Dog anatomy

Dog anatomy comprises the anatomical study of the visible parts of the body of a domestic dog. 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 at the shoulder, in length along the head and body, and weighed only. The heaviest dog was an English Mastiff named Zorba, which weighed. The tallest known adult dog is a Great Dane that stands at the shoulder.
File:Anatomy dog.png|300 px|thumb|External anatomy of a typical dog: 1. Head 2. Muzzle 3. Dewlap 4. Shoulder 5. Elbow 6. Forefeet 7. Croup 8. Leg 9. Hock 10. Hind feet 11. Withers 12. Stifle 13. Paws 14. Tail

Anatomy

Muscles

The following table lists the limb muscles of the dog.

Muscle	Location	Origin	Insertion	Nerve	Action
Descending superficial pectoral	chest and shoulders	first sternebra	greater tubercle of the humerus	cranial pectoral nerves	adducts the forelimb
Transverse superficial pectoral	chest and shoulders	second and third sternebrae	greater tubercle of the humerus	cranial pectoral nerves	adducts the forelimb
Deep pectoral	chest and shoulder	ventral sternum	lesser tubercle of the humerus	caudal pectoral nerves	extends the shoulder joint
Sternocephalicus	chest and neck	sternum	temporal bone	accessory nerve	turns the head and neck
Sternohyoideus	anterior chest	Sternum	Basihyoid bone	ventral branches of the cervical spinal nerves	moves the tongue caudally
Sternothyoideus	neck	first costal cartilage	thyroid cartilage	ventral branches of the cervical spinal nerves	moves the tongue caudally
Omotransversarius	anterior back and neck	spine of the scapula	wing of the atlas	accessory nerve	advances the forelimb and flexes the neck
Trapezius	anterior back midline	supraspinous ligament	spine of the scapula	accessory nerve	elevates and abducts the forelimb
Rhomboideus	back of the neck to the shoulders	nuchal crest of the occipital bone	scapula	ventral branches of the spinal nerves	elevates the forelimb
Latissimus dorsi	back and shoulder	thoracolumbar fascia	teres major tuberosity of the humerus	thoracodorsal nerve	flexes the shoulder joint
Serratus ventralis	back of the neck to shoulders	transverse processes of the C3 to C7 vertebrae	scapula	ventral branches of the cervical spinal nerves	support the trunk and depress the scapula
Deltoideus	shoulder	acromial process of the scapula	deltoid tuberosity	axillary nerve	flexes the shoulder
Infraspinatus	shoulder	infraspinatus fossa	greater tubercle of the humerus	suprascapular nerve	extends and flexes the shoulder joint
Teres minor	shoulder	infra glenoid tubercle of the scapula	teres minor tuberosity of the humerus	axillary nerve	flexes the shoulder and rotates the forelimb laterally
Supraspinatus	shoulder	supraspinous fossa of the scapula	greater tubercle of the humerus	suprascapular nerve	extends and stabilizes the shoulder joint
Subscapularis	shoulder	subscapular fossa	greater tubercle of the humerus	subscapular nerve	rotates the forelimb medially
Teres major	shoulder	scapula	teres major tuberosity of the humerus	axillary nerve	rotates the forelimb medially
Coracobrachialis	shoulder	coracoid process of the scapula	crest of the lesser tubercle of the humerus	musculocutaneous nerve	adducts, extends, and stabilizes the shoulder
Tensor fasciae antebrachium	upper forelimb	fascia covering the latissimus dorsi	olecranon	radial nerve	extends the elbow
Triceps brachii	upper forelimb	caudal border of the scapula	olecranon tuber	radial nerve	extends the elbow and flexes the shoulder
Anconeus	lower forelimb	humerus	proximal end of the ulna	radial nerve	extends the elbow
Biceps brachii	upper forelimb	supraglenoid tubercle	ulnar and radial tuberosities	musculocutaneous nerve	flexes the elbow and extends the shoulder
Brachialis	upper forelimb	lateral surface of the humerus	ulnar and radial tuberosities	musculocutaneous nerve	flexes the elbow
Extensor carpi radialis	lower forelimb	supracondylar crest	metacarpals	radial nerve	extends the carpus
Common digital extensor	lower forelimb, carpus	lateral epicondyle of the humerus	distal phalanges	radial nerve	extends the carpus and digits 3, 4, and 5
Extensor carpi ulnar	lower forelimb	lateral epicondyle of the humerus	metacarpal 5 and the accessory carpal bone	radial nerve	abducts and extends the carpal joint
supinator	forelimb	lateral epicondyle of the humerus	radius	radial nerve	rotates the lower forelimb laterally
Abductor pollicis longus	lower forelimb	ulna	metacarpal 1	radial nerve	abducts digit 1 and extends the carpal joint
pronator teres	lower forelimb	medial epicondyle of the humerus	medial border of the radius	median nerve	rotate lower forelimb medially and flex the elbow
Flexor carpi radialis	lower forelimb	medial epicondyle of the humerus	palmar side of metacarpals 2 and 3	median nerve	flexes the carpus
Superficial digital flexor	lower forelimb	medial epicondyle of the humerus	palmar surface of the middle phalanges	median nerve	flexes the carpus and the metacarpophalangeal and proximal interphalangeal joints
Flexor carpi ulnar	lower forelimb	olecranon	accessory carpal bone	ulnar nerve	flexes the carpus
Deep digital flexor	lower forelimb	medial epicondyle of the humerus	palmar surface of the distal phalanges	median nerve	flexes the carpus, metacarpophalangeal joints, and interphalangeal joints
Pronator quadratus	lower forelimb	radius and ulna	?	median nerve	pronates the paw
Biceps femoris	upper hindlimb	ischiatic tuberosity	patellar ligament	sciatic nerve	extends the hip, stifle, and hock
Semitendinosus	hindlimb	ischiatic tuberosity	tibia	sciatic nerve	extends the hip and hock, flexes the stifle
Semimembranosus	hindlimb	ischiatic tuberosity	femur and tibia	sciatic nerve	extends the hip and stifle
Sartorius	hindlimb	ilium	patella and tibia	femoral nerve	flexes the hip and both flexes and extends the stifle
Gracilis	hindlimb	pelvic symphysis	cranial border of the tibia	obturator nerve	adducts the hindlimb, flexes the stifle and extends the hip and hock
Pectineus	upper hindlimb	iliopubic eminence	caudal femur	obturator nerve	adducts the hindlimb
Adductor	upper hindlimb	pelvic symphysis	lateral femur	obturator nerve	adducts the hindlimb and extends the hip
Tensor fasciae latae	upper hindlimb	tuber coxae of the ilium	lateral femoral fascia	cranial gluteal nerve	flexes the hip and extends the stifle
Superficial gluteal	hips	lateral border of the sacrum	third trochanter	caudal gluteal nerve	extends the hip and abducts the hindlimb
Middle gluteal	hips	ilium	greater trochanter	cranial gluteal nerve	abducts the hip and rotates the hindlimb medially
Deep gluteal	hips	ischiatic spine	greater trochanter	cranial gluteal nerve	extends the hip and rotates the hindlimb medially
Internal obturator	hips	pelvic symphysis	trochanteric fossa of the femur	sciatic nerve	rotates the hindlimb laterally
Gamelli	hips	lateral surface of the ischium	trochanteric fossa	sciatic nerve	rotates the hindlimb laterally
Quadratus femoris	hips	ischium	intertrochanteric crest	?	extends the hip and rotate the hindlimb laterally
External obturator	hips	pubis and ischium	condyloid process of mandible	obturator nerve	rotates the hindlimb laterally
Quadriceps femoris	upper hindlimb	femur and ilium	tibial tuberosity	femoral nerve	extends the stifle and flexes the hip
Ilipsoas	upper hindlimb	ilium	lesser trochanter	femoral nerve	flexes the hip
Cranial tibial	lower hindlimb	tibia	plantar surfaces of metatarsals 1 and 2	peroneal nerve	flexes the tarsus and rotates the paw laterally
Popliteus	lower hindlimb	lateral condyle of the femur	tibia	tibial nerve	rotates the leg medially
Long digital extensor	lower hindlimb	extensor fossa of the femur	extensor processes of the distal phalanges	peroneal nerve	extends the toes and flexes the tarsus
Peroneus longus	lower hindlimb	tibia and fibula	fourth tarsal bone and the plantar aspect of the metatarsals	peroneal nerve	flexes the tarsus and rotates the paw medially
Gastrocnemius	lower hindlimb	supracondylar tuberosities of the femur	tuber calcanei	tibial nerve	extends the tarsus and flexes the stifle
Superficial digital flexor	lower hindlimb	lateral supracondylar tuberosity of the femur	tuber calcanei and bases of the middle phalanges	tibial nerve	extends the tarsus and flexes the stifle
Deep digital flexor	lower hindlimb	Fibula	plantar surface of the distal phalanges	tibial nerve	extends the tarsus and flexes the toes

Skeleton

The vertebrae have muscles attached to the pedicles, the laminae, the spinous, transverse, and articular processes, the vertebral and intervertebral foramina, the atlas, axis, dens, and ventral lamina.

Skull

In 1986, a study of skull morphology found that the domestic dog is morphologically distinct from all other canids except the wolf-like canids. The difference in size and proportion between some breeds are as great as those between any wild genera, but all dogs are clearly members of the same species. In 2010, a study of dog skull shape compared to extant carnivorans proposed that "The greatest shape distances between dog breeds clearly surpass the maximum divergence between species in the Carnivora. Moreover, domestic dogs occupy a range of novel shapes outside the domain of wild carnivorans."
The domestic dog compared to the wolf shows the greatest variation in the size and shape of the skull that ranges from 7 to 28 cm in length. Wolves are dolichocephalic but not as extreme as some breeds of dogs, such as greyhounds and Russian wolfhounds. Canine brachycephaly is found only in domestic dogs and is related to paedomorphosis. Puppies are born with short snouts, with the longer skull of dolichocephalic dogs emerging in later development. Other differences in head shape between brachycephalic and dolichocephalic dogs include changes in the craniofacial angle , morphology of the temporomandibular joint, and radiographic anatomy of the cribriform plate.
One study found that the relative reduction in dog skull length compared to its width was significantly correlated to both the position and the angle of the brain within the skull, regardless of the brain size or the body weight of the dog.

Canid	Carnassial	Canine
Wolf	131.6	127.3
Dhole	130.7	132.0
African wild dog	127.7	131.1
Greenland Dog	117.4	114.3
Coyote	107.2	98.9
Side-striped jackal	93.0	87.5
Golden jackal	89.6	87.7
Black-backed jackal	80.6	78.3

Respiratory system

The respiratory system is the set of organs responsible for the intake of oxygen and the expelling of carbon dioxide. As dogs have few sweat glands in their skin, the respiratory system also plays an important role in body thermoregulation.
Dogs are mammals with two large lungs that are further divided into lobes. They have a spongy appearance due to the presence of a system of delicate branches of the bronchioles in each lung, ending in closed, thin-walled chambers called alveoli. The presence of a muscular structure, the diaphragm, exclusive to mammals, divides the peritoneal cavity from the pleural cavity, besides assisting the lungs during inhalation.
Inbreeding dogs can cause brachycephalic airway syndrome. The dog's face can have a shortened skull, facial and nasal bones, stenotic nares, a hypoplastic trachea, and everted laryngeal saccules.

Digestive system

The organs that make up the canine digestive system are the same as those in most other mammals, including a mouth, esophagus, stomach, small and large intestines, rectum, anus, liver, and pancreas.

Physical characteristics

Sixty percent of the dog's body mass falls on the front legs.
The dog has a cardiovascular system. The dog's muscles provide the dog with the ability to jump and leap. Their legs can propel them to leap forward rapidly to chase and overcome prey. They have small, tight feet and walk 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.
The dog's muzzle size will vary with the breed. Dogs with medium muzzles, such as the German Shepherd Dog, are called mesocephalic and dogs with a pushed in muzzle, such as the Pug, are called brachycephalic. 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.
All living Canidae have a ligament connecting the spinous process of their first thoracic vertebra to the back of the axis bone, which supports the weight of the head without active muscle exertion, thus saving energy. This ligament is analogous in function 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.
Dogs have disconnected shoulder bones 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 in the rear, the dog is said to be "double dewclawed."

Size

Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier that stood only at the shoulder, in length along the head and body, and weighed only. The largest known adult dog was an English Mastiff, which weighed. The tallest known adult dog is a Great Dane that stands at the shoulder.
In 2007, a study identified a gene that was proposed to be responsible for dog size. The study found a regulatory sequence next to the gene Insulin-like growth factor 1, which, together with the gene and regulatory sequence, "is a major contributor to body size in all small dogs." Two variants of this gene were found in large dogs, making a more complex reason for the large breed size. The researchers concluded that this gene's instructions to make dogs small must be at least 12,000 years old and it is not found in wolves. Another study has proposed that lap dogs are among the oldest existing dog types.

Coat

Domestic dogs often display the remnants of countershading, 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 counter-shaded animal will have dark coloring on its upper surfaces and light coloring below. This reduces the general visibility of the animal. In this pattern, many breeds will have the occasional "blaze", stripe, or "star" of white fur on their chests or undersides.
A study found that the genetic basis that explains coat colors in horse coats and cat coats did not apply to dog coats. The project took samples from 38 different breeds to find the gene responsible for dog coat color. One version produces yellow dogs and a mutation produces black dogs. All dog coat colors are modifications of black or yellow. For example, the white in white miniature schnauzers is a cream color, not albinism.
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.

Tail

There are many different shapes of dog tails: straight, straight up, sickle, curled and cork-screw. In some breeds, the tail is traditionally docked to avoid injuries. It can happen that some puppies are born with a short tail or no tail in some breeds. The T-box gene mutation is responsible for bobtail breeds having no tail to short tail. Dogs have a violet gland or supracaudal gland on the dorsal surface of their tails.

Footpad

The dog's footpad is a fatty tissue locomotive-supporting organ, present at the bottom of the four legs, consisting of digital pads, a metacarpal pad, and a carpal pad, with dewclaw near the footpad. When a dog's footpad is exposed to the cold, heat loss is prevented by an adaptation of the blood system that recirculates heat back into the body. It brings blood from the skin surface and retains warm blood on the pad surface.

Senses

Vision

Like most mammals, dogs have only two types of cone photoreceptors, 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 between red and green, making their color vision equivalent to red–green color blindness in humans. 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 occurs around 480 nm, the part of the spectrum that appears blue-green to humans. For dogs, wavelengths longer than the neutral point cannot be distinguished from each other, and all appear yellow.
Dogs use color instead of brightness to differentiate between light or dark blue/yellow. They are less sensitive to differences in gray shades than humans and can also detect brightness with about half the accuracy of humans. The dog's visual system has evolved to aid in hunting. Dogs have been shown to be able to discriminate between humans at a range of between ; however, this range decreases to if the object is stationary. 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. A test has estimated poodles' visual acuity to have a Snellen rating of 20/75, a relatively low score compared to humans' vision.
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 the 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 is associated with an adaptation toward superior night vision.
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°. 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 found that dogs can distinguish between complex visual images such as those 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.

Hearing

The frequency range of dog hearing is between 16–40 Hz and up to 45–60 kHz, which means that dogs can detect sounds 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. Dogs can lose their hearing from age or an ear infection.

Smell

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.

Taste

Dogs have around 1,700 taste buds compared to humans, with around 9,000. The sweet taste buds in dogs respond to furaneol. 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 are 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 is the part of the tongue that they curl 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 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.

Touch

Dogs have specialized whiskers known as vibrissae, sensing organs present above the dog's eyes, below their jaw, and on their muzzle. Vibrissae are more rigid, 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.

Magnetic sensitivity

A study found that dogs may prefer, when they are off the leash and the Earth's magnetic field is calm, to urinate and defecate with their bodies aligned on a north-south axis. Dogs are sensitive to changes in the Earth's magnetic field polarity. No significant differences between males and females in angular preferences were found. Some studies have detected cryptochrome 1 in some dogs' photoreceptors' blue-sensitive cones.

Temperature regulation

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 set of nasal turbinates, an elaborate set of bones and associated soft-tissue structures in the nasal cavities. These turbinates allow for heat exchange between small arteries and veins on their maxilloturbinate surfaces in a counter-current heat-exchange system. Compared to the ambush predation of cats, dogs are capable of prolonged chases due to these turbinates. This same turbinate structure helps conserve water in arid environments. The water conservation and thermoregulatory capabilities of these turbinates in dogs may have allowed dogs to survive in the Arctic environment and other cold areas of northern Eurasia and North America, which are dry and cold.