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Dog Anatomy

This article has been produced as a guide only.
For expert help and advice always consult your vet or breeder.

The anatomy of dogs varies tremendously from breed to breed, more than in any other animal species, wild or domesticated. And yet there are basic physical characteristics that are identical among all dogs, from the tiny Chihuahua to the giant Irsh Wolfhound.

Physical characteristics
Like most predatory mammals, the dog has powerful muscles, a cardiovascular system that supports both sprinting and endurance, and teeth for catching, holding, and tearing.

The dog's ancestral skeleton provided the ability to run and leap. Their legs are designed to propel them forward rapidly, leaping as necessary, to chase and overcome prey. Consequently, they have small, tight feet, walking on their toes; their rear legs are fairly rigid and sturdy; the front legs are loose and flexible, with only muscle attaching them to the torso.

Although selective breeding has changed the appearance of many breeds, all dogs retain the basic ingredients from their distant ancestors. Dogs have disconnected shoulder bones (lacking the collar bone of the human skeleton) that allow a greater stride length for running and leaping. They walk on four toes, front and back, and have vestigial dewclaws (dog thumbs) on their front and rear legs just like monkeys' thumbs. In some cases these claws are missing due to surgery, the rear dewclaws sometimes being removed to prevent the possibility of them being ripped off, or catching on something and breaking, especially in dogs with loose dewclaws. This practice is illegal in some countries.

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 Hound.

Dog coats
Dogs exhibit a diverse array of coat textures, colors, and markings, and a specialized vocabulary has evolved to describe them. Originally, dogs all had dense fur with an undercoat and long muzzles and heads, although both of these features have been altered in some of the more extremely modified breeds, such as the Mexican Hairless Dog and the English Bulldog.

Color
Coat colors range from pure white to solid black and many other variations. One often refers to a specific dog first by coat color rather than by breed; for example, "a blue merle" or "a chocolate Lab". Coat colors include:

  • Black: Usually pure black but sometimes grizzled.
  • Brown: From mahogany through very dark brown.
  • Red: Reminiscent of reddish woods such as cherry or mahogany; also tawny, chestnut, orange, rusty, liver, and red-gold.
  • Yellow: From pale cream to a deep yellowish-gold tan.
  • Gold: From pale apricot to rich reddish-yellow.
  • Gray: Pale to dark gray, including silver; can be mixed with other colors or various shades to create sandy pepper, pepper, grizzle, blue-black gray, or silver-fawn.
  • Blue: A dark metallic gray, often as a blue merle or speckled (with black).
  • Sable: Black-tipped hairs; the background color can be gold, silver, gray, or tan.
  • White: Distinct from albino dogs.

Pattern
Coat patterns include:

  • Two-color coats, such as Black and tan, red and white. The coat has both colors but in clearly defined and separated areas; usually the top and sides are darker and lower legs and underside are the lighter color.
  • Tricolor: Consisting of three colors, usually black, tan, and white or liver, tan, and white.
  • Brindle: A mixture of black with brown, tan, or gold, usually in a "tiger stripe" pattern.
  • Harlequin: "Torn" patches of black on white.
  • Merle: Marbled coat with darker patches and spots of the specified color.
  • Particolor: Two-colored coat with the colors appearing in patches in roughly equal quantities.

Texture
Coat textures vary tremendously. Some coats make the dogs more cuddly and others make them impervious to cold water. Densely furred breeds such as most sled dogs and Spitz types can have up to 600 hairs per inch, while fine-haired breeds such as the Yorkshire Terrier can have as few as 100, and the "hairless" breeds such as the Mexican Hairless have none on parts of their bodies. The texture of the coat often depends on the distribution and the length of the two parts of a dog's coat, its thick, warm undercoat (or down) and its rougher, somewhat weather-resistant outer coat (topcoat, also referred to as guard hairs). Breeds with soft coats often have more or longer undercoat hairs than guard hairs; rough-textured coats often have more or longer guard hairs. Textures include:

  • Double-coated: Having a thick, warm, short undercoat (or down) that is usually dense enough to resist penetration by water and a stronger, rougher weather-resistant outer coat (topcoat), also referred to as guard hairs. Most other coat types are also double-coated.
  • Single-coated: Lacking an undercoat.
  • Smooth-coated: "Smooth" to the eye and touch.
  • Wire-haired: Also called broken-coated. The harsh outer guard hairs are prominent, providing excellent weather protection for hunting dogs such as the Border Terrier or Wirehaired Pointing Griffon.
  • Long-haired: Hair longer than an inch or so.
  • Short-haired: Hair around an inch or so long.
  • Corded coat: for example the Puli.

Parts of the body
A special vocabulary has been developed to describe the shapes of various body parts including the ears and tail.

Ears
Dogs' ears come in a variety of sizes, shapes, lengths, positions on the head, and amounts and types of droop. Every variation has a term, including:

  • Bat ear: Erect, broad next to the head and rounded at the tip.
  • Button ear: A smaller ear where the tip folds forward nearly to the skull, forming a V, as in the Jack Russell Terrier.
  • Cropped ear: Shaped by cutting; see docking.
  • Drop ear: An ear that folds and droops close to the head, such as most scent hounds'. Also called a pendant ear.
  • Natural: Like a wolf's.
  • Prick ear: Erect and pointed; also called pricked or erect.
  • Rose ear: A very small drop ear that folds back; typical of many sight hounds and the English Buldog.
  • Semiprick ear: A prick ear where the tip just begins to fold forward, as in the Rough Collie.

Tails
Like ears, tails come in a tremendous variety of shapes, lengths, amounts of fur, and tailsets (positions). Among them:

  • Corkscrew: Short and twisted, such as a Pug's.
  • Docked: Shortened by surgery or other method, usually two or three days after birth.
  • Odd: Twisted, but not short. Uncommon. Tibetan Terriers have odd tails.
  • Saber: Carried in a slight curve like a saber.
  • Sickle: Carried out and up in a semicircle like a sickle.
  • Squirrel: Carried high and towards the head, often with the tip curving even further towards the head.
  • Wheel: Carried up and over the back in a broad curve, resembling a wheel.

Sight
Dogs were thought to be dichromats and thus, by human standards, color blind. New research is now being explored that suggests that dogs may actually see some colour, but not to the extent that humans do. It has also been suggested that dogs see in varieties of purple/violet and yellow shades. Because the lenses of dogs' eyes are flatter than humans', they cannot see as much detail; on the other hand, their eyes are more sensitive to light and motion than humans' eyes. Some breeds, particularly the best sighthounds, have a field of vision up to 270° (compared to 180° for humans), although broad-headed breeds with their eyes set forward have a much narrower field of vision, as low as 180°.

Hearing
Dogs detect sounds as low as the 16 to 20 Hz frequency range (compared to 20 to 70 Hz for humans) and as high as 70,000 to 100,000 Hz (compared to 20,000 Hz 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.

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). Some breeds have been selectively bred for excellence in detecting scents, even compared to their canine brethren. What information a dog actually detects when he is scenting is not perfectly understood; although once a matter of debate, it now seems to be well established that dogs can distinguish two different types of scents when trailing, an air scent from some person or thing that has recently passed by, as well as a ground scent that remains detectable for a much longer period. The characteristics and behavior of these two types of scent trail would seem, after some thought, to be quite different, the air scent being intermittent but perhaps less obscured by competing scents, whereas the ground scent would be relatively permanent with respect to careful and repetitive search by the dog, but would seem to be much more contaminated with other scents. In any event, it is established by those who train tracking dogs that it is impossible to teach the dog how to track any better than it does naturally; the object instead is to motivate it properly, and teach it to maintain focus on a single track and ignore any others that might otherwise seem of greater interest to an untrained dog. An intensive search for a scent, for instance searching a ship for contraband, can actually be very fatiguing for a dog, and the dog must be motivated to continue this hard work for a long period of time.

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.

It is not uncommon for puppies to have their ears cropped or straightened, tails docked, or in the case of the Chow Chow, to have their eyefolds stitched back so that they can see. Many of these are done in accordance with breed standards for many Kennel Clubs. Some countries like Italy have banned this practice as an act of animal cruelty.

Temperature Regulation
A common misconception is that dogs do not sweat. Primarily, dogs regulate their body temperature in a completely different way, through their tongue. That is why after a dog has been running or on a hot day, its mouth will be seen wide open with the tongue hanging out. This form of cooling maximizes heat lost while conserving moisture, because it carries heat from the hottest part of the body, the interior core of the thorax, unlike sweating, which cools the already coolest part of the body, the skin or in less intuitive, more scientific terms, this higher efficiency of thermal loss relative to moisture conservation arises because heat flow is proportional to temperature gradient. In addition, dogs effectively sweat through the pads of their feet, since they are not furred. On a warm day and after exercise, a dog's naturally wet footprints might be visible on a smooth floor.

Dogs possess a rete mirabile in the carotid sinus at the base of their neck, a complex of intermingled small arteries and veins which acts as a heat exchanger 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 literally drops dead from overheating.

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