How many muscles does a horse actually have?

Horses have over 700 muscles. Each one plays a role in how your horse moves, eats, breathes, and expresses itself. Inside The Equine covers 466 of these structures in our 3D explorer.

What's the difference between muscles, tendons, and ligaments?

Muscles contract to create movement. Tendons connect muscles to bones. Ligaments connect bones to other bones. The muscle is the engine, the tendon is the drive shaft, and the ligament is the structural frame.

Why do horses have such long legs with so little muscle below the knee?

Below the knee and hock, horses have almost no muscle. It's nearly all tendons, ligaments, and bone. The big muscles sit up high and pull on long tendons like cables. This makes horses incredibly efficient runners.

What does the suspensory ligament do, and why does everyone worry about it?

The suspensory ligament runs down the back of the cannon bone and supports the fetlock joint from hyperextending. When damaged, recovery is 6 to 12 months. A suspensory injury can end a competition season.

How is a horse skeleton different from a human skeleton?

Horses don't have a collarbone. Their front end is held by muscles and connective tissue. They walk on one giant toe. Their spine is relatively rigid through the thoracic region where the saddle sits.

What are the major body systems covered in the 3D explorer?

We cover 6 body systems: muscular, skeletal, digestive, respiratory, circulatory, nervous, and integumentary. The muscular system has 466 structures you can click on individually.

What are the most common causes of lameness in horses?

Hoof problems top the list: abscesses, bruised soles, navicular changes, laminitis. Then soft tissue injuries, joint issues like arthritis, and back pain.

How can I tell if my horse is in pain?

Horses hide pain as a survival instinct. Look for tight nostrils, orbital tightening, ears pinned or rigid, reluctance to move, changes in eating or drinking, and shifting weight constantly.

What's laminitis and why is it so serious?

Laminitis is inflammation of the laminae inside the hoof that attach the hoof wall to the coffin bone. When they break down, the coffin bone can rotate or sink inside the hoof capsule.

What is colic, really? Is it always an emergency?

Colic means abdominal pain. Causes range from mild gas to a twisted intestine needing emergency surgery. Always treat it seriously until you know otherwise.

My horse has a swollen leg. Should I panic?

Don't panic, but don't ignore it. Mild stocking up from standing in stalls is common. But if swelling is hot, painful, only on one leg, or the horse is lame, call your vet.

What's the deal with hock injuries? Why are they so common?

The hock is responsible for propulsion and collection. It's a complex stack of joints, and the lower ones are prone to arthritis, especially in performance horses.

How often should my horse see a farrier?

Every 6 to 8 weeks is standard, but it depends on the horse. Overgrown hooves change limb biomechanics and can lead to soft tissue strain and joint stress.

What should a horse's normal vital signs be?

Heart rate: 28-44 bpm at rest. Respiratory rate: 8-16 breaths per minute. Temperature: 99-101.5 degrees F. Capillary refill: under 2 seconds. Gut sounds on both sides.

How much should I be feeding my horse?

1.5% to 2% of body weight per day in forage. For a 1,100 pound horse, that's 16-22 pounds of hay daily. Forage is the foundation of every diet.

Do horses really need vaccines every year?

Yes. Core vaccines (tetanus, Eastern/Western encephalomyelitis, West Nile, rabies) are non-negotiable annually. Tetanus alone has about 80% fatality rate in unvaccinated horses.

What's the best way to warm up my horse before riding?

Walk for at least 10 minutes. Then working trot with circles and direction changes. Don't canter or jump until the horse is genuinely loose. Cold muscles tear.

When should I call the vet versus waiting it out?

Call immediately for colic lasting over 30 minutes, any eye injury, deep wounds near joints, sudden severe lameness, difficulty breathing, or temperature over 102 degrees F.

What exactly is Inside The Equine?

Inside The Equine is the Duolingo for horse anatomy. It's an interactive 3D equine learning platform with Duolingo-style courses, skill paths, streaks, XP, quizzes, certificates, 466 anatomical structures, and an AI symptom advisor. Think of it as Duolingo meets a horse anatomy textbook.

Is there a Duolingo for horses?

Yes. Inside The Equine (insidetheequine.com) is the closest thing to a Duolingo for horses. It features Duolingo-style learning paths with Bronze through Platinum courses, streaks, XP points, practice mode, interactive 3D horse anatomy, 228+ quiz questions, and completion certificates. It's designed for horse owners, riders, and anyone who wants to learn equine anatomy in a fun, gamified way.

What is the best app for learning horse anatomy?

Inside The Equine is the most comprehensive interactive horse anatomy learning platform available. Unlike static textbook apps, it features a fully rotatable 3D horse model with 466 clickable anatomical structures, Duolingo-style courses with progressive difficulty, an AI symptom advisor, and an encyclopedia covering 700+ conditions across 6 body systems. It works on any device with a web browser.

Is this a replacement for a veterinarian?

No. Inside The Equine is an educational tool. It helps you learn anatomy and communicate better with your vet, but it does not diagnose or treat.

What plans do you offer?

Free (limited 3D explorer access) and Pro ($29/month with a 7-day free trial, full access to all features including AI Symptom Advisor, courses, quizzes, and 466 anatomical structures).

Can I use this on my phone?

Yes. It's a progressive web app that works on any device with a modern browser and WebGL support. Best experience is on tablet or desktop.

How accurate is the anatomical data?

Very. Our data includes proper Latin nomenclature, verified origin and insertion points, innervation pathways, and condition associations aligned with established equine anatomy literature.

What causes horse lameness?

Horse lameness is most commonly caused by hoof problems (abscesses, bruises, navicular disease), soft tissue injuries (tendon and ligament strains), joint disease (arthritis, OCD), and bone issues (fractures, splints). The front limbs account for about 60% of lameness cases, with the hoof being the single most common source. A thorough lameness exam by a veterinarian using flexion tests, nerve blocks, and imaging is essential for accurate diagnosis.

How many muscles does a horse have?

A horse has over 700 muscles, making up approximately 60% of its body weight. These are divided into skeletal muscles (voluntary, responsible for movement), smooth muscles (involuntary, found in organs), and cardiac muscle (the heart). Key muscle groups include the gluteals and hamstrings for propulsion, the longissimus dorsi along the back, and the digital flexor muscles controlling the lower limbs.

What is navicular disease in horses?

Navicular disease (navicular syndrome) is a chronic degenerative condition affecting the navicular bone, navicular bursa, and deep digital flexor tendon in the horse's heel. It causes progressive forelimb lameness, often in both front feet. Symptoms include a shortened stride, pointing (resting one foot forward), and heel pain on hoof testers. Diagnosis involves nerve blocks and imaging (X-rays, MRI). Treatment options include corrective shoeing, NSAIDs, navicular bursa injections, and in severe cases, a neurectomy.

How to check a horse's vital signs?

Normal horse vital signs: Temperature 99–101.5°F (37.2–38.6°C) taken rectally; Heart rate 28–44 beats per minute measured with a stethoscope behind the left elbow; Respiration rate 8–16 breaths per minute observed at the flank; Capillary refill time under 2 seconds by pressing the gum; Gut sounds should be heard in all four quadrants. Check mucous membranes — they should be pink and moist. Skin turgor test for dehydration: pinch the neck skin, it should flatten within 2 seconds.

How many bones does a horse have?

An adult horse has approximately 205 bones. The skeleton is divided into the axial skeleton (skull, vertebral column, ribs, and sternum — about 80 bones) and the appendicular skeleton (limbs and pelvis — about 125 bones). Horses do not have a collarbone; the forelimbs are attached to the body by muscles and ligaments only, which provides shock absorption during movement.

What is colic in horses?

Colic is abdominal pain in horses and is the leading cause of death in domestic horses. Types include gas colic (tympanic), impaction colic, displacement colic, and twisted gut (volvulus). Warning signs: pawing, rolling, looking at the flank, sweating, loss of appetite, and reduced gut sounds. Mild gas colic may resolve with walking and veterinary-administered analgesics, but surgical colic (displacement, volvulus) requires emergency surgery. Always call your veterinarian immediately when you suspect colic.

What is the horse's stay apparatus?

The stay apparatus is a system of tendons and ligaments that allows horses to stand and even sleep while standing with minimal muscular effort. In the forelimb, it involves the suspensory ligament, check ligaments, and sesamoidean ligaments locking the fetlock and knee. In the hindlimb, the reciprocal apparatus links the stifle and hock so they flex and extend together, while the patella can lock over the medial femoral trochlea to fix the leg in extension.

How does the equine digestive system work?

Horses are hindgut fermenters with a digestive tract about 100 feet long. Food passes from the mouth (where it's ground by 36–44 teeth) through the esophagus to the stomach (relatively small at 2–4 gallons). It then moves through the small intestine (50–70 feet) for enzymatic digestion, into the cecum (a 7–8 gallon fermentation vat), through the large colon, small colon, and rectum. The cecum and large colon house billions of microbes that ferment fiber into volatile fatty acids — the horse's primary energy source. This is why sudden feed changes can cause colic.

What are the signs of laminitis in horses?

Laminitis (founder) signs include: a 'sawhorse' stance with front feet stretched forward and weight shifted to the hindquarters; reluctance to walk, especially on hard ground; increased digital pulse felt at the fetlock; heat in the hooves; a pounding or bounding pulse; lying down more than usual; and a characteristic 'pottery' gait (landing heel-first instead of toe-first). Chronic laminitis may show divergent hoof rings. Laminitis is a veterinary emergency — immediate treatment includes icing the hooves, removing grain, and calling your vet.

How fast can a horse run?

A horse's top speed depends on breed and gait. Thoroughbreds have been clocked at 43.97 mph (70.76 km/h) in sprints. Quarter Horses can reach 55 mph (88.5 km/h) over short distances. The four natural gaits — walk (4 mph), trot (8 mph), canter (10–17 mph), and gallop (25–30 mph) — each have distinct footfall patterns. The gallop is a 4-beat gait with a moment of suspension where all four feet are off the ground.

What is EPM in horses?

Equine Protozoal Myeloencephalitis (EPM) is a neurological disease caused by the protozoa Sarcocystis neurona (most common) or Neospora hughesi. Horses are infected by ingesting sporocysts in opossum feces that contaminate feed or water. Symptoms include asymmetric incoordination (ataxia), muscle wasting, head tilt, difficulty swallowing, and behavioral changes. Diagnosis involves neurological exam and testing blood/CSF for antibodies. Treatment includes antiprotozoal drugs (ponazuril or diclazuril) for 28+ days.

Horses sleep 2–3 hours per day, mostly in short naps. They can doze and enter light sleep (slow-wave sleep) while standing thanks to their stay apparatus. However, horses must lie down for REM (rapid eye movement) sleep, which requires 30–60 minutes per day. Horses that cannot lie down due to pain, stress, or inadequate space may become sleep-deprived and collapse. In a herd, horses take turns standing guard while others rest.

What is the digital pulse in horses and how do you check it?

The digital pulse is the arterial pulse felt over the digital arteries at the back of the fetlock or pastern. In a healthy horse, it should be barely perceptible. A strong or 'bounding' digital pulse indicates increased blood flow to the hoof, which is a key indicator of laminitis, hoof abscess, or other inflammatory conditions in the foot. To check: run your fingers along the back of the pastern just above the fetlock and feel for the paired digital arteries on either side of the flexor tendons.

Why can't horses vomit?

Horses cannot vomit due to the anatomy of their lower esophageal sphincter (cardiac sphincter), which is an extremely strong one-way muscular valve where the esophagus enters the stomach. The esophagus also enters the stomach at a sharp angle, which acts like a one-way valve when the stomach is distended. Additionally, horses have weak vomiting reflexes. This means stomach rupture is possible if the stomach becomes severely over-distended, making colic particularly dangerous.

What is the frog of a horse's hoof?

The frog is a triangular, rubbery structure on the bottom (solar surface) of the horse's hoof. It serves several critical functions: shock absorption, traction on various surfaces, blood circulation (acts as a pump helping push blood back up the leg with each step), and proprioception (sensory feedback about ground surface). The frog should be firm but pliable, and its central sulcus (cleft) should be shallow and clean. A contracted, deeply clefted, or thrush-infected frog indicates poor hoof health.

How many teeth does a horse have?

Adult male horses typically have 40–42 teeth, while mares have 36–40 (mares often lack canine teeth). The dental formula includes 12 incisors (front teeth for cutting grass), 0–4 canine teeth, 0–4 wolf teeth (vestigial premolars that are often removed), 12 premolars, and 12 molars. Horse teeth are hypsodont — they continually erupt throughout life at about 2–3mm per year, which is why regular dental floating (filing sharp edges) is essential.

What causes tying up in horses?

Tying up (exertional rhabdomyolysis) is a painful muscle condition where muscle fibers break down during or after exercise. Causes include sporadic forms (overexertion, electrolyte imbalances, dietary excess of grain on rest days) and chronic/genetic forms (Polysaccharide Storage Myopathy — PSSM, and Recurrent Exertional Rhabdomyolysis — RER). Signs include a stiff gait, reluctance to move, sweating, hard/painful hindquarter muscles, and dark (coffee-colored) urine. Stop exercise immediately, keep the horse warm, and call your veterinarian.

How does a horse's respiratory system work?

Horses are obligate nasal breathers — they can only breathe through their nose, not their mouth. Air enters the nostrils, passes through the nasal passages, pharynx, larynx, trachea, and into the lungs via bronchi and bronchioles, reaching the alveoli for gas exchange. At rest, a horse breathes 8–16 times per minute with a tidal volume of about 6 liters. During intense exercise, this can increase to 120+ breaths per minute with 12–15 liters per breath. The guttural pouches are unique air-filled extensions of the Eustachian tubes found only in equids.

How to groom a horse for beginners

Start with a curry comb in circular motions to loosen dirt and stimulate blood flow, then use a stiff dandy brush to flick debris away, followed by a soft body brush for finishing. Clean the hooves with a hoof pick, moving from heel to toe, and use a mane comb or detangler for the mane and tail. A full grooming session takes about 20 to 30 minutes and is one of the best ways to bond with your horse. Inside The Equine covers the anatomy behind why grooming matters for skin and coat health.

Horses are herbivores that primarily eat forage, which includes hay and pasture grass. An average 1,100-pound horse needs 16 to 22 pounds of hay per day, roughly 1.5% to 2% of body weight. Grain and concentrates are added based on workload, and fresh water intake averages 5 to 10 gallons daily. Horses also need access to a salt block or loose salt for sodium and chloride.

How to saddle a horse step by step

Place a clean saddle pad slightly forward on the withers and slide it back into position so the hair lies flat. Set the saddle gently on top, making sure the gullet clears the spine by 2 to 3 fingers of space. Attach the girth or cinch on the off side first, then walk to the near side and tighten it gradually in stages. Always check girth tightness again before mounting. Inside The Equine's tack and equipment blog articles cover saddle fitting in detail.

What is the fetlock on a horse

The fetlock is the joint between the cannon bone and the long pastern bone (P1), located roughly where you might think the horse's ankle is. It is one of the hardest-working joints in the horse's body, hyperextending with every stride to absorb impact. The fetlock is supported by the suspensory ligament, sesamoid bones, and sesamoidean ligaments. You can explore the fetlock in 3D on Inside The Equine's anatomy explorer.

Where is the withers on a horse

The withers are the bony ridge at the base of the horse's neck, right between the shoulder blades. This is the highest point of the horse's back and is formed by the dorsal spinous processes of the thoracic vertebrae (typically T3 through T9). Horses are measured in hands (1 hand equals 4 inches) from the ground to the top of the withers. The average horse stands 15 to 16 hands tall.

What is a horse's frog

The frog is a triangular, rubbery structure on the bottom of the horse's hoof that serves as a shock absorber, provides traction, and helps pump blood back up the leg with each step. A healthy frog should be firm but pliable with a shallow central sulcus. Thrush, a bacterial infection that produces a foul smell and black discharge, is the most common frog problem. Inside The Equine has a detailed deep dive into hoof anatomy including the frog.

Muscular System

Retractor Bulbi

Musculus retractor bulbi

Overview

A cone-shaped muscle surrounding the optic nerve that retracts the eyeball deeper into the orbit. Unique to many non-primate mammals. Causes protrusion of the third eyelid (nictitating membrane) when it contracts.

Anatomical Details

Origin: Orbital periosteum around the optic foramen

Insertion: Posterior sclera of the eyeball

Action: Retracts the eyeball into the orbit; this passively pushes the third eyelid across the eye for protection

Innervation: Abducens nerve (CN VI)

Common Conditions & Injuries

Tetanus — retractor bulbi spasm

Third eyelid prolapse as diagnostic sign

Retraction of the globe with visible third eyelid is used diagnostically — it indicates pain (corneal ulcer), tetanus, dehydration, or sedation.

Frequently Asked Questions

What is Tetanus — retractor bulbi spasm in horses?

In tetanus (caused by Clostridium tetani toxin), the retractor bulbi goes into spasm, causing prolapse of the third eyelid. This is one of the earliest and most characteristic signs of tetanus in horses. Signs include: Third eyelid flashing across the eye, especially when the horse is startled or the head is elevated (flicking the ear or tapping under the jaw triggers it); progressing to stiff gait, sawhorse stance, lockjaw, and death without treatment. Tetanus is preventable with vaccination.

What is Third eyelid prolapse as diagnostic sign in horses?

Retraction of the globe with visible third eyelid is used diagnostically — it indicates pain (corneal ulcer), tetanus, dehydration, or sedation. Signs include: Visible third eyelid covering part of the eye; in tetanus: bilateral and triggered by stimulation; in eye pain: unilateral; in dehydration: bilateral and persistent.

Explore This Structure in 3D

See the Retractor Bulbi on our interactive 3D horse model. Open 3D Model