Dog anatomy includes the same internal structures that are in humans. Details of structures vary tremendously from breed to breed, more than in any other animal species, wild or domesticated, as dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lb) and was 250 cm (98 in) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.
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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 jump and leap. Their legs can propel them forward rapidly, leaping as necessary to chase and overcome prey. Consequently, they have small, tight feet, walking on their toes (thus having a digitigrade stance and locomotion); their rear legs are fairly rigid and sturdy; the front legs are loose and flexible, with only muscle attaching them to the torso.
The dog's muzzle size will come with the breed. The sizes of the muzzle have different names. Dogs with longer muzzles, such as the German shepherd dog, are called mesocephalic and dogs with a pushed in muzzle, such as the pug, are called brachacephalic.
All dogs (and all living Canidae) have a ligament connecting the spinous process of their first thoracic (or chest) vertebrae to the back of the axis bone (second cervical or neck bone), which supports the weight of the head without active muscle exertion, thus saving energy. This ligament is analogous in function (but different in exact structural detail) to the nuchal ligament found in ungulates. This ligament allows dogs to carry their heads while running long distances, such as while following scent trails with their nose to the ground, without expending much energy.
Although selective breeding has changed the appearance of many breeds, all dogs retain the basic characteristics of their 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 on their front legs and on their rear legs. When a dog has extra dewclaws in addition to the usual one the rear, the dog is said to be "double dewclawed".
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.
Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lb) and was 250 cm (98 in) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.
Researchers have identified a particular piece of genetic material that is common to every small-dog breed and, in turn, is probably responsible for making them tiny. The study, published in 2007, found a regulatory sequence (not in the coding sequence) next to the gene IGF1; together the gene and regulatory sequence are known as a haplotype that "is a major contributor to body size in all small dogs." Medium and large size dogs do not usually have the regulatory sequence, although the small-size sequence was found in the Rottweiler breed. The study included 3,241 dogs from 143 breeds. The researchers concluded 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 (small dogs) are among the oldest dog types.
Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Dogs are predators and scavengers, possessing sharp teeth and strong jaws for attacking, holding, and tearing their food. Although selective breeding has changed the appearance of many breeds, all dogs retain basic traits from their distant ancestors. Like many other predatory mammals, the dog has powerful muscles, fused wristbones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing.
Domestic dogs often display the remnants of counter-shading, a common natural camouflage pattern. The general theory of countershading 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 countershaded 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.
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. Dogs have a violet gland or supracaudal gland on the dorsal (upper) surface of their tails.
There is 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.
Like most mammals, dogs have only two types of cone photoreceptor, making them dichromats. These cone cells are maximally sensitive between 429 nm and 555 nm. Behavioural studies have shown that the dog's visual world consists of yellows, blues and grays, but they have difficulty differentiating red and green making their color vision equivalent to red–green color blindness in humans (deuteranopia). When a human perceives an object as "red", this object appears as "yellow" to the dog and the human perception of "green" appears as "white", a shade of gray. This white region (the neutral point) occurs around 480 nm, the part of the spectrum which appears blue-green to humans. For dogs, wavelengths longer than the neutral point cannot be distinguished from each other and all appear as yellow.
Dogs use color instead of brightness to differentiate light or dark blue/yellow. They are less sensitive to differences in grey shades than humans and also can detect brightness at about half the accuracy of humans.:page140
The dog's visual system has evolved to aid proficient hunting. While a dog's visual acuity is poor (that of a poodle's has been estimated to translate to a Snellen rating of 20/75), their visual discrimination for moving objects is very high; dogs have been shown to be able to discriminate between humans (e.g., identifying their human guardian) at a range of between 800 and 900 metres (2,600 and 3,000 ft), however this range decreases to 500–600 metres (1,600–2,000 ft) if the object is stationary.
Dogs have a temporal resolution of between 60 and 70 Hz. This means that domestic dogs are unlikely to perceive modern TV screens in the same way as humans because these are optimized for humans at 50–60 Hz.:page140 Dogs can detect a change in movement that exists in a single diopter of space within their eye. Humans, by comparison, require a change of between 10 and 20 diopters to detect movement.
As crepuscular hunters, dogs often rely on their vision in low light situations: They have very large pupils, a high density of rods in the fovea, an increased flicker rate, and a tapetum lucidum. The tapetum is a reflective surface behind the retina that reflects light to give the photoreceptors a second chance to catch the photons. There is also a relationship between body size and overall diameter of the eye. A range of 9.5 and 11.6 mm can be found between various breeds of dogs. This 20% variance can be substantial and is associated as an adaptation toward superior night vision.:page139
The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations. Many long-nosed breeds have a "visual streak"—a wide foveal region that runs across the width of the retina and gives them a very wide field of excellent vision. Some long-muzzled breeds, in particular, the sighthounds, have a field of vision up to 270° (compared to 180° for humans). Short-nosed breeds, on the other hand, 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 broad-headed breeds with short noses have a field of vision similar to that of humans.
Most breeds have good vision, but some show a genetic predisposition for myopia – such as Rottweilers, with which one out of every two has been found to be myopic. Dogs also have a greater divergence of the eye axis than humans, enabling them to rotate their pupils farther in any direction. The divergence of the eye axis of dogs ranges from 12–25° depending on the breed.
Experimentation has proven that dogs can distinguish between complex visual images such as that of a cube or a prism. Dogs also show attraction to static visual images such as the silhouette of a dog on a screen, their own reflections, or videos of dogs; however, their interest declines sharply once they are unable to make social contact with the image.:page142
The frequency range of dog hearing is between 16-40 Hz (compared to 20–70 Hz for humans) and up to 45–60 kHz (compared to 13–20 kHz for humans), which means that dogs can detect sounds far beyond the upper limit of the human auditory spectrum.
Dogs have ear mobility that allows them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate, raise, or lower a dog's ear. A dog can identify a sound's location much faster than a human can, as well as hear sounds at four times the distance.
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.
While the human brain is dominated by a large visual cortex, the dog brain is dominated by a large olfactory cortex. Dogs have roughly forty times more smell-sensitive receptors than humans, ranging from about 125 million to nearly 300 million in some dog breeds, such as bloodhounds. This is thought to make its sense of smell up to 40 times more sensitive than a human's.:246 These receptors are spread 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).
The dog has mobile nostrils that helps it determine the direction of the scent. Unlike humans, the dog does not need to fill up his lungs as he continuously brings the odor into his nose in bursts of 3-7 sniffs. The dog's nose has a bony structure inside that humans don't have, which allows the air that has been sniffed to pass over a bony shelf and many odor molecules stick to it. The air above this shelf is not washed out when the dog breathes normally and so the scent molecules accumulate in the nasal chambers and the scent builds with intensity, allowing the dog to detect the most faint of odors.:247
One study into the learning ability of dogs compared to wolves indicated that dogs have a better sense of smell than wolves when locating hidden food, but there has yet been no experimental data to support this view.
The wet nose, or rhinarium, is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents.
Dogs have around 1,700 taste buds compared to humans with around 9,000. The sweet taste buds in dogs respond to a chemical called furaneol which is found in many fruits and in tomatoes. It appears that dogs do like this flavor and it probably evolved because in a natural environment dogs frequently supplement their diet of small animals with whatever fruits happen to be available. Because of dogs' dislike of bitter tastes, various sprays, and gels have been designed to keep dogs from chewing on furniture or other objects. Dogs also have taste buds that are tuned for water, which is something they share with other carnivores but is not found in humans. This taste sense is found at the tip of the dog's tongue, which the part of the tongue that he curls to lap water. This area responds to water at all times but when the dog has eaten salty or sugary foods the sensitivity to the taste of water increases. It is proposed that this ability to taste water evolved as a way for the body to keep internal fluids in balance after the animal has eaten things that will either result in more urine being passed, or will require more water to adequately process. It certainly appears that when these special water taste buds are active, dogs seem to get an extra pleasure out of drinking water, and will drink copious amounts of it.
The main difference between human and dog touch is the presence of specialised whiskers known as vibrissae. Vibrissae are present above the dog’s eyes, below their jaw, and on their muzzle. They are sophisticated sensing organs. Vibrissae are more rigid and embedded much more deeply in the skin than other hairs, and have a greater number of receptor cells at their base. They can detect air currents, subtle vibrations, and objects in the dark. They provide an early warning system for objects that might strike the face or eyes, and probably help direct food and objects towards the mouth.
Dogs prefer, when they are off the leash and Earth's magnetic field is calm, to urinate and defecate with their bodies aligned on a north-south axis.
Primarily, dogs regulate their body temperature through panting, and sweating via their paws. Panting moves cooling air over the moist surfaces of the tongue and lungs, transferring heat to the atmosphere.
Dogs and other canids also possess a very well-developed set of nasal turbinates, an elaborate set of bones and associated soft-tissue structures (including arteries and veins) in the nasal cavities. These turbinates allow for heat exchange between small arteries and veins on their maxilloturbinate surfaces (the surfaces of turbinates positioned on maxilla bone) in a counter-current heat-exchange system. Dogs are capable of prolonged chases, in contrast to the ambush predation of cats, and these complex turbinates play an important role in enabling this (cats only possess a much smaller and less-developed set of nasal turbinates).:88 This same complex turbinate structure help conserve water in arid environments. The water conservation and thermoregulatory capabilities of these well-developed turbinates in dogs may have been crucial adaptations that allowed dogs (including both domestic dogs and their wild prehistoric ancestors) to survive in the harsh Arctic environment and other cold areas of northern Eurasia and North America, which are both very dry and very cold.:87
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