Dog Anatomy: Interactive 3D Model and Labeled Diagrams

A dog is a four-legged carnivoran mammal that walks on its toes. Its body has a head with a long nasal cavity and powerful jaws, a flexible neck of seven vertebrae, a deep chest that holds the heart and lungs, an abdomen with a simple stomach and a short intestine, and four limbs. The forelimbs are attached to the trunk by muscles only, because the clavicle is reduced to a tiny remnant. The hind limbs are joined to the spine through the pelvis and drive the dog forward.

This page describes a medium mesaticephalic dog (a Labrador or Beagle type, about 57 cm at the shoulder). Dogs vary more in size and shape than almost any other domestic animal, from Chihuahuas to Great Danes, but almost every breed has the same organs and the same named bones. Where breeds differ, for example in skull shape, leg length, ear carriage or the number of tail vertebrae, the text says so.

Directions: cranial means towards the head and caudal towards the tail; rostral is used on the head for "towards the nose". Dorsal is the back, ventral the belly. On the legs, proximal is towards the body and distal towards the paw; the back of the front paw is palmar and of the hind paw plantar. Left and right are always the dog's own.

The external parts of a dog, often called the "points of the dog", are the named regions of the body surface. Breeders, judges and vets use them to describe conformation. Click them on the model with the coat at full opacity.

Other external structures are the nose leather (planum nasale), the whiskers (vibrissae), the ear flaps (pinnae), the paw pads and claws, the anus and the external genitalia. Whiskers are stiffer than ordinary hairs and rooted more deeply, with many touch-sensitive nerve endings at their base.

Inside a dog, the chest holds the heart and lungs, separated from the abdomen by the dome-shaped diaphragm. Just behind the diaphragm lie the large liver and the stomach, with the spleen along the stomach's left side. The small intestine fills the lower abdomen, the kidneys sit under the lumbar spine, and the bladder and reproductive organs lie at the entrance to the pelvis.

Switch on Focus selected in the viewer and click an organ to see it on its own, with everything else ghosted.

This is the text version of the 3D model: all 182 labeled structures, grouped by body system, with what each one does. Structures marked "males only" or "females only" appear when you switch sex in the viewer.

The canine muscular system has several hundred named muscles. The ones you see first, just under the skin and the thin cutaneous trunci muscle, are the superficial muscles of the head, neck, shoulder, trunk and limbs shown below. In the forelimb, muscles such as the trapezius, rhomboids, serratus ventralis and pectorals form a muscular sling that attaches the leg to the body.

• Brachiocephalicus: A long muscle from the head and neck to the arm. Pulls the forelimb forward, or bends the neck when the leg is planted.
• Trapezius: A thin triangular sheet over the neck and withers. Holds the scapula against the trunk and swings it.
• Latissimus dorsi: A broad, fan-shaped muscle over the side of the chest. Pulls the forelimb backwards in the stride.
• Triceps brachii: The large muscle mass behind the upper arm. Extends (straightens) the elbow so the dog can bear weight.
• Middle gluteal muscle: The largest gluteal muscle, over the wing of the ilium. Extends the hip and drives the hind limb.
• Biceps femoris: The largest superficial muscle of the outer thigh. Extends the hip and hock; one of the hamstring muscles.
• Gastrocnemius: The calf muscle at the back of the lower hind leg. Extends the hock and pushes off the ground.
• Common calcanean tendon: The dog's Achilles tendon. Transmits the pull of the calf and hamstring muscles to the heel bone.

The brachiocephalicus is typical of carnivores: the remnant of the clavicle, a tendinous band called the clavicular intersection, splits it into a cleidocephalicus part running to the head and neck and a cleidobrachialis part running to the arm. In the hip, the superficial gluteal and the biceps femoris both attach to the sacrotuberous ligament, and the sciatic nerve runs dorsal and caudal to the hip joint deep to them. Vets avoid this area for injections into the back of the thigh.

A dog's skeleton has roughly 320 bones; the exact number depends mainly on the length of the tail. The vertebral formula is C7, T13, L7, S3 (fused into the sacrum) and about 20 caudal vertebrae. There are 13 pairs of ribs, a breastbone of eight sternebrae, and limb bones that match ours in name but not in proportion: dogs stand on their toes, so the long bones of the paw (metacarpals and metatarsals) form part of the leg.

Every bone in the diagram can be clicked in the 3D model with the Skeletal system on. For bone-by-bone detail, the dental formula, skull bones and breed differences, see the dog skeleton guide.

A dog's front leg runs from the shoulder blade through the upper arm, elbow, forearm, wrist (carpus) and pastern to the paw. The rear leg runs from the hip through the thigh, knee (stifle), lower leg, ankle (hock) and rear pastern to the hind paw. The front legs carry more of the body weight when the dog stands; the hind legs provide most of the forward push.

The dog's knee is the stifle, and its "backwards knee" lower down the hind leg is really the hock, the ankle. Dogs and humans share the same joints; the dog simply stands with its heel off the ground.

A dog's paw has four weight-bearing toes, each with a digital pad and a claw, a large heart-shaped metacarpal (front) or metatarsal (hind) pad behind them, and on the front leg a carpal pad higher up. The dewclaw is digit I, on the inner side of the leg; it has its own claw but does not touch the ground.

Each claw grows continuously around the last toe bone, the distal phalanx. Inside is the quick, a living core of blood vessels and nerves that grows as the claw grows. Cutting into it hurts and makes the claw bleed, sometimes heavily. When claws are trimmed regularly, the quick stays short; once a claw has grown long, it can only be trimmed back a little at a time. Most dogs need a trim every three to four weeks.

Pads are thick, keratinised skin over cushions of fat. They grip, absorb shock and protect the bones and joints of the paw. The carpal pad acts as a brake when a dog stops or comes down a slope.

The dog's head is built around the skull, which is divided into the braincase (cranium) and the face. Wide zygomatic arches give the jaw-closing masseter and temporalis muscles a large area to attach, and the jaw joint works mainly as a hinge for a strong scissor bite. Adult dogs have 42 teeth (2 × I 3/3, C 1/1, P 4/4, M 2/3); the carnassial pair, upper fourth premolar and lower first molar, slices food like scissors.

The tongue sits on the floor of the mouth, anchored to the hyoid bones. Saliva comes from four pairs of salivary glands: parotid, mandibular, sublingual and zygomatic. See the labeled dog skull for the individual bones.

A dog's sense of smell starts at the moist planum nasale and the two comma-shaped nostrils, but the real work happens deep in the nasal cavity. Thin, scroll-shaped bones called nasal conchae (turbinates) fill the muzzle. Their front part warms and cleans the air; the caudal ethmoidal conchae carry the olfactory epithelium, where scent molecules bind to receptor cells. Dogs also have a second, separate chemical sense: the paired vomeronasal organ, which lies between the nasal and oral cavities near the vomer bone. (Kokocińska-Kusiak, A. et al. (2021). Canine olfaction: physiology, behavior, and possibilities for practical applications. Animals 11, 2463 (PMC8388720).)

The same review notes that a much smaller share of the dog's olfactory receptor genes are inactive pseudogenes (about 20%) than in humans (more than 50%). Popular figures such as '220 million receptors' or 'a million times better' are estimates that vary between sources, so treat them as rough comparisons. The Merck Veterinary Manual's owner guide gives the 'about a million times stronger' figure. (Merck Veterinary Manual (owners): Description and physical characteristics of dogs.)

Each eye has an upper and lower eyelid plus a third eyelid, the nictitating membrane: a pale fold in the inner (medial) corner that sweeps across the eye and protects it. A gland at its base makes a large part of the tear film. The American College of Veterinary Ophthalmologists puts it at 33–66% of tears, which is why vets replace a prolapsed gland ('cherry eye') rather than remove it: removal predisposes the eye to dry eye. (American College of Veterinary Ophthalmologists: Cherry eye.)

Behind the retina, the tapetum lucidum reflects light back through the light-sensitive cells. It makes the eyes glow in a torch beam and helps dogs see in dim light. Dogs have two cone types, so their colour vision is dichromatic: they see blues and yellows well but confuse reds and greens. With eyes set at the sides of the head, their total field of view is about 250°, wider than a human's. (American Kennel Club: What do dogs see?.)

A dog's ear has three parts: the outer ear (the pinna and the L-shaped ear canal), the middle ear (eardrum and three ossicles inside the tympanic bulla) and the inner ear (cochlea and balance organs). Dogs move each pinna on its own to locate a sound, and hear high-pitched sounds far better than people.

The dog's ear canal is L-shaped. From the opening at the base of the pinna, the vertical canal runs down the side of the head; it then bends to become the horizontal canal, which runs inwards to the eardrum (tympanic membrane). Lifting the ear flap straightens the canal, which is why ear drops are given with the pinna held up. (VCA Animal Hospitals: Applying ear drops to dogs.)

Behind the eardrum, the middle ear sits in the tympanic bulla, a bony bubble on the underside of the skull. It holds the three ossicles and connects to the throat through the auditory tube. The inner ear contains the cochlea for hearing and the vestibular system for balance. The canal is much longer than a human's, and ear problems are common in dogs. (Merck Veterinary Manual (owners): Ear structure and function in dogs.)

In otitis externa the canal lining swells and narrows, and medication may not reach the horizontal canal. Breeds with heavy, floppy, hairy ears are more prone; VCA reports that allergy is involved in up to 43% of cases. (VCA Animal Hospitals: Ear infections in dogs (otitis externa).)

A dog's heart has four chambers, like ours: two atria that receive blood and two ventricles that pump it. It lies in the lower chest between the forelimbs, roughly level with the elbows, with its apex angled down, back and to the left. The mitral and aortic valves are on the left side; the tricuspid and pulmonary valves on the right. At rest it beats about 70–120 times a minute (Merck Veterinary Manual), faster in small dogs.

• Right atrium: The receiving chamber for blood from the body. Receives blood from the venae cavae and passes it to the right ventricle.
• Right ventricle: The pumping chamber on the cranial and right side of the heart. Pumps oxygen-poor blood into the pulmonary trunk and lungs.
• Left atrium: The receiving chamber for blood from the lungs. Receives oxygen-rich blood from the pulmonary veins and passes it to the left ventricle through the mitral valve.
• Left ventricle: The thick-walled pumping chamber that forms the apex of the heart. Pumps oxygen-rich blood into the aorta and around the body.
• Aorta: The main artery leaving the left ventricle. Distributes oxygen-rich blood to the whole body.
• Brachiocephalic trunk: The first and larger branch of the aortic arch, which supplies the head, neck and right forelimb. Gives off the left and right common carotid arteries and ends as the right subclavian artery.
• Left subclavian artery: The second branch of the aortic arch, supplying the left forelimb and the left side of the neck and chest wall. Carries oxygen-rich blood to the left forelimb; it continues as the axillary artery.
• Pulmonary trunk and arteries: The large artery from the right ventricle to the lungs. Carries oxygen-poor blood to the lungs.

Blood returns from the body in the cranial and caudal venae cavae to the right atrium, is pumped by the right ventricle to the lungs, returns oxygenated to the left atrium, and leaves the left ventricle through the aorta. The aortic arch of the dog has just two large branches: the brachiocephalic trunk, which gives off both common carotid arteries and the right subclavian artery, and the left subclavian artery. Mitral valve disease is the most common acquired heart disease in older small-breed dogs.

Air enters through the nostrils, passes through the nasal cavity and pharynx, and reaches the larynx (voice box) in the throat. The trachea runs down the underside of the neck, held open by C-shaped cartilage rings, and divides above the heart into two main bronchi. The oesophagus runs beside it, on the left in the neck, and the thyroid glands lie on each side of the trachea just behind the larynx.

The lungs have lobes: the left lung has a cranial lobe (with cranial and caudal parts) and a caudal lobe; the right lung has cranial, middle, caudal and accessory lobes. The diaphragm, the main breathing muscle, bulges forward into the chest; when it contracts it flattens and draws air in. A resting dog takes about 18–34 breaths a minute.

Food travels from the mouth down the oesophagus, through the diaphragm into the stomach, then into the small intestine (duodenum, jejunum and ileum), the large intestine (cecum, colon and rectum) and out through the anus. The liver and pancreas pour bile and enzymes into the duodenum. The dog's gut is relatively short, as expected for a carnivore that also eats plant material.

• Stomach: A simple, J-shaped stomach on the left of the cranial abdomen. Stores and begins to digest food with acid and enzymes.
• Liver: The largest gland, with six main lobes, against the diaphragm. Makes bile, processes nutrients, stores energy and removes toxins.
• Gallbladder: A small sac that stores bile. Releases bile into the duodenum to digest fat.
• Pancreas: A V-shaped gland with right and left lobes. Makes digestive enzymes and the hormones insulin and glucagon.
• Jejunum: The long, coiled middle part of the small intestine. Digests food and absorbs nutrients.
• Colon: The large intestine: ascending, transverse and descending parts. Absorbs water and salts and forms faeces.
• Anal sacs: Two small scent glands beside the anus. Make a strong-smelling secretion that marks faeces for territory.

The dog has a simple, single-chambered stomach. It lies mostly on the left behind the liver, with the cardia where the oesophagus enters, a dome-shaped fundus, a body along the left flank, and the pyloric part that crosses to the right to join the duodenum. In gastric dilatation-volvulus (GDV, 'bloat'), the stomach fills with gas and can rotate on itself, cutting off its own blood supply. Risk is linked to a deep, narrow chest (a high thoracic height-to-width ratio); Great Danes, Weimaraners, St Bernards and setters are among the breeds most affected. (American College of Veterinary Surgeons: Gastric dilatation-volvulus.)

Two bean-shaped kidneys filter the blood under the lumbar spine. The right kidney lies further forward, at about T13–L2, tucked into a hollow of the liver; the left is further back and moves more. Urine runs down the ureters to the bladder in the caudal abdomen and leaves through the urethra, which in males runs the length of the penis and opens at the tip inside the prepuce.

Male and female dogs differ in their reproductive organs and in how they urinate. The male's penis lies in a sheath (prepuce) along the belly, and his testes hang in a scrotum between the thighs. The female's vulva sits below the anus, and she has two rows of mammary glands with teats.

Switch the 3D viewer between Female and Male to see each set of organs in place.

The stifle is the dog's knee: the joint between the femur, the tibia and the patella. Inside it, two cruciate ligaments cross between the femur and the tibia. The cranial cruciate ligament (CCL, the dog's equivalent of the human ACL) is one of the most important stabilisers of the joint, and its gradual degeneration and rupture is one of the most common reasons for hind-limb lameness in dogs. (American College of Veterinary Surgeons: Cranial cruciate ligament disease.)

Because the ligament stops the tibia sliding forward, a ruptured CCL lets the tibia shift ('cranial drawer') each time the dog bears weight. The menisci, cartilage pads between the bones, are often damaged as well. VCA describes CCL rupture as the most common knee injury in dogs. (VCA Animal Hospitals: Cruciate ligament rupture in dogs.)

Four sesamoid bones sit around the canine stifle: the patella in the quadriceps tendon, two fabellae in the heads of the gastrocnemius behind the femoral condyles (the lateral one is longer), and a small popliteal sesamoid behind the top of the tibia. (Mauragis, D. Small animal radiography: stifle joint and crus. Today's Veterinary Practice.)

In the male, the testes sit in the scrotum and the epididymis on each testis connects it to the ductus deferens. The prostate is the only accessory sex gland of the male dog. The penis has two unusual features: a bone, the os penis, which keeps it rigid, and the bulbus glandis, which swells inside the female during mating and produces the 'tie'. (University of Illinois ANSC 207: Reproductive anatomy of the male dog.)

In the female, the paired ovaries lie behind the kidneys. The uterine horns join to form a short uterine body in front of the cervix, which connects the uterus to the vagina; the vagina opens at the vulva. (University of Illinois ANSC 207: Reproductive anatomy of the female dog.)

The bitch is monoestrous: she has one heat (oestrus) at a time with a long resting interval, roughly every 6–7 months, at any time of year. Puberty comes at about 6–24 months depending on breed size. (Merck Veterinary Manual: Features of the reproductive cycle (table).)

Most dogs have four to five pairs of mammary glands. Mammary tumours are common in intact females, which is one reason spaying is discussed early in life. (Merck Veterinary Manual: Mammary tumors in dogs.)

The brain fills the cranial cavity: the large cerebrum in front, the cerebellum behind it for coordination, and the brainstem joining the spinal cord. The olfactory bulbs at the front are large. The spinal cord runs through the vertebral canal and, in a medium-sized dog, ends around the sixth to seventh lumbar vertebra, with the nerve roots of the cauda equina continuing beyond. The brachial plexus supplies the forelimb and the sciatic nerve the hind limb.

Because the cord lies directly above the intervertebral discs, a herniated disc can press on it and cause pain, weakness or paralysis. This is common in short-legged, long-backed breeds such as the Dachshund (see the Dachshund skeleton).

The tail is the continuation of the spine: about 20 caudal vertebrae, larger and with small processes near the base and simple rods towards the tip, moved by several groups of muscles. Dogs use the tail for balance when turning and for communication. Under the base of the tail lie the anus and, on each side of it at about the 4 and 8 o'clock positions, the anal sacs, whose strong-smelling secretion marks the faeces.

Seen from below (the ventral view), a dog shows the chest floor and sternum between the forelimbs, the belly with its two rows of teats in females or the prepuce in males, the inguinal region between the thighs, the scrotum or vulva, and the pads of all four paws. Inside, the ventral view is the classic dissection view: the liver lobes against the diaphragm, the stomach on the left, the coils of jejunum, and the bladder at the pelvic inlet. Rotate the 3D model and look up from below with the coat faded.

Vets feel several lymph nodes at every check-up: the mandibular nodes under the angle of the jaw, the superficial cervical (prescapular) nodes in front of the shoulder, and the popliteal nodes behind the stifle. They are small and smooth in a healthy dog; enlargement of several at once is a common first sign of lymphoma. The spleen, thyroid and adrenal glands are not normally felt but are shown in the model.

1. What is the dog's knee called, and which joint is the "backwards knee"?
2. Name the four parts of the canine heart and the vessel that leaves the left ventricle.
3. Which organ lies along the left side of the stomach?
4. Why is a dog's ear canal described as L-shaped?
5. What is the quick, and why should you not cut it?
6. How many lobes does each lung have?
7. Name two anatomical differences between male and female dogs.

Answers: 1 stifle; hock (ankle). 2 right and left atria and ventricles; aorta. 3 spleen. 4 vertical canal then horizontal canal. 5 the vessel and nerve in the claw; it bleeds and hurts. 6 left 2 (cranial, caudal), right 4 (cranial, middle, caudal, accessory). 7 for example os penis and prostate in males; uterus and mammary glands in females.