Dog Skeleton: Labeled 3D Model of the Canine Skeleton

A dog's skeleton has roughly 320 bones. It is divided into the axial skeleton (skull, hyoid, vertebral column, ribs and sternum), the appendicular skeleton (the bones of the four limbs, including the pelvis) and one visceral bone, the os penis of males. There is no single exact total because the number of tail vertebrae, and the presence of rear dewclaws, varies from dog to dog and breed to breed.

Directions: cranial is towards the head, caudal towards the tail, dorsal the back and ventral the belly. On a limb, proximal is towards the body and distal towards the paw; medial faces the midline and lateral faces outwards.

The labeled dog skeleton below shows every major bone from the left side, the standard view in veterinary anatomy books. Use it as a "label the dog skeleton" worksheet: cover the labels and name each bone, then check in the 3D model.

The axial skeleton is the central axis of the body: the skull, the hyoid apparatus that hangs the tongue and larynx from it, the vertebral column from the atlas to the tip of the tail, the 13 pairs of ribs and the sternum. It protects the brain, spinal cord, heart and lungs and anchors the muscles of the back, neck and breathing.

• Atlas (C1): The first neck vertebra: a ring with broad, wing-like transverse processes. Its joint with the skull lets the head nod.
• Axis (C2): The second and longest neck vertebra, with a tall, blade-like spinous process. Its dens (a peg) projects into the atlas, forming the pivot joint for turning the head.
• Cervical vertebrae (C3–C7): The remaining five neck vertebrae; dogs have seven cervical vertebrae in all. Support and move the neck and protect the spinal cord.
• Thoracic vertebrae (T1–T13): Thirteen vertebrae, each carrying a pair of ribs. Anchor the ribs; their tall spinous processes form the withers.
• Lumbar vertebrae (L1–L7): Seven large vertebrae of the loin with long transverse processes. Bear the strong back muscles and allow flexion in the gallop.
• Sacrum: Three sacral vertebrae fused into one bone. Joins the spine to the pelvis at the sacroiliac joints.
• Caudal (tail) vertebrae: The vertebrae of the tail, usually about 20, becoming simple rods towards the tip. Support the tail and its muscles.
• Ribs: Thirteen pairs of curved bones forming the rib cage. Protect the heart and lungs and move in breathing.
• Sternum (breastbone): The breastbone: eight sternebrae from the manubrium to the xiphoid process. Anchors the sternal ribs and the pectoral muscles.

The dog's vertebral column has five regions. Dogs have seven cervical vertebrae (the first two are the atlas and axis), thirteen thoracic vertebrae, seven lumbar vertebrae and three sacral vertebrae fused into the sacrum. The number of caudal (tail) vertebrae varies widely between individuals and breeds. (WikiVet: Bones – Dog Anatomy.)

Comparative tables give the dog's formula as C7, T13 (range 12–14), L7 (sometimes 6), S3 and Cd 20–23. (Vetflow: Columna vertebralis (vertebral column) – vertebral formulas by species.)

The anticlinal vertebra, where the spinous processes switch from leaning backwards to leaning forwards, is a useful radiographic landmark. In a study of 100 dogs it was T11 in 75–89% of dogs depending on the definition used, and small breeds were more likely to have T10, so it should not be assumed to be T11 in every dog. (Baines, E. A. et al. (2009). Identification of the anticlinal vertebra in dogs. Veterinary Radiology & Ultrasound 50, 14–16.)

Dogs have 13 pairs of ribs: 9 sternal (true) ribs whose cartilages reach the sternum, and 4 asternal ribs. The cartilages of the asternal ribs join the cartilage in front, forming the costal arch. (IMAIOS vet-Anatomy: Ribs.)

The sternum is made of eight sternebrae. The first is the manubrium and the last is the xiphoid process. In dogs and cats the sternebrae stay separate, joined by cartilage, rather than fusing into one bone. (IMAIOS vet-Anatomy: Sternebrae.)

Each typical vertebra has a body underneath, a vertebral arch around the spinal canal, a dorsal spinous process, paired transverse processes and articular processes that link it to its neighbours. Between the bodies lie the intervertebral discs. The spinal cord ends at about L6–L7 in a medium-sized dog; the nerve roots of the cauda equina continue through the sacrum.

The dog skull is made of the cranium, which encloses the brain, and the facial skeleton, which carries the nose, eyes and teeth. In adults the bones are joined at sutures, many of which close with age. The main bones visible from the side are listed below; the ethmoid, vomer, pterygoid and the deep parts of the sphenoid lie inside.

Dogs have an incomplete bony orbit: the ring around the eye is closed behind only by the orbital ligament between the zygomatic process of the frontal bone and the zygomatic arch. The very wide zygomatic arches and the sagittal crest anchor the temporalis and masseter, the muscles that close the jaw.

Dog skulls are grouped by the relative length of the face. Brachycephalic breeds, such as the Pug and Bulldog, have a foreshortened facial skeleton; mesaticephalic (mesocephalic) breeds, such as the Labrador and Beagle, have a muzzle and cranium of medium proportions; dolichocephalic breeds, such as the Greyhound and Collie, have a long, narrow face. Packer and colleagues showed that brachycephaly is not an either/or trait: the risk of obstructive airway disease rises steadily as the muzzle gets shorter relative to the cranium. (Packer, R. M. A. et al. (2015). Impact of facial conformation on canine health: brachycephalic obstructive airway syndrome. PLOS ONE 10, e0137496.)

The permanent dental formula of the dog is 2 × (I 3/3, C 1/1, P 4/4, M 2/3) = 42: in each half of the upper jaw three incisors, one canine, four premolars and two molars, and in each half of the lower jaw the same plus a third molar. The upper fourth premolar and lower first molar are the large carnassial teeth.

Not a functional one. In the dog the clavicle is rudimentary: a small plate about 1 cm long, not connected to the rest of the skeleton. It lies inside the brachiocephalicus muscle at the clavicular intersection, the tendinous line that divides that muscle into its cleidocephalicus and cleidobrachialis parts. Because no bone links the forelimb to the trunk, the shoulder is held by muscles alone, which lengthens the stride. (IMAIOS vet-Anatomy: Clavicle.)

WikiVet describes the canine clavicle as reduced to a fibrous area that may still be visible on radiographs of some dogs. (WikiVet: Bones – Dog Anatomy.)

Adult dogs have 42 permanent teeth: 12 incisors, 4 canines, 16 premolars and 10 molars. Vets name them with the modified Triadan system, which numbers each tooth by quadrant (1 = upper right, 2 = upper left, 3 = lower left, 4 = lower right) and position. (Merck Veterinary Manual: Dentition and dental nomenclature of animals.)

Puppies have 28 deciduous teeth. Baby teeth come in at about 3–5 weeks, permanent teeth start at about 4–5 months, and the adult set is complete at about 6–7 months. There are no deciduous molars. (Merck Veterinary Manual (owners): Dental development of dogs.)

The front leg (thoracic limb) is built from the scapula, the humerus, the radius and ulna, seven carpal bones, five metacarpals and the phalanges of five digits. It is joined to the trunk only by muscles; there is no functional collarbone.

• Scapula: The shoulder blade: a flat, triangular bone with a ridge (spine) on its lateral surface. Attaches the forelimb to the trunk through muscles only, and forms the shoulder joint.
• Humerus: The upper arm bone, between the shoulder and the elbow. Transmits weight and carries the arm muscles.
• Radius: The main weight-bearing bone of the forearm. Carries most of the weight to the carpus.
• Ulna: The forearm bone behind the radius, forming the point of the elbow. The triceps pulls on the olecranon to extend the elbow.
• Carpal bones: Seven small wrist bones in two rows. Let the wrist bend and absorb shock.
• Accessory carpal bone: A carpal bone that projects backwards like a heel at the wrist. A lever for the flexor muscles of the carpus.
• Metacarpal bones: Five bones of the front pastern; I is short (the dewclaw), II–V are long. Support the paw at the correct angle.
• Phalanges of the front paw: The toe bones: proximal, middle and distal phalanges in digits II–V; two in the dewclaw. The distal phalanx ends in the ungual process, which sits inside the claw.

The hind leg (pelvic limb) begins at the hip bone (os coxae), made of three fused bones: the ilium, ischium and pubis. The two hip bones and the sacrum form the pelvis. Below the hip joint come the femur, the patella, the tibia and fibula, seven tarsal bones, the metatarsals and the phalanges.

• Ilium: The largest part of the hip bone, with a wide wing that joins the sacrum. Transfers the push of the hind limb to the spine.
• Ischium: The caudal part of the hip bone, ending in the ischial tuberosity (point of the buttock). Anchors the hamstring muscles.
• Pubis: The ventral, cranial part of the hip bone, forming the front of the pelvic floor. Anchors the adductor and abdominal muscles.
• Femur: The thigh bone: the longest bone of the hind limb. Carries weight and the large thigh muscles.
• Patella (kneecap): The kneecap: a large sesamoid bone in the tendon of the quadriceps. Increases the leverage of the quadriceps when it straightens the stifle.
• Fabellae and popliteal sesamoid: Small sesamoid bones behind the stifle. Act as pulleys for the calf muscle tendons.
• Tibia: The shin bone: the main bone of the lower hind leg. Carries the weight; the cruciate ligaments attach on its flat top (tibial plateau).
• Fibula: A thin bone beside the tibia. Gives attachment to muscles and forms the lateral malleolus of the hock.
• Talus: The ankle bone that forms the hinge of the hock with the tibia. Its pulley-shaped trochlea lets the hock flex and extend.
• Calcaneus: The heel bone, which forms the point of the hock. A lever for the calf muscles, which pull on it through the common calcanean (Achilles) tendon.
• Tarsal bones: The remaining five small hock bones: the central tarsal and tarsals I–IV. Form the lower joints of the hock and absorb shock.
• Metatarsal bones: The long bones of the rear pastern: II–V, with I usually small or absent. Support the hind paw.

For the soft-tissue anatomy of the stifle, including the cruciate ligaments, see dog knee anatomy.

The dog's carpus (wrist) has seven bones. In carnivores the radial and intermediate carpal bones are fused into a single intermedioradial carpal bone; the proximal row also contains the ulnar and accessory carpal bones, and the distal row carpal bones I–IV. (IMAIOS vet-Anatomy: Carpal bones.)

The tarsus (hock) also has seven bones: the talus and calcaneus in the proximal row, the central tarsal bone, and tarsal bones I–IV in the distal row. (IMAIOS vet-Anatomy: Tarsal bones.)

Each toe of digits II–V has three phalanges: proximal, middle and distal. The distal phalanx ends in the ungual process, a hook of bone covered by the claw. The dewclaw (digit I) is shorter and has two phalanges. Behind each knuckle (the metacarpophalangeal or metatarsophalangeal joint) sits a pair of small proximal sesamoid bones, with a small dorsal sesamoid on the other side. Dogs walk on their toes (a digitigrade stance), so the metacarpals and metatarsals form the pastern.

Sesamoid bones grow inside tendons where they run over a joint. The dog has the patella (kneecap), two fabellae behind the stifle, a popliteal sesamoid, the paired sesamoids behind each knuckle of the paws and small dorsal sesamoids. Males also have the os penis, a bone inside the penis with a groove for the urethra underneath; it keeps the penis rigid for mating.

• Patella (kneecap): The kneecap: a large sesamoid bone in the tendon of the quadriceps. Increases the leverage of the quadriceps when it straightens the stifle.
• Fabellae and popliteal sesamoid: Small sesamoid bones behind the stifle. Act as pulleys for the calf muscle tendons.
• Sesamoid bones of the paws: Small bones in tendons behind each knuckle joint of the paws. Reduce friction and act as pulleys for the flexor tendons.
• Os penis: A bone inside the penis. Keeps the penis rigid for mating before full erection.

Every bone and bone group in the 3D model, with its location and job.

A Dachshund has the same bones as any other dog, but its limb bones are short and curved because it is a chondrodystrophic breed: cartilage growth at the growth plates is altered. The same trait makes its intervertebral discs degenerate and calcify early. The American College of Veterinary Surgeons reports that chondrodystrophic breeds account for the vast majority of disc ruptures, with Dachshunds alone making up 45–70% of all cases; most ruptures are in the thoracolumbar spine. (American College of Veterinary Surgeons: Intervertebral disc disease.)

1. Write the dog's vertebral formula.
2. Which bone forms the point of the elbow, and which forms the point of the hock?
3. Name the three bones that fuse to form each hip bone.
4. How many pairs of ribs does a dog have, and how many are sternal?
5. Which two teeth are the carnassials?
6. Where are the fabellae?

Answers: 1 C7 T13 L7 S3 Cd≈20. 2 olecranon of the ulna; tuber of the calcaneus. 3 ilium, ischium, pubis. 4 13; 9 sternal and 4 asternal. 5 upper fourth premolar and lower first molar. 6 behind the femoral condyles, in the heads of the gastrocnemius.

See the organs, muscles, paw and ear on the dog anatomy page.