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Canine Design

How the dog moves, sees, hears, smells, breaths, eats and digests its food helps you understand how its body functions and why it needs to needs to eat like a carnivore!

Despite the changes the dog has undergone since the beginning of evolution, it is still a carnivore – a wolf in canine clothing! The dog was designed to run fast, to capture and kill its prey. Even though urban dogs rarely have the opportunity to hunt, they still possess the basic anatomy and physiology that allows them to consume the same foods as their ancestors and wild equivalents.

The Digestive System
Dogs evolved eating the carcasses of other animals and have digestive systems that reflect this history. The overall length of their digestive tract is short relative to the size of the dog. The stomach is simple compared to plant eating animals. Because the dog is designed to eat easily digested animal foods, he does not need a complex digestive system.

Plant material is much lower than meat in energy content and an herbivore must consume a large quantity in order to satisfy its energy requirements. Herbivores spend much more time eating than do carnivores. Carnivores obtain most of their food by eating other animals, and their digestion relies largely on enzymes rather than “fermentation” by micro-organisms. This is a key consideration when compared to the digestion of plant-eating animals. Herbivorous animals that consume plants containing a high proportion of cellulose (fiber) digest their food sequentially throughout the first three chambers of their stomach whereas animals that eat relatively soft vegetation do not need a multi-chambered stomach as they ferment the fiber portion of their diets in their hindguts (colon). The small intestine of plant-eating animals needs to be very long to allow adequate time for absorption of nutrients from fibrous food. To give you an idea, the ratio of intestinal length to total body length is 6:1 for dogs, 10:1 for rabbits and 20:1 for some herbivores.

The simplest way to understand the dog’s internal “food processor” is through the component parts of the intestinal tract, of which each has a specific job starting with the mouth. Once a dog is aware that food will soon be available, the digestive process swings into action. Saliva is produced in the mouth by the salivary glands, which signals the impending meal. The dog’s saliva does not play an active role in the initial stages of digestive function; rather it serves to lubricate the food for transport to the stomach. The autonomic system controls both the volume and type of saliva secreted. A dog fed dry dog food produces saliva that is fairly thin and watery, while a dog on a diet of animal foods secretes saliva with much more mucus. The mucus in the saliva is extremely effective in binding masticated food into a slippery bolus that enables passage easily through the esophagus. The starch digesting enzyme amylase is not present or only in very small quantities in the saliva of dogs. Conversely, the saliva of humans and plant eating animals does contain amylase, which is necessary for initiating the breakdown of vegetable matter in the mouth to begin digestion.

Once the dog takes food into its mouth, the tongue proceeds to move it to the back of the mouth, where it is lifted through the pharynx, over the larynx and into the esophagus, which opens to receive it and closes behind it. This is a highly complex maneuver involving dozens of tiny muscles and finely-tuned nervous control can be summed up in one word – swallowing.

In the esophagus, a muscular process called peristalsis squeezes food along the gut like toothpaste in a tube. The esophagus has thicker walls and is stretchier than the intestine, so dogs can swallow fairly large solid food items such as chunks of meat. The esophagus moves the food through the chest into the stomach. A valve called the cardiac sphincter opens momentarily to let the bolus of food into the stomach then closes tightly behind it.

The stomach serves two purposes. First, it is a storage reservoir, capable of mixing and holding large amounts of foodstuff. The other purpose is to subject food to a concentrated solution of hydrochloric acid, which is necessary to facilitate protein breakdown.

The wall of the dog’s stomach is divided into two roughly equal areas – the fundic region and the pyloric region. The fundic region contains the fundic or gastric glands which produce the hydrochloric acid and an enzyme which produces pepsin for breaking down protein in the food. The dog’s stomach is extremely acidic with a pH of 1-2, which ensures that most pathogenic bacteria are destroyed.

After being held in the stomach for three to four hours, small amounts of bolus are moved into the duodenum. This is the first part of the small intestine. Considered the “workhorse” of the digestive tract, it is smooth and straight, like a stove pipe, and takes a short, direct route. In contrast, the human colon turns back and forth along a convoluted pathway with many twists and turns. The dog’s colon is designed to allow a short transit time for digested animal foodstuff, without much need for fiber to move things along. Microbial digestion of any fiber present occurs in the large intestine of the dog, but to such a small extent that the colon can be removed and the dog can survive perfectly well.

Glands in the duodenum produce a thick, alkaline secretion which begins neutralizing the acid bolus from the stomach and also protects the intestine from the acid. Other important digestive enzymes enter the duodenum from the pancreas. The arrival of food material in the duodenum promotes the secretion of pancreatic juices. Apart from digestive enzymes, it also produces insulin to help store glucose.

By the time the bolus has passed through the small intestine, the enzymes have completed their work and the food has been broken down into its component parts, the bulk of it being absorbed. Much of the water present in the remaining material is reabsorbed in the large intestine before it passes into the rectum and is finally voided as feces. Fecal characteristics in dogs are significantly affected by the quantity and type of digestible and indigestible matter that is present in the diet.

The Mouth and Teeth
All dogs have a wide mouth opening in relation to the size of their head – an obvious advantage for catching prey. The jaw consists of a simple hinge joint that lies in the same plane as the teeth and a large primary muscle on the side of the head for operating the jaw. The lower jaw cannot move forward and has very restricted side-to-side motion which limits thorough mastication of foods. Consequently, the dog tends to “wolf” its food rather than chew it.

The incisor and canine teeth are the primary tools canids use to subdue prey. The canines are sharp and pointed enabling them to puncture and cling. The incisors are short, and play an important role in grasping and holding on to prey. The arrangement of the incisors allows them to be used independently from the canines; to pull at meat, to remove delicate tissues from a carcass, or to nip at small food items such as berries or blades of grass.

The carnassial pairs, the upper fourth premolar and the lower first molar are used to cut through hide and meat. Each tooth from this pair has two shearing edges separated by a V-shaped notch on each edge of the cutting blade. As the jaw closes, the upper and lower blades shear past each other, slicing food between the converging notches, which are self-sharpening. The carnassial pair is multipurpose, as the rear portion of the lower first molar is adapted as a crushing or grinding surface, paired with a smaller upper molar. They are also used for cutting tendons, cracking and crushing small bones, and for gnawing large bones. In plant eaters, these side and back teeth are very flat, serving to grind, mash and chew the food.

The dog’s tongue is covered with small bump-like projections. These bumps give the tongue its rough feeling and contain the taste buds. The taste buds are part of the nervous system and nerve fibers lead from them to the taste center in the brain. The most potent compounds for stimulating the taste buds in dogs are amino acids that also taste sweet to humans, especially L-cysteine, L-proline, L-lysine, and L-leucine. Taste bud responses probably help distinguish various nutritional qualities among meats.

The Skeleton
The dog’s skeleton provides a superb framework for its body. A robust skull, with deep, protective pockets for the eyes and ears, surrounds the brain. Folds of the nasal membrane are attached to delicate bone, housed inside the muzzle. The vertebrae in the neck and back have extensions to which powerful muscles are attached. The shoulder blades are unattached to the rest of the skeleton, allowing great flexibility for running. Long ribs form a cage to protect the heart, lungs, and liver. The shoulders and hips act as pivots, enabling the limbs to move gracefully and accurately. The system is held together by strong, elastic ligaments and tendons and a complex of muscles adapted for endurance.

The skeleton consists of bony levers and lubricated joints, held together by fibrous ligaments, elastic tendons, and powerful muscles. Bone is a latticed structure of hard, calcified struts, filled in its hollow core with bone marrow. Bone needs nourishment and this is supplied by blood vessels that enter the shaft of the bone through a hole called the nutrient foramen. The growth plate itself is nourished by a profuse supply of tiny arteries. If a bone breaks, its fibrous surface produces new bone cells to bridge the gap of the fracture.

The limits of the dog’s size differences appear to be similar to the natural range of size variety in the entire canine family. A long limb bone begins in the puppy as a hollow, tubular cartilage structure. As the dog grows, the cartilage is replaced by bone. The outside of this tubular bone, the fibrous periosteum, produces new bone cells and increases the thickness of bone. On the inside, old bone is constantly remodeled, keeping the bony wall, or cortex, uniformly thick. Growth in length occurs near the ends of the long bones, at the growth (epiphyseal) plates. These areas of cartilage are converted to bone. Hormones have a powerful influence on these growth plates – the earlier that sexual maturity occurs the sooner the bone length is set.

Bones are joined together at cartilaginous joints, which act like shock absorbers. Each joint is surrounded by a joint capsule, which is filled with lubricating joint fluid. The cartilaginous ends of bones have smooth surfaces, allowing easy movement and absorbing concussive force as the dog steps down on its legs while moving. The articular cartilage of the joint obtains nourishment from the growth plate’s blood supply and from the synovial joint fluid.

Ligaments and Tendons
Ligaments anchor bones to each other and permit movements in specific directions while preventing excessive movements that might injure delicate parts of the anatomy. Excess weight creates tension on some ligaments and can cause them to tear. Tendons are the elastic tips by which muscles attach themselves to the skeleton. The dog’s tendons are superbly constructed and tendon injuries are rare.

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, dogs’ muscles vary very little between breeds. 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 the 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. The domestic dog often has rather soft muscles through insufficient exercise.

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 breeds – long nose breeds have a fairly straight arch while in short nosed breeds it is curved. The jaw muscles are very powerful. It is said that a 44lb dog can exert a bite of 365lbs; the pressure of an average human bite is 44-66lbs. The cranium (upper part of the dog’s skull), varies between breeds. For instance, in the Chihuahua, a high-domed shape has been specially selected over years of breeding. The ‘stop’ is the point where the sagittal crest ends and the skull outline drops down to the nasal bones. Some breeds, such as the Boxer, are required by breed standards to have a pronounced stop, while others like the Greyhound are not. At the back of the skull, the sagittal crest ends in the occipital bone, which gives the basset hound its peak.

The Brain
The dog’s brain differs from the human brain, mainly in the cerebrum; a human has much more gray matter than a dog. Although both need to coordinate and control bodily functions and movements, a human does this with more sophistication. Most of the dog’s brain is involved with senses and recognition. Very little of the brain is available for association of ideas. Interestingly, the area of the dog’s brain responsible for the sense of smell has 40 times the number of cells of the equivalent area of a human’s brain.

The Eyes
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 dog’s eye is made up of three layers. From to back, these are the sclera, the uvea, and the retina. The sclera incorporates the transparent cornea at the front of the eye. The uvea consists of three parts – choroids, 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 a camera. The ciliary body, a ring of tissue behind the iris, is the point of attachment for the suspensory 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. The retina is the light-sensitive inner layer of the eye. It contains light-sensitive cells 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 color. 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 color blind.

The Eyelids
The dogs’ 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 is 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.

Dogs’ have eyelashes on both upper and lower lids. They have a third lid on each eye, known as the haw or nictating membrane. This is mainly hidden under the lower lid and just a small part of the pigmented edge is visible in the corner of the eye close to the nose. In some breeds, it is more prominent. The membrane acts like a windshield wiper for the eye – a defense mechanism to sweep away foreign bodies. When the eye is suddenly drawn back into its socket or becomes sunken through age or disease, the third eyelid becomes more prominent. If it appears suddenly and remains visible, this may be an indication of disease or slight pain.

Dogs see better in the dark than humans, partly because a dog’s retina is dominated by rods that are sensitive to low light and partly because of the tapetum lucidum, which lies underneath the rod cells and reflects concentrated light back through them. This is a help to wild dogs who are likely to be hunting in poor light conditions. As a further aid to hunting, dogs are particularly sensitive to seeing movement in the distance. They perceive stationary objects relatively poorly, though, which makes some individuals seem clumsy.

The way a dog’s eyes are positioned on its head, combined with good muscles for moving them around, gives the dog a comparatively wide field of view. However, the variation in the shape of dogs’ heads alters the placement of the eyes somewhat modifying the field of vision between breeds.

The Ears and Hearing
The two most highly developed senses of a dog are its hearing and its sense of smell. Dogs’ ears vary tremendously in appearance, but they all have excellent hearing and can detect very high frequency sounds inaudible to man. The external ear, the pinna, 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 toward the eardrum (tympanic membrane).

The dog’s middle ear incorporates the tympanic membrane and the tympanic cavity, within which are the smallest bones in the body, the auditory ossicles. 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 sensitive to sound by amplification, yet the ossicles also protect the inner ear against violent vibrations from very loud noises by restricting the range of their movements.

Further inside the ear are the sound sensitive spiral cochlea 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 human.

The Nose and the 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. In fact, almost every pet owner can testify to at least one embarrassing incident where their dog was sniffing somewhere it shouldn’t! The dog’s sense of smell gives it all kinds of information and is 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 humans do. Part of the increased sensitivity of a dog’s nose is due to its having a large olfactory area. This 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 inch. The nose is kept moist by the secretions of special cells. These are stimulated when new odors are detected. Scents, present as small particles, are dissolved by the secretions and brought into contact with the sensory cells.

The pharynx is the area at the back of the mouth where the trachea and the esophagus begin. The soft palate, a floppy extension at 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 of 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 disorder.

The Chest
The boundaries of the chest are the ribcage 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 lower point just touching the ribcage. Both of these organs, the heart, and the lungs, move within the chest; to avoid them interfering with each other or sticking together, each is surrounded by its own slippery encasing. Also traversing the chest is the tubular esophagus, which carries’s 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 ventricles which then 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” that send coordinated impulses to the muscles telling them when to contract and when to relax.

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 divides into bronchi, which subdivide in their turn. Eventually, the air is led into the sacs with blood vessels in their walls. This is where the exchange of gasses occurs, the blood taking in new oxygen and carbon dioxide being released.

The Abdomen
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 is divided into three basic parts: the urogenital system, including the kidneys and the reproductive tract; the spleen; and the digestive tract, including the intestine, liver, and pancreas.

The term “urogenital system” is used to cover two systems – excretory and reproductive. 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 form a complex filtration system, consisting of units called nephrons. The function of the kidneys is to filter the blood and potentially toxic substances from the blood. A clear fluid is produced from the blood by filtering out blood cells. The fluid passes into ducts which remove sodium into the tissues. This sodium draws out water from other parts of the duct, concentrating the urine. Other waste products are excreted into the urine at various points. The urine passes into collecting ducts, then into the kidney pelvis. The most important waste product in urine is urea, produced in the liver from the breakdown of excess amino acids. If urea builds up in the body, it causes serious problems. Each kidney has a ureter to carry the urine from the kidney to the bladder. Peristaltic waves carry the urine into the bladder.

The largest single organ in the body of all animals is the liver, a very important chemical factory. In the dog, it performs several functions which are linked to the blood, food storage and removal of toxins. The spleen is close to the stomach and its main function is storage and recycling of red blood cells. The pancreas performs two vital functions. It produces insulin which helps the body obtain energy from glucose. This is known as an endocrine process. The pancreas also produces digestive enzymes – and exocrine process.


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