The Anatomy of the Dog

Dog anatomy includes identical internal structures to those that are inside of a human. Details of structures vary tremendously from breed to breed, more than in any other species of animals, in the wild or domesticated, and yet there basic physical characteristics that are identical among all dogs even as dogs vary from the tiniest Chihuahua who stands at about five inches to the giant Irish wolfhound who stands about 32 inches high at the withers, or top of the shoulders.

External anatomy (topography) of a typical dog includes : 1.Stop – 2.Muzzel – 3.Dewlap (throat, neck skin) – 4.Shoulder – 5.Elbow – 6.Forefeet – 7.Croup (rump) – 8.Leg (thigh and hip) – 9.Hock – 10.Hind feet – 11.Withers – 12.Stifle – 13.Paws 14.Tail

Skeleton of a domestic dog

The skeletal frame of the dog consists of 319 bones.

The Skeleton is formed from a highly specialized substance known as bone, a complex of living cells embedded in a material formed mainly from a compound calcium-phosphorus salt reinforced by a meshwork of protein fibers. Through-out an animal’s life, bone is constantly being removed, reformed and remodeled in order to perform its several functions. Besides supporting the body weight, bone provides rigid points for muscle attachment and for a system of levers which, when activated by muscles, effect movement of the body. Bone also protects soft structures, such as the heart and lungs, and provides some calcium for bodily needs. It is the source of bone marrow in which new red and white blood cells are formed.

The dogs knee

A Dog knee includes the joint, which connects three different bones;

Femur: A dog femur is also called the thigh bone, which connects the knee to the dog hip.
Tibia: The dog tibia is also called the canine shin-bone and it forms part of the leg. It connects the knee to the dog ankle.
Patella: The patella is also called the “dog knee cap.” It not only protects the knee joint, but also restricts the back and forth movement of the tibia and the femur during walking.

Ligaments are fibrous tissues, which hold the set of three bones together in the dog knee joint. These ligaments are flexible in nature, thus the movement of the knee joint is facilitated without losing the integrity of the joint. A normal dog knee joint has different types of ligaments, which holds the bones together. These ligaments are;

Anterior cruciate ligament (ACL)
Lateral collateral ligament (LCL)
Posterior cruciate ligament (PCL)
Medial collateral ligament (MCL)
Cranial cruciate ligament (CrCL) – (equivalent to ACL)
Caudal cruciate ligament (CaCL) – (equivalent to PCL)

Physical characteristics

Similar to most predatory mammals the dog’s muscles are very powerful, they possess a cardiovascular system that supports both sprinting and endurance, and the teeth for catching, tearing, and holding.

The dog’s ancestral skeleton provided them with the ability to jump and leap. Their legs are designed to propel them forward rapidly allowing them to leap as necessary, to chase and overcome their prey. Consequently, that leaves them having small tight feet which requires them to walk on their toes there for the rear legs are fairly rigid and sturdy but the front legs are loose yet flexible, with only muscle attaching there pairs to the torso.

Dogs have 42 teeth. Six pairs of sharp incisor teeth are in front of the mouth, flanked by two pairs of large canine (“dog”) teeth. The other teeth are premolars and molars. The incisors and the canines are very important because the dog bites and tears at its food with these teeth. Each eye of a dog has three eyelids, the main upper and lower lids and a third lid hidden between them in the inner corner of the eye. The third eyelid can sweep across the transparent cornea of the eye and clean it like a windshield wiper.

Although selective breeding has changed many factors in the appearance of breeds, all dogs still retain the basic DNA from their distant ancestors. Dogs have disconnected shoulder bones and unlike the human body they are lacking a collar bone which allows them to have a greater stride length for tasks such as running and leaping. They walk on four toes, front and back, and have concealed dewclaws also known as dog thumbs on their front legs and sometimes on their rear legs .When a dog has extra dewclaws on the back legs in addition to the usual one on each front leg, the dog is said to be “double dew clawed”. There has been some debate about whether a dewclaw helps dogs to gain traction when they run because, in some dogs, the dewclaw makes contact when they are running and the nail on the dewclaw often wears down in the same way that the nails on their other toes do, from contact with the ground. However, in many dogs the dewclaws never make contact with the ground; in this case, the dewclaw’s nail never wears away, and it is then often trimmed to keep it to a safe length. The dewclaws are not dead appendages. They can be used to lightly grip bones and other items that dogs hold with the paws. However, in some dogs these claws may not appear to be connected to the leg at all except by a flap of skin; in such dogs the claws do not have a use for gripping as the claw can easily fold or turn. There is also some debate as to whether dewclaws should be surgically removed. The argument for removal states that dewclaws are a weak digit, barely attached to the leg, so that they can rip partway off or easily catch on something and break, which can be extremely painful and prone to infection. Others say the pain of removing a dewclaw is far greater than any other risk. For this reason, removal of dewclaws is illegal in many countries. There is, perhaps, an exception for hunting dogs, who can sometimes tear the dewclaw while running in overgrown vegetation. If a dewclaw is to be removed, this should be done when the dog is a puppy, sometimes as young as 3 days old, though it can also be performed on older dogs if necessary (though the surgery may be more difficult then). The surgery is fairly straight-forward and may even be done with only local anesthetics if the digit is not well connected to the leg. Unfortunately many dogs can’t resist licking at their sore paws following the surgery, so owners need to remain vigilant. In addition, for those dogs whose dewclaws make contact with the ground when they run, it is possible that removing them could be a disadvantage for a dog’s speed in running and changing of direction, particularly in performance dog sports such as dog agility.

The dog’s ancestor was about the size of a Dingo, and its skeleton took about 10 months to mature. Today’s toy breeds have skeletons that mature in only a few months, while giant breeds such as the Mastiffs take 16 to 18 months for the skeleton to mature. Dwarfism has affected the proportions of some breeds’ skeletons, as in the Basset Hounds.

Knowledge of basic anatomy also helps when competing in dog shows or contests.


Researchers have identified a unique piece of genetic material that is common to every small dog breed and in turn is most likely what is responsible for making them tiny. The study that was published in 2007, found a regulatory sequence (not a gene) next to the gene IGF1; together the gene and regulatory sequence are known as a haplotype that is a major factor to body size in all small dogs. Medium and large size dogs do not usually possess the regulatory sequence, although the small-size sequence was found in the Rottweiler breed. The study included 3,241 dogs from 143 different breeds. The researcher’s came to the conclusion that the genetic instructions to make dogs small must be at least 12,000 years old, and it is not found in wolves. Another study has shown that lap dogs better known as small breeds are among the oldest dog types.

Modern day dog breeds show more variation in their size, appearance, and behavior than any other domesticated animal. Within the range of extremes, dogs generally share attributes with their wild ancestors the wolves. Dogs are predators and scavengers, they possess sharp teeth and strong jaws for attacking, holding, and tearing their food. Although selective breeding has caused a change in the appearance of many breeds, however, all dogs retain basic traits from their distant ancestors. Like many other predatory mammals, the dog has powerful muscles, fused wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing.


The eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. The names are clues as to their basic structures and functions, but a closer look at the components of each layer will make understanding the mechanism of sight much easier.

Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans. Different breeds of dogs have different eye shapes and dimensions, and they also have different retina configurations. Dogs with long noses have a “visual streak” which runs across the width of the retina and gives them a very wide field of excellent vision, while those with short noses have an “area centralis” — a central patch with up to three times the density of nerve endings as the visual streak — giving them detailed sight much more like a human’s.

Some breeds, particularly the sight hounds, have a field of vision up to 270° (compared to 180° for humans), although broad-headed breeds with short noses have a much narrower field of vision, as low as 180°.


Puppies are born unable to hear. They are unresponsive to even loud noises. The ear canals remain closed, unable to carry sound to the eardrum until the puppy is about ten days of age. In some individuals, the ear canals may open slightly sooner or later but it averages about ten days. The canals become fully open by three weeks of age. As a result of the ear canals ‘opening up,’ most puppies will begin to hear sounds at about fourteen days of age, with functional hearing by twenty-one days of age. It is very difficult to assess possible hearing impairment until the puppy is at least four weeks of age, at which time deafness, if present, may be noticed and evaluated.

According to, the frequency range of a dog hearing is approximately 40 Hz to 60,000 Hz. Dogs detect sounds as low as the 16 to 20 Hz frequency range (compared to 20 to 70 Hz for humans) and above 45 kHz (compared to 13 to 20 kHz for humans), and in addition have a degree of ear mobility that helps them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate and raise or lower a dog’s ear. Additionally, a dog can identify a sound’s location much faster than a human can, as well as hear sounds up to four times the distance that humans are able to. Those with more natural ear shapes, like those of wild canids like the fox, generally hear better than those with the floppier ears of many domesticated species.


A dog’s scent organ (inside his nose) is about four times larger than a human’s, and a dog’s sense of smell is about 50-100 times more powerful than ours. Although all dogs have a powerful sense of smell, some breeds have a greater talent for sniffing out things. A few examples are Basset Hounds, Bloodhounds, and Beagles, which are considered ‘scent hounds.’ If you live with a scent hound, you know how difficult it is to get his mind focused on anything but odors.

The nose of a dog is highly sensitive. Scent hounds, especially the Bloodhounds , are bred for their keen sense of smell. Dogs have nearly 220 million smell-sensitive cells over an area about the size of a pocket handkerchief (compared to 5 million over an area the size of a postage stamp for humans). According to, dogs can sense odors at concentrations nearly 100 million times lower than humans can. According to, the percentage of the dog’s brain that is devoted to analyzing smells is actually 40 times larger than that of a human. Some dog breeds have been selectively bred for excellence in detecting scents, even compared to their canine brethren.


Dogs have two types of hair in their coats. There are short fluffy hairs called secondary hairs. Other names for secondary hairs include under fur and undercoat. The second type of hair is the longer and stiffer outer hairs called primary hairs. Primary hairs are also referred to as guard hairs, outer hairs, or outer coat. Dogs also have a third type of hair: the whisker. Whiskers are called tactile hairs because they help the dog sense his surroundings.

The hair of a dog does not grow continuously, but in cycles, similar to our eyebrows. Anagen is the first phase, in which the hair is produced. The new hair grows alongside the old hair, which is subsequently lost. Catagen is an intermediate stage in the cycle, and telogen is the resting phase in which the follicle is basically dormant. The hair follicles are not all in the same phase at the same time, which is why we do not see a lot of bald dogs!

Domestic dogs often display the remnants of counter-shading, a common natural camouflage pattern. The general theory of counter shading is that an animal that is lit from above will appear lighter on its upper half and darker on its lower half where it will usually be in its own shade. This is a pattern that predators can learn to watch for. A counter shaded animal will have dark coloring on its upper surfaces and light coloring below. This reduces the general visibility of the animal. One reminder of this pattern is that many breeds will have the occasional “blaze”, stripe, or “star” of white fur on their chest or undersides.

Dogs diverged from a now-extinct Asian wolf between 12,000 and 15,000 years ago, according to recent DNA studies. In that time the long nose and heavy grey-colored double coat of the wolf has changed into the wide variety of dog shapes and coats and colors seen today. The change was due at first to genetic changes that occurred as the original dogs learned to tolerate the presence of humans, as shown in the research on foxes by Dmitri Belyaey in his Farm-Fox Experiment . The research found that a genetic change to tameness brought along other unexpected changes as well; one notable change was in the coats, changed from a typical fox coat to a spotted coat resembling a dog’s coat. As ancient dogs learned to live near humans and became less like wolves, their appearance changed as well, long before any selective breeding was done by people.

A Stanford University School of Medicine study published in Science in October, 2007 found the genetics that explain coat colors in other mammals such as in horse coats and in cat coats, did not apply to dogs. The project took samples from 38 different breeds to find the gene (a beta defensin gene) responsible for dog coat color. One version produces yellow dogs, and a mutation produces black. All dog coat colors are modifications of black or yellow. For example, the white in white miniature schnauzers is a cream color, not albinism (a genotype of e/e at MC1R.)

Modern dog breeds exhibit a diverse array of fur coats, including dogs without fur, such as the Mexican Hairless Dog. Dog coats vary in texture, color, and markings, and a specialized vocabulary has evolved to describe each characteristic.


There are many different shapes for dog tails: straight, straight up, sickle, curled, cork-screw. In some breeds, the tail is traditionally docked to avoid injuries (especially for hunting dogs). It can happen that some puppies are born with a short tail or no tail in some breeds.

Puppy characteristics

Puppies often have characteristics that do not last beyond early puppyhood. Eye color often changes from blue to its adult color as the puppy matures. The coat color may change: Kerry Blue Terrier puppies have black coats at birth and change to blue with maturity, and Dalmatians are white and gain their spots with age. The ear shape will also often change, especially with erect-eared breeds such as the German shepherd dog which have soft ears at birth, but the cartilage strengthens with age. Labrador Retrievers and other swimming dogs, start of with a very fluffy puppy coat, and over time the water proof layer grows. Puppies that are going to grow into larger dogs, often will have over sized paws to begin with, and then the rest of them grow to fit.

Temperature regulation

It is a common misconception that dogs do not sweat They do sweat, mainly through the footpads, but only a small fraction of a dog’s excess heat is lost this way. Primarily, dogs regulate their body temperature through panting. Panting moves cooling air over the moist surfaces of the tongue and lungs, rejecting heat to the atmosphere.

Dogs possess a rete mirabile, a complex of intermingled small arteries and veins, in the carotid sinus at the base of their neck. This acts to thermally isolate the head, containing the brain, the most temperature-sensitive organ, from the body, containing the muscles, where most of the heat is generated. The result is that dogs can sustain intense physical exertion over a prolonged time in a hot environment, compared to animals which lack this apparatus; thus, a dog chasing a jackrabbit through the desert may not be able to outrun the rabbit, but it can continue the chase until the rabbit slows due to overheating.