Have you ever found yourself wondering how many teeth a dog has? Or whether their front legs have elbows or knees? It sounds like you could use a crash course in canine anatomy. Don’t worry, we’re not going to get too lost in the nitty gritty details or look at things at a cellular level, but we will leave you with a good understanding of the important features and functions of your dog’s body.
So let’s take a tour of the dog, from nose to tail.
Vital Statistics of the Dog
| Scientific name: | Canis lupus familiaris |
| Average lifespan: | 10 to 14 years |
| Weight: | From 3 lbs to over 200 lbs |
| Number of bones: | 319-321 (males have one additional bone, the os penis) |
External Anatomy
Skeletal Anatomy
The Skull
Would you believe that there are 50 bones that make up the skull, including the hyoid apparatus (small bones that support the larynx)? The majority of these bones are fused before birth. As a result, what we see is the skull as a single unit, with the upper jaw (maxilla) attached to the lower jaw (made up of the fused left and right mandible bones), by the hinged temporomandibular joint.
Because of selective breeding, the size and shape of the dog skull varies significantly across the breeds, from the long nose of the Afghan Hound to the snub-nosed Pug. For this reason, there are three categories used to describe skull anatomy:
- Doliocephalic: long skull
- Mesocephalic: mid-length or standard-length skull
- Brachycephalic: short skull
Brachycephalic Health Problems
Many dogs that are characterized as brachycephalic, like Pugs, Bulldogs, Shih Tzus, and French Bulldogs, suffer from varying levels of brachycephalic obstructive airway syndrome (BOAS). This is due to problematic proportions; as the muzzle length got shorter, the internal structures did not necessarily condense in proportion to their shrinking size. The result is a soft palate that’s a little too long, a tongue that’s a bit too large, and a trachea that is slightly too narrow.
The Spine
Supporting the head is the neck, comprising 7 cervical vertebrae. The first vertebrae is called the Atlas, named after the Titan who, according to Greek mythology, was condemned to hold up the earth, or in this case, the skull! The second vertebra is named the axis because it allows the head to turn, much like the Earth spins on its axis.
After the cervical vertebrae, there are:
- 13 thoracic vertebrae that support the chest (thorax) and forelimbs
- 7 lumbar vertebrae that support the abdomen and hindlimbs
- 3 sacral vertebrae that help support the pelvis and hindlimbs
- 20-23 coccygeal vertebrae of the tail
Each vertebral segment is designed to support the muscles and structures around it and form a protective column through which the spinal cord runs. Between each vertebra is a cushioning disk. In dogs suffering from intervertebral disk disease (IVDD), these disks may become calcified under pressure and push up against the spinal cord, resulting in pain, weakness, or paralysis.
TRIVIA QUESTION:
CAN YOU GUESS HOW MANY CERVICAL VERTEBRAE THERE ARE IN
- Humans?
- Blue whales?
- Giraffes?
Keep reading for the answer!
The Limbs
The positioning and gait of the canine limbs is known as digitigrade, as they bear weight on their digits (toes) when walking.
The Forelimb
The front legs of the dog are anatomically similar to the human arms, with a few key differences. For example, the shoulder blades (scapulae) are positioned on the side of the ribs rather than over the back, and digit 1 (our thumb), called the dew claw, is small and does not move freely.
The Hindlimb
Coming to the back leg, the hips are an area where intensive selective breeding has resulted in a condition called hip dysplasia to become prominent in many breeds. It occurs when the socket located on the pelvis (called the acetabulum) is too shallow for the head of the femur, or the head of the femur is not the right shape to fit well in the acetabulum. Sometimes, it is both.
The canine knee is called the stifle, a joint that is prone to injuries like cruciate ligament rupture (the equivalent of a torn ACL in humans), and patella luxation, which occurs when the kneecap is not sitting where it should.
The first digit (the equivalent of our big toe) is absent on most dogs, although some are still born with small, ‘vestigial’ hind dew claws that may be floppy attachments or fully formed digits.
The Paws
Each paw has four digital pads and a thick, leathery metacarpal (front limb) or metatarsal pad that allows the dog to walk on surfaces that are rough, cold, hot, icy, and uneven. The front paws also have a digital pad or spur at the base of the dew claw, and a carpal pad, also known as a stopper pad.
Despite their tough exterior, these pads are quite sensitive underneath, and cannot tolerate very hot or extremely cold surfaces for very long. As part of their regular grooming, you should take time to check and clean the paws, making sure the pads aren’t dry and cracked. If your dog tends to spend more time on soft carpet than rugged terrain, you may need to consider protecting them if you suddenly launch them into the great outdoors!
The Nails
Though they are the bane of every dog owner’s existence, a dog’s claws are vital for proper locomotion, exploration, and protection. However, most dogs live a more pampered life these days, and it often falls to us to keep those nails from overgrowing. Becoming familiar with the anatomy of the claw, as well as the correct location for trimming them, can be a real life-saver. But if in doubt, your vet practice or groomer will be able to help.
How many cervical (neck) vertebrae does a human, blue whale, or giraffe have?
ANSWER: It’s the same as the dog: 7!
With the exception of sloths and manatees, all mammals have 7 vertebrae in their neck.
Canine Dentition
Compared to humans, who have 32 adult teeth and 20 baby (deciduous) teeth, most dogs have 42 adult teeth and 28 deciduous teeth. Puppy teeth start to erupt when they are around 3 weeks old, and should all be present when they are 6 weeks old. Then, when they are about three months old, these deciduous teeth begin to loosen and fall out and will be completely replaced by their permanent (adult) teeth by the time they are six months old. This is why puppies at this age seem to be hell-bent on chewing everything!
| Function | Adult (42) | Puppy (28) | |||||
| Upper | Lower | Total | Upper | Lower | Total | ||
| Incisors | Nibbling | 6 | 6 | 12 | 6 | 6 | 12 |
| Canines | Piercing/killing prey | 2 | 2 | 4 | 2 | 2 | 4 |
| Premolars | Chewing and cutting | 8 | 8 | 16 | 6 | 6 | 12 |
| Molars | Chewing and grinding | 4 | 6 | 10 | – | – | – |
Dog teeth evolved for killing prey, tearing through flesh, chewing through muscle & tendons, and crunching bones. Of course, over several millennia of domestication, our pets’ teeth are rarely required to fulfill such a role, which is very fortunate considering the size and arrangement of some breed’s mouths! Without such rigorous use, these teeth will accumulate tartar and calculus, leading to gingivitis, caries (tooth decay), and infection, so it’s up to us to provide our four-legged family members with appropriate foods, chew toys, and brushing to prevent dental disease before it starts.
TRIVIA QUESTION:
ARE DOGS CLASSED AS
A) Herbivores
B) Carnivores
C) Omnivores
D) Facultative Carnivores
E) All of the Above
Keep reading for the answer!
Eyes
Apart from being soulful and hard to resist, canine eyes have some specific differences to our own.
Tapetum lucidum: the reflective layer that sits behind the retina, at the back of the eye, that makes the eye glow at night. This layer amplifies any light coming into the eye, giving animals better night vision.
Nictitating membrane: more commonly known as the third eyelid, this extra layer of protection helps keep dirt, debris, and sharp branches from damaging the cornea.
Rods and cones: these are the cells that make up the retina. Rods detect light levels and motion, and cones differentiate color. Humans have three types of cones, allowing us to see a wider spectrum of colors and distinguish more details and shapes. Dogs only have two cone types, but they have more rods, which allow them to see better at night, track prey, and see objects moving more than half a mile away.
Ears
The ear consists of the external ear – ear flap (pinna), ear cartilage, ear canal, middle ear, and inner ear, which contains the cochlea and vestibular apparatus. The L-shape of the vertical and horizontal ear canals helps to funnel sound waves into the ear, whilst keeping debris out (although this is not always successful!). The size, position, and shape of the pinnae have changed over time through breeding. Hunting dogs tend to have alert, pricked up ears, while scenting dogs have long, floppy ears that help waft scents up towards the nose.
More recently, ear appearance has changed according to aesthetic preferences and fashion, and those that haven’t evolved the preferred appearance, like Dobermans, Great Danes, or Pitbulls, often have them amputated to achieve the desired result, in a practice that is, fortunately, falling out of favor.
Nose
You can’t hear the words ‘wet nose’ without immediately thinking ‘dog.’ The leathery outer part of the canine nose (nasal planum) usually feels moist and cool, and many think that a dry or warm nose are indicators of illness, but this isn’t necessarily the case.
The nasal planum is often kept moist by mucus from the nostrils and saliva from the tongue. It is also one of the few places on a dog’s body surface with active sweat glands. We believe that keeping the nose wet helps prevent dirt and debris from going in, but it may also encourage microscopic olfactory particles to stick to the nose.
Although a dry nose may be a sign of dehydration or disease of the nasal planum, it can also just be a result of dry, windy weather. Equally, a warm nose can result from a fever, but it may also feel warm in hot weather or after exercise, so it’s important to look at the whole picture before jumping to conclusions about what your dog’s nose might be trying to tell you.
The Digestive System
With ancestors like wolves and other wild dog species, it’s no surprise that the digestive system of the dog is similar to most carnivore species, with a simple stomach, fairly short small intestine, simple large intestine (colon), and pancreas.
Interestingly, the process of domestication has had an impact on the canine digestive tract, with modern-day dogs having gut microbiota that have adapted to eating more starchy foods.
ANSWER:
ARE DOGS CLASSED AS
A) Herbivores
B) Carnivores
C) Omnivores
D) Facultative Carnivores
E) All of the Above*
*Depending on who you talk to or which studies you read, dogs are usually classified as facultative carnivores or omnivores. Through domestication, the canine digestive system has evolved from one of a complete carnivore to one that can digest a wide variety of foods. Some dogs still consume diets that are almost exclusively based on meat, but recent research has shown that dogs fed on a carefully formulated plant-based diet are thriving. This should not be confused with the herbivorous diet of cattle or horses, but one that is only possible through careful balancing of alternative protein sources.
It seems that, after spending thousands of years evolving alongside humans, dogs have also developed a digestive plasticity like our own. Always talk to your vet about which diet is best for your dog.
The Tail
We have finally reached the end of our anatomy journey, appropriately at the tail. With up to 23 vertebrae, the tail is the longest section of the dog’s spine, but this varies between breeds and individuals. There are powerful muscle attachments from the sacral vertebrae and pelvis that allow that tail to wag with joy, drop in fear, or stand tall at attention. The spinal cord does not enter the tail. Instead, dozens of nerves fan out from the base of the cord, called the cauda equina (Latin for “horse tail”), to supply the tail.
Some dogs develop a tail strain injury known as “swimmer’s tail,” where the nerves and muscles become fatigued from excessive wagging or using their tail as a rudder when swimming. Another relatively common tail problem is colloquially termed “happy tail syndrome,” which occurs when the tail tip is damaged by hitting hard surfaces. The lack of sensation at the end of the tail means that dogs can have a bleeding cut at the end of the tail without feeling it – a rather messy problem!
Stumpy or coiled tails are the result of a genetic mutation that has been targeted by selective breeding. In some breeds, particularly the French Bulldog and Pug, this has inadvertently led to an increased frequency in vertebral malformations further up the spine, as well as pain where the tail has curled too tightly on itself.
Class Dismissed!
We’ve covered quite a lot of information here, but there is still plenty more to learn. Hopefully, this will help you understand your dog’s body a little better. Time to show off your new anatomy knowledge!
Featured Image Credit: Julia Suhareva, Shutterstock
