Stay apparatus

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A draft horse sleeping while standing up Cheval ardennais expo.jpg
A draft horse sleeping while standing up

The stay apparatus is an arrangement of muscles, tendons, and ligaments that work together so that an animal can remain standing with virtually no muscular effort. [1] It is best known as the mechanism by which horses can enter a light sleep while still standing up. [2] The effect is that an animal can distribute its weight on three limbs while resting a fourth in a flexed, non-weight-bearing position. The animal can periodically shift its weight to rest a different leg, and thus all limbs are able to be individually rested, reducing overall wear and tear. The relatively slim legs of certain large mammals, such as horses and cows, would be subject to dangerous levels of fatigue if not for the stay apparatus. [3]

The lower part of the stay apparatus consists of the suspensory apparatus, which is the same in both front and hind legs, while the upper portion of the stay apparatus is different between the fore and hind limbs. [4]

In the front legs, the stay apparatus engages when the animal's muscles relax. [2] The upper portion of the stay apparatus in the forelimbs includes the major attachment, extensor, and flexor muscles and tendons. [4] In essence, the accessory check ligaments act as tension bands, they stabilize the carpus (called the "knee" in horses), fetlock and bones of the foot. In the upper portion, the shoulder and elbow joints have several musculo-tendinous structures that keep these joints in passive extension. [1]

In the hind limbs, the major muscles, ligaments and tendons work with the reciprocal joints of the hock and stifle, [4] which are a reciprocal apparatus that forces the hock and stifle to flex and extend in unison. The medial patellar ligament "locks" the patella ("kneecap") in place and this prevents flexion in both the stifle and the hock. [1] At the stifle joint, a "hook" structure on the inside bottom end of the femur cups the patella and the medial patella ligament, prevents the leg from bending. [5]

Cattle have a stay apparatus which allows them to rest individual limbs, [3] but cattle generally do not sleep standing up. [6]

The ancient Dinohippus had a rudimentary stay apparatus Dinohippus leidyanus foot bones.JPG
The ancient Dinohippus had a rudimentary stay apparatus

Anatomical structures important in the stay apparatus include:

The most common of the ancient, now-extinct wild horse species in North America, Dinohippus , had a distinctive passive stay apparatus that helped it conserve energy while standing for long periods. Dinohippus was the first horse to show a rudimentary form of this characteristic, and its existence provided additional evidence of the close relationship between Dinohippus and the modern Equus. [8]

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<span class="mw-page-title-main">Ulna</span> Medial bone from forearm

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<span class="mw-page-title-main">Humerus</span> Long bone of the upper arm

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The stifle joint is a complex joint in the hind limbs of quadruped mammals such as the sheep, horse or dog. It is the equivalent of the human knee and is often the largest synovial joint in the animal's body. The stifle joint joins three bones: the femur, patella, and tibia. The joint consists of three smaller ones: the femoropatellar joint, medial femorotibial joint, and lateral femorotibial joint.

<span class="mw-page-title-main">Fascial compartments of arm</span> Anatomical compartments

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<span class="mw-page-title-main">Elbow</span> Joint between the upper and lower parts of the arm

The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa, and the lateral and the medial epicondyles of the humerus. The elbow joint is a hinge joint between the arm and the forearm; more specifically between the humerus in the upper arm and the radius and ulna in the forearm which allows the forearm and hand to be moved towards and away from the body. The term elbow is specifically used for humans and other primates, and in other vertebrates it is not used. In those cases, forelimb plus joint is used.

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The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons, and ligaments that support the weight of the equine body. They include two apparatuses: the suspensory apparatus, which carries much of the weight, prevents overextension of the joint and absorbs shock, and the stay apparatus, which locks major joints in the limbs, allowing horses to remain standing while relaxed or asleep. The limbs play a major part in the movement of the horse, with the legs performing the functions of absorbing impact, bearing weight, and providing thrust. In general, the majority of the weight is borne by the front legs, while the rear legs provide propulsion. The hooves are also important structures, providing support, traction and shock absorption, and containing structures that provide blood flow through the lower leg. As the horse developed as a cursorial animal, with a primary defense mechanism of running over hard ground, its legs evolved to the long, sturdy, light-weight, one-toed form seen today.

References

  1. 1 2 3 4 Ferraro, Gregory L.; Stover, Susan M.; Whitcomb, Mary Beth. "Suspensory Ligament Injuries in Horses" (PDF). Center for Equine Health. School of Veterinary Medicine, University of California, Davis. Archived from the original (PDF) on February 18, 2015. Retrieved 19 May 2016.
  2. 1 2 Pascoe, Elaine. "How Horses Sleep". Equisearch.com. Archived from the original on 2007-09-27. Retrieved 2007-03-23.
  3. 1 2 3 Asprea, Lori; Sturtz, Robin (2012). Anatomy and physiology for veterinary technicians and nurses a clinical approach. Chichester: Iowa State University Pre. pp. 109–111. ISBN   9781118405840.
  4. 1 2 3 Harris, p. 253
  5. "Horseware Ireland North America - The worlds leading equine product leader for horse and rider".
  6. "Animal MythBusters | Manitoba Veterinary Medical Association". www.mvma.ca. Archived from the original on 2014-01-25.
  7. Watson, JC; Wilson, AM (January 2007). "Muscle architecture of biceps brachii, triceps brachii and supraspinatus in the horse". J. Anat. 210 (1): 32–40. doi:10.1111/j.1469-7580.2006.00669.x. PMC   2100266 . PMID   17229281.
  8. Florida Museum of Natural History