. The same basic pattern of innervation was found for the interossei.

So far… facts; now the speculation. I have no way to follow up these ideas, but perhaps someone looking for an interesting dissertation in physiology might be interested.

These small muscles are richly innervated, suggesting that they are active. Can one speculate that they are acting as peripheral proprioceptive sensors? With the evolution to monodactyly the muscular bodies of the forearm and gaskin are a long way from the insertions of their tendons. Perhaps the small muscles are better positioned to monitor proprioceptive data arising below the carpus and tarsus. No amount of anatomical speculation can answer that question. There is no help from pathology either since I have not seen or seen reports of damage to the small muscles with subsequent gait abnormality. Anesthesia and or extirpation of the lumbricales with sophisticated monitoring of gait on a treadmill offers one approach. Specific anesthesia or extirpation of the interossei would be quite difficult though the tendons could be transected near the fetlock.

Perhaps clinicians/practitioners who have used local anesthesia for help in diagnosing problems of the proximal end of the interosseus might have some salient observations. I am not sanguine, however, that any of those esteemed individuals will have read this far!

^[1] I give fair warning that this essay will be of no practical interest to most of you. It is a rumination for possible consideration by anatomists and scholarly students of horse locomotion.

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