Like many animals, a dog is a miraculous amalgam of organs and systems which carry out specific jobs as well as well as interrelating to keep the dog “running efficiently”. Although anatomy is a huge subject, a concise consideration of how the dog moves, sees, hears, smell, breathes, eats and digests its food helps us to understand why it is built the way it is. Despite the changes undergone since the time when it needed to fend for itself in the wild, the domestic dog is still basically a carnivore and adapted as such. The dog was designed to run fast, to capture and kill its prey as part of a pack. It retains astonishing senses of hearing and smell – both superior to man’s.
The Basic Design of the Dog
Dogs come in all shapes and sizes, but the basic design of the “standard” dog equips it for being a carnivorous animal. A dog is designed to chase, capture, kill and eat its prey. It has a mark of carnivore – the huge carnassials teeth. And yet, the dog is not such a refined carnivore as some, the cat for instance. Dogs have retained a few molar teeth for chewing and grinding. Domestic cats, on the other hand, have reduced their molars to the point that they have very little chewing ability.
Although the teeth are specially modified, the general skeleton of a carnivore is fairly primitive. Carnivores have not emulated herbivores in reducing their number of toes and converting them to a hoof. A dog needs to be agile, capable of rapid changes of direction and able to use its claws as weapons. The wild herbivores may be speedy, but their movement is essentially forward and they’re not as adept at recovering from falls.
Over the years, selection of dogs to develop the various breeds has modified the anatomy considerably, giving the many variations on the basic shape.
Its basic design allows for sufficient bursts of speed for the dog as a pack hunter, whereas a solitary hunter like a cheetah needs the refinement of extra speed. In addition, the wild dog is well muscled for endurance and long-distance foraging.
Natural and Artificial Selection
In the wild, if a certain physical element become beneficial to a species over a period of time, then those animals with that particular modification become more successful at surviving and therefore more numerous. Charles Darwin, the originator of this idea, called it “Natural Selection”. Darwin developed his theories after watching finches and other animals on the Galapagos Island.
Over the years of domestication, man has selected various characteristics in the dog which fulfill a particular requirement. He has interceded (some would say inferred) and encouraged certain characteristics that wouldn’t succeed in the wild. Since man is a part of nature and he changed the environment, it is perhaps permissible that he has modified the dog to fit into his world rather than excluding it and forcing its extinction. One thing is certain – most of us find the world enriched with your furry canine friends. Whatever your lifestyle, there is dog to suit you.
It is interesting to speculate what might happen to the dog in years to come. Future developments are in hands of breeders and Kennel Clubs. The old familiar breeds will probably change gradually as working dogs become refined into show animals, although the former hopefully won’t be lost. New “old” breeds will probably be introduced from far-flung corners of the world and congenital problems eradicated to produce healthier dogs.
The Dog’s Skeleton
There are two major types of bone in the dog’s skeleton: long bones (tubular bones like the limb bones and spine), and flat bones (the skull, pelvis and shoulder bones). Although the basic design is unaltered since the early days the limbs have been considerably modified between the breeds. Just think of difference between the shapes of a Dachshund and a St Barnard. The reason is that man has bred dogs selectively with bones of different lengths and thickness.
How the Skeleton Works
The skeleton is a system of bony levers moved by muscles which are anchored at crucial point on the bones. The bones are linked together which act like shock absorbers. Bones have a complicated structure which give great stability and yet allows movement. They are anchored by ligaments which permit a given degree of movement in specific directions.
Each joint is surrounded by a joint capsule which contains the joint lubricant fluid. The ends of the bone involved in the joint are covered in cartilage – a smooth surface which helps the joint move easily and helps to absorb any concussion as the dog’s weight comes down on the leg.
How Bones Grow and Develop
Long bones begin in the fetus as cartilage structures, which are replaced by true bone in the latter weeks of pregnancy. A limb bone can be considered as tubular structure with a joint or articulation at each end. The parts of the bone shaft not involved in the joint are covered with a tough, fibrous periosteum. In young, growing dogs the inner layer of the periosteum is actively growing and producing bone. On the inside of the tubular bone, to prevent it to become too thick and heavy, the older bone is reabsorbed and remodeled, keeping the actual bony wall same thickness.
Once the dog has stopped growing the periosteum becomes relatively inactive, although if a fracture occurs and needs repairing, it can become active again in that area. To avoid this process weakening the bone, the inside is filled with fine bony struts. The spaces between these are filled in the young animal with bone marrow, replaced by fat as the dog gets older.
Growth in length occurs in regions of the bone near the joints called growth plates. These growth plates are areas where cartilage is still being produced as an advancing layer behind the growth plate. The cartilage is converted to bone, and so the bone grows is length. In most dogs all growth in length of bones is complete by ten months.
Fuel for Growth
Bone growth requires fuel, and this is provided by blood vessels. The main shaft of each bone is supplied by one or two large nutrient arteries which enter the bone through a hole in the shaft. The epiphysis received blood from a ring of arteries inside the joint capsule. These arteries penetrate the whole of the epiphysis to feed the growing bone. They also supply nutrition to the inside layer of the articular cartilage; the rest of its nutrition comes from the synovial fluid inside the joint.
Muscles and Movement
Collectively, the dog’s muscles are the largest organ in its body. Although selective breeding has brought about great changes in body shape and skeleton, dog’s muscles vary very little between breeds.
How Muscles Work
Most of the muscles are attached to bones. The flat bones are the main anchorage points for the muscles responsible for moving the legs. When muscles contract, the bones to which they are joined are brought closer together, and when they relax, the bones can move apart again. Extra bending of limbs and extension of joints is carried out by muscles running down the legs and attached to the long bones at critical points to obtain maximum leverage. At the point of contact with the bone, the muscles become fibrous tendons.
The wild dog is well-muscled – it needs to be in order to hunt for its food. Man’s best friend – the domestic dog – often has rather soft muscles through insufficient exercise. Of today’s breeds, the Husky is close to the wolf and shares its strength and endurance. Working Husky teams can pull a load of twice their own weight all day at up to three miles per hour.
How the Dog Moves
The apparatus of dog’s locomotion consists of bones, joints, muscles and nerves. The nervous system initiates and co-ordinates muscular activity. It sends messages to the muscles, which work to move the limb bones.
The action of the dog’s limbs can be linked to the spokes of the wheel, each in turn exerting pressure against the ground, then being rotated until able to repeat the process. The larger a wheel, the more ground it covers in one revolution and the longer a dog’s limbs, the greater it stride. The further forward its center of gravity, the faster a dog can move, because its hind legs aren’t supporting too much weight and are more readily available for propulsion. This is true of breeds noted for their speed and agility such as Greyhounds and Borzois. With gundogs, breeders usually try to achieve a happy medium in the center of mouths.
Most of the forward drive comes from the powerful thrust of the hind paws against the ground. Considerable force has to act through the hind legs, so the articular surfaces of the bones fit closely together and are held in position by complex system of muscles and ligament.
Dogs aren’t as good as cats at jumping and climbing. This is partly because they can’t control their claws or twist their legs in the way a cat does. Dogs can be trained to jump obstacles by using their own weight to gain momentum when running. But a dog’s power is really developed for endurance running rather than the sudden muscle contraction needed for the action of jumping.
There are three basic skull shapes in dogs:
· Dolichocephalic – long nosed breeds like the Rough Collie, Afghan hound and Fox Terriers.
· Brachycephalic – short, snub-nosed breeds like the Pug, Bulldog and Pekinese.
· Mesocephalic – A group including dogs which fall between the other two extremes.
Part of the Skull
The features of the skull tend to vary with the overall shape and type of the skull. The eye sits in the space called the orbit, within the zygomatic arch. The two zygomatic arches govern the total width of the skull. They vary in shape between the breeds have a fairly straight arch while in short-nosed breeds it is much curved.
The shape of the jaw varies quite considerably between breeds. The official breed standards include requirements for the “bite” of each dog.
The jaw muscles are very powerful. It is said that a 20 kg mongrel can exert a bite of 165 kg; the pressure of an average human bite is 20-30 kg.
The upper part of the dog’s skull, it houses the brain and also varies between breeds. In the Chihuahua, a high domed shape is specially selected over years of breeding.
Unfortunately this has led in certain cases to people breeding from dogs with heredity brain deformities such as hydrocephalous.
This is the point where the sagittal crest ends and skull outline drops down to the nasal bones. Some breeds, such as the Boxer, are required by the breed standards to have a pronounced stop, while others like the Greyhounds and Bull Terrier are not.
At the back of the skull, the sagittal crest end in the occipital bone, this gives the Basset Hound its peak. This feature does not usually appear in puppies until the age of nine to ten weeks.
The dog’s brain differs from men mainly in the cerebrum; man has much more grey matter than a dog. Although both need to co-ordinate and control bodily functions and movements, man does this with more sophistication. Most of a dog’s brain is involved with senses and recognition. Very little of the brain is available for association of the ideas. A dog can be taught to recognize a coin, but would never understand the concept of money and how many cans of the dog’s food the coin would buy.
A large breed like the St Bernard which is similar in weight to a man has a brain about 15 percent the weight of a man’s brain. Interestingly, the area of the dog’s brain responsible for the sense of smell has 40 times the number of cell of the equivalent area of a man’s brain.
A dog’s teeth adapt it for being a carnivore. It has large, strong shearing teeth which it uses to chew through tough materials. In addition, this last premolar in the upper jaw has become elongated and develop ridge which overlaps with the first molar on the lower jaw when the dog bites. The long, pointed and slightly curved incisors, often called ‘dog teeth’, are useful stabbing weapons for catching and holding prey.
In its basic structure, the dog’s eye is much like a human’s but there are a few differences which mean that the dog has a different type and range of vision.
The eye is split into two main sections by the lens. As a dog grows, the lens grows too, being produced from a living layer around the outside of it, called the lens capsule.
The Three “Coats” of the Eye
The dog’s eye is made up of three layers. From front to back, these are the sclera, uvea and the retina. The sclera incorporates the transparent cornea at the front of the eye. The uvea consists of three parts – choroid, iris and ciliary body. The choroid contains a reflective layer called the tapetum.
The iris (a muscular ring) is controlled by the nervous system and moderates the amount of light entering the eye, like the aperture of the camera. The ciliary body is the point of attachment for the suspensor ligament which holds and moves the lens. It also plays a part in focusing the image on the retina and secretes fluid for nourishing the cornea.
This is the light-sensitive inner layer of the eye. It contains light-sensivitive lens of two types – rods and cones. Rods are very sensitive and work well in low light levels. They only appreciate black and white. Cones operate under good lighting conditions and can appreciate colour. In a dog’s retina, only about five percent of the cells are cones and the remainders are rods, so a dog is probably largely colour blind, seeing in black, white and shades of grey.
The dog’s eyelids have a number of special features. Under the upper lid is the lacrimal gland which produces tears to keep the cornea moist and prevent it from drying out and becoming inflamed. To avoid tears flowing down the face continually, there’s a special drainage system. Both top and bottom lids have a short duct at the inner corner; the two ducts fuse to form a single lacrimal duct for each eye, leading tears to the nasal cavity. Various problems can cause blockage of these ducts and it is important to treat such problems seriously.
A dog has eyelashes on both upper and lower lids. If these points the wrong way, they may hurt the eye. The eyelids deformities known as entropion and ectorpion are fairly common and affect certain breeds more than others.
The Ear and Nose
The most highly developed senses of a dog are its hearing and its sense of smell. Both are superior to men’s and adapt the dog as hunter.
Ears and Hearing
Dogs’ ears vary tremendously in appearance, but they all have excellent hearing and can detect very high frequency sounds inaudible to man. Ears range in shape from large, floppy, sleepy-looking Basset Hound ears, to the pert, pricked ears of some terrier. Cocker spaniels’ ears are floppy and very hairy – merge into the head. In contrast, French Bulldogs’ ears are covered with short hair and stand proud, like radar dishes.
Parts of the Ear
Although the outer ear varies so much between breeds, the structure and function of the middle and inner ears is the same for every dog.
The external ear is a cartilage framework, covered with muscles and skin. In most dogs the pinna is fairly mobile, its muscles moving it to follow sounds. The pinna leads into the external auditory canal – a short tube which runs vertically, then turns horizontally towards the ear drum.
The Middle Ear
The dog’s middle ear incorporates the tympanic membrane, and the tympanic cavity, within which are the smallest bones in the body. Because of their shape and function, these are known as the hammer, anvil and stirrup. The three bones are linked, and operate as a system of levers. Sounds received by the inner ear make the eardrum vibrate. This moves the ossicles which transmit the sound to the inner ear.
This system helps to make the ear sensitivity to amplification, yet the ossicles protect the inner ear against violent vibrations from very loud noises by restricting the range of their movements.
The Inner Ear
Further inside the ear are the sound-sensitive spiral and the organs of balance associated with the semicircular canals. The semicircular canals can detect movement; the saccule and utricle give information on the alignment of the head. This arrangement is the same as that in the cat and in man.
The Nose and Sense of Smell
One of the most remarkable features of the dog is its sense of smell. All dogs have an innate desire to sniff everything – places, people, and other dogs. The dog’s sense of smell gives it all kinds of information and about one million times more sensitive than our own. A dog also has 40 times the number of brain cells involved in scent recognition than the number in a human. Man makes use of his ability in dogs by training them as sniffer dogs (to find drugs and bombs), detectors of gas leaks, or truffle hounds.
Part of the increased sensitivity of a dog’s nose is due to its having a much larger sensory area. In man, this is about three square cm, but in the average dog it is 130 square cm. The sensory area is folded many times over, creating ridges which form a trapping mechanism for capturing smells. The sensory cells are more closely packed, giving more cells per square centimeter.
The pharynx is the area at the back of the mouth where the trachea and the esophagus begin. The soft palate – a floppy extension to the roof of the mouth – hangs down and divides the pharynx in two.
The dog is basically a nose breather, with the soft palate closing off the mouth. By circulating most the normal air supply to the lung through the nasal passages, the dog filters, warms and moistens the air before it reaches the lungs. Mouth breathing becomes far more important to a dog when the air temperature is high, if it has been exercising or if it has a nasal disease.
Problems of Short-nosed Breeds
In the more short-nosed breeds, the soft palate cause respiration problems because it is effectively pushed further back into the head which constricts the pharynx, making mouth breathing very difficult for some dogs.
It can be dangerous to hold the mouth of some of these short nosed breeds closed because when excited they cannot breath effectively through the nose alone. A combination of nose and mouth breathing is needed to prevent the soft palate sticking in the larynx.
The boundaries of the chest are the rib-cage and the diaphragm. Most of the dog’s chest is occupied by its lungs. The heart sits in the center of the chest with its lover point just touching the rib-cage. Both of these organs – the heart and lungs – move within the chest; to avoid them interfering with each other or sticking together, each is housed in its own slippery sac.
Also traversing the chest is the tabular esophagus, carrying food from the mouth to the digestive system in the abdomen.
The dog has a “standard” four-chambered mammalian heart. Two atria empty blood into the powerful ventricle which drive the blood around. The right ventricle pumps blood to the lungs to eliminate carbon dioxide and to pick up oxygen. This blood from the lungs returns to the left atrium which empties it into the left ventricle to be pumped around the body.
The resistance to the heart’s pumping is greater in the bulk of the body than it is in the lungs, so the left ventricle is larger and stronger than the right. Built into the wall of the heart are two “pacemakers” which sends co-ordinate impulses to the muscles them when to contract and when to relax.
The Trachea and Lungs
The entrance to the trachea is the larynx, which is made up of several cartilage segments. The vocal cords sit just within the opening.
The trachea is a tube, made up of rings of cartilage. It leads down to the lungs, where it is divides into bronchi, which subdivide in their turn. Eventually, the air is led into the alveoli – small, membranous sacs with blood vessels in their walls. This is where the exchange of gases occurs, the blood taking in new oxygen and carbon dioxide being released.
Behind the dog’s diaphragm is the body cavity called the abdomen. This is the home of several complex organs, concerned with internal maintenance, converting food into usable material, excreting waste, filtering and storing blood, and reproduction. The abdomen divides into three parts:
· The urogenital system, including the kidneys and the reproductive tract
· The spleen
· The digestive tract, including the intestine, liver and pancreas
The Excretory System
The term “urogenital system” is used to cover two systems – excretory system and reproduction. The two kidneys hang from the roof of the dog’s abdomen, close to the last of its ribs. Each kidney has a cortex, medulla and an area called the pelvis. The cortex and medulla from a complex filtration system, consisting of units called nephrons.
The kidneys filter the blood to remove unwanted and potentially toxic substances from the blood:
1. A clear fluid is produced from the blood by filtering out blood cells.
2. The fluid passes into ducts which remove sodium into the tissues.
3. This sodium draws out water from other parts of the duct, concentrating the urine.
4. Other waste products are excreted into the urine at various points.
5. The urine passes into collecting ducts, then into the kidney pelvis.
The most important and dangerous waste product in urine is urea, produced in the liver from the breakdown of excess amino acids. If urea builds up into the body, it causes serious problems, leading to death.
Each kidney has a ureter to carry the urine from the kidney to the bladder. Peristaltic waves (like those which move food in the intestine) carry the urine into the bladder.
The Skin and Coat
A dog’s skin consists of two basic layers – outer layer and inner layer. The outer layer is not nearly so thick in a dog as in humans – the dog’s coat performs the protective function for which man needs his thick skin.
The inner layer contains blood vessels, skin glands and hair follicles, from which the hair grows up through the outer layer. In man, the dermis and epidermis are linked by interlocking ridges to give flexibility to the skin. The dog has very few of these ridges except on the thick skin of its nose and foot pads. Obviously a dog has far more hair follicles than a man, and these helps fuse the two layers.
Including one primary, or “guard” hair belonging to the coarser outer coat and several secondary hairs constituting the softer undercoat. Most of the follicles have a small muscle attached. Because of the acute angle of the attachment of this muscle, its contraction causes the dog’s hair to “stand on end”.
Special Features of the Dog
Apart from all the various systems which keep it alive and healthy, there are certain aspects which make a dog unmistakably a dog! A mental picture of a dog conjures up a cheerful creature with a wagging tail, tongue perhaps hanging out, and giving the odd lively bark. These are not just ornamental features – they’re all useful parts of the dog’s way of life.
The dog’s bark is one of its ways of signaling to people or to other dogs. There is a strong feeling against the surgical operation of de-barking (banned in the UK) – it robs a dog of a useful means of self-expression. The dog’s tail is another important “mood indicator”. A wagging tail signifies pleasure; other positions of the tail show fear, submission or aggression.
Other characteristics include the tactile whiskers (used for feeling in the dark) and the special anal sacs which allow the dog to scent-mark its territory.