The dog has three types of muscle

  • Smooth muscle:  not striated - Controls movement of viscera/inside
    [The Noniated Muscles: (In-Voluntary) 
    Noniated Muscles are a reflex action, which is mostly associated with the Smooth and Cardiac Muscles, which are considered Involuntary or Autonomic they primarily play the part for digestive, respiratory, circulatory, 
    and euro-genital systems.]

  • Cardiac muscles:  striated - makes up the bulk of the heart tissue
    [The Striated Muscle forms nearly 1/2 of your pet’s entire body weight.
    Skeletal muscles for the most part are considered Voluntary. 
    Skeletal muscles have the ability for High Elasticity as they provide Contractual support allowing the Dog to perform Lateral, backwards and forward movements with little strain. These muscles for the most part come in pairs and are attached by their extremities to "Two-More" bones. When in action one muscle is fixed this is called the "Origin", movable is called the "Insertion".
    Dogs are very agile-flexible this allows for a greater range of movement and stamina. Voluntary muscles are distinguished from one another by various names, which refer to there primary action such as: 
    (Flexor) bending action. (Extensor) straightening action. Other associations are: Depressors, Constrictors, and Dilators etc.]

  • Skeletal muscles:   striated - makes up the rest

The function of muscles is to provide movement to the dog’s body. Muscles are made up of two types smooth and striated. Smooth muscles are within the internal organs i.e. intestines, stomach and bladder. The dog cannot voluntary or consciously control these muscles or the cardiac muscles. Skeletal muscles are under conscious control they aid walking, eating or wagging the tail also eye movement, head movement etc.. The dog can control skeletal muscle which can be contracted or relaxed at will. Most muscles consist of bundle of long cells & muscle fibre. Muscles attached to the bones by tendons. The stomach has a very thin covering of muscle. Thigh muscles give explosive energy for chasing. The Hip muscles support the back legs. Tail muscles allow the tail to raise, curl and wag. The neck muscles let the head turn over 220 degrees. Extrinsic muscles are those that you see and are voluntary muscles. The intrinsic muscles control the internal force and action, and extrinsic muscles provide range of movement.

Types of muscles

  • Strap muscles:   parallel Fibres – The fibres are arranged parallel to the tendon of insertion this results in a greater range of shortening and provides greater movement velocity distance per time. Examples of these muscles are the sternothyroid and sternothoid in the neck.

  • Pennate muscle:   unipennate – The fibres are arranged at an angle to the direction in which the tendons moves. This results in a greater area of muscle fibres along axes of contraction and produces more strength.

  • Unipennate:   ulnar & radial heads of digital flexors muscles

  • Bipennate:   infraspinatus muscle attached to the scapula

  • Multipennate:   humeral head of the deep digital flexor muscles, deltoid muscle

Most muscles consist of bundle of long cells & muscle fibre. Muscles attached to the bones by tendons. The stomach has a very thin covering of muscle. Thigh muscles give explosive energy for chasing. The Hip muscles support the back legs. Tail muscles allow the tail to raise, curl and wag. The neck muscles let the head turn over 220 degrees. 

Skeletal Muscle Groups

  • Muscles of mastication:  operate temporomandibular joint.
    The temporomandibular joint connects the lower jaw, called the mandible, to the temporal bone. To close the jaw the dog uses temporal and masseter muscles to open the jaw dogs use digastricus muscles and gravity

  • Muscles of facial expressions:   move nose, lips, eye lids, ears and skin

  • Muscles of the pharynx:   (striated muscle) three parts nasopharynx, oropharynx & laryngopharynx

Tongue muscles 

  • Intrinsic:  forms tongue this muscles curls and bends the tongue

  • Extrinsic:  moves the tongue in and out

  • Genioglossus:  protracts the tongue

  • Hyoglossus:  retracts the tongue

  • Styloglossus:  retracts the tongue

  • Lyssa:  fibrous tissue enveloping fat & muscle

Eye Muscles

  • Extrinsic eye muscles:  made up of seven muscles

  • Oculomotor:  dorsal rectus, medial rectus, ventral rectus and ventral oblique muscles

  • Trochlear:  dorsal oblique muscle

  • Abducent:  Lateral rectus and retractor bulbi muscles


These connect the muscles to the bones they are tough fibrous bonds of collagen, a fibrous protein called tendons. They may be flat as in the toes or round as in the triceps that connect to the elbow. The tendons must be strong but able to glide freely to ensure easy movement.

Osteology ‘The study of bones’

The dog’s skeleton averages 319 bones. The skeleton is composed of three types

  • Appendicular skeleton:  portion of the skeletal system includes all of the bones contained in the limbs (appendages) of the animal. ... click to view

  • Axial skeleton:  the bones of the skull, spine, ribs and sternum. The axial portion of the skeletal system is located along the longitudinal axis of the animal. ... click to view

  • Visceral skeleton:  bone that forms part of an organ (such as the middle ear ossicles). The bones of the visceral skeleton are formed in soft organs. They are not present in all species of animals. Examples of visceral skeleton bones include: os penis - bone in the penis of dogs

Bones are rigid living organs that have their own supply of blood vessels and nerves. They are made up of minerals, primarily calcium and phosphorous. They make the frame structure of the body. They also protect the delicate internal organs. The skull protects the brain and eyes. The breastplate and ribs protect the heart and lungs. The other bones provide support and locomotion i.e. legs, tail and neck. The bones in the ear provide sound transmission and help the dog to hear. Bones come in different shapes and sizes:-

  • Long bones:  length is greater than the diameter found in the limbs. Bones commonly found in the appendicular skeleton, consisting of a diaphysis and an epiphysis.

  • Short bones:  approximately equivalent dimensions found in the carpus, metacarpus and tarsus, metatarsus.

  • Flat bones:  e.g scapula and many bones in the skull where they surround and protect the eye, ear, sinuses, and brain and they are found in the pelvis where they provide for the attachment of muscles and long bones.

  • Irregular bones:  short and multiple bones e.g. vertebrae and all bones of the skull that are not of the flat type, and three parts of the hip bone.

  • Sesamoid bones:  small bones within tendons e.g. knee cap (patella). Small cuboidal shaped bones associated with tendons and ligaments. These bone reduce wear and tear on the tendons and as they pass over a articulation or prominence. 

The outside of a bone is called the cortex made up of minerals and protein. The inside of the bone is marrow. The bones are held together by specialised connective tissues called ligaments. This is called an articulation or joint. Bone ends would grate causing wear and tear if not for the cartilage. Although cartilage is tough it can be damaged by joint stress and trauma. Joints of the hip and shoulder are called ball & socket. The rest of the limb joints are of the hinge type they flex and extend in a plane from front to back. Dogs and humans skeletal anatomy have much in common. One difference – humans carry weight on their hip and the dog carries 75% of weight on his shoulder joints and front end.

Common bone features 

  • Articular surface:  a joint surface
    • Condyle:   a large rounded articular surface 

      Head:  the rounded proximal articular surface of many long bones -- united to shaft by "neck"
    • Facet:  a flat articular surface 

  • Processes:  lumps and bumps on bones
    • tuber, tubercle, tuberosity, trochanter

  • Holes or Depressed Areas
    • Foramen:  a hole in a bone
    • Fossa:  a depressed area on a bone

Arthrology ‘The study of joints’

Ligaments connect bone to bone generally found across joints. Joints are where two bones meet the ends covered by a layer of smooth cartilage.

Fibrous joints:  immobile joints joined by fibrous tissue.

  • Suture  undulating seams between bones of the skull 

  • Gomphosis  tooth in an alveolus, united by periodontal ligament (see teeth diagram)

  • Syndesmosis bones joined by ligaments, e.g., radius, ulna, tibia and fibula long bones running parallel.

Cartilaginous joints:  immobile joints joined by cartilage.

  • Symphysis: fibrocartilage union e.g. pelvic symphysis, mandibular symphysis

  • Synchondrosis:  hyaline cartilage union, e.g., physis 

Synovial joints:  mobile joints, fibrous tissue enclosing a synovial cavity

  • Simple joint:  formed by two bones, e.g., shoulder joint 

  • Compound joint:  formed by more than two bones, e.g. elbow joint 

  • Hinge joint movement in one plane 

  • Ball & socket joint:  capable of circumduction 

  • Plane joint:  gliding action, e.g., vertebral articular 



Domestic dog, carnivorous mammal, generally considered the first domesticated animal. The domesticated dog has coexisted with human beings as a working partner and household pet in all eras and cultures since the days of the cave dwellers. It is generally believed that the direct ancestor of the domestic dog is the wolf, originally found throughout Europe, Asia, and North America. Remains of a dog, estimated to be 10,500 years old, have been found in Idaho. 

TAXONOMY Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Canidae Genus: Canis Species: Canis familiaris 


Little is known about wild dogs of the past but that they are carnivores: hunters and scavengers. This means that they are secondary consumers in web chains. Even though they are carnivores they sometimes accept eating green plants. The ecology of dogs right know is that it helps the human in many fields of life. Since the cave dweller times, dogs have been domesticated by humans and it has helped him to hunt, in herding, protection, etc. It has been very important as a work animal and as a psychological support for humans. The habitat of the dog is where it's owner lives. Different dogs have different adaptations to their ancestral habitat but nowadays, this is not applicable. 



The skeleton of the dog is the articulated structure, moved by the muscles, that holds the dog's body and protects some organs and the nervous system. It also functions as mineral and blood deposit of the body. The skeleton of a dog is made up of approximately 321 bones: 134 form the axial skeleton (skull, vertebrae, ribs, etc.), and 186 form the appendicular skeleton (appendages). An extra bone has to be added for male dogs which is the penile bone. The dog is a digigraded animal (it walks with it's toes). It lies on it's third phalanges which are protected by palm cushions. The dog's toes are arranged in an angle which gives more facility of rest after running or other activities. The teeth of the dog is composed of 42 teeth which include canines, molars, incisors, etc.


Joints permit the movement of the bones. There are three types of joints in a dog: fixed joints, movable joints, and semi-movable joints. Fixed joints, such as the ones in the skull don't permit any movement but keep the bones together. The semi-movable joints are those that permit a little movement. They are represented in the spinal column. The movable joints are those present in the rest of the bones. Inside of this group of joints there are various types: the hinge, the ball-and-socket, the pivot, and the gliding joints. The most movable joints are present in the appendages. Joints are held together by a fibrous wrapping, the joint capsule, which reinforced by ligaments. Muscles and tendons also help keeping the bones together. 


There are three types of muscles in a dog: the skeletal muscle, cardiac muscle, and smooth muscle. The skeletal muscle works in pairs, a flexor and an extensor. It permits the movement of the skeleton and it also moves the skin of the dog (cutaneal muscle is very developed in dogs). The cardiac muscle is the muscle which is exclusively in the heart. The smooth muscle is the one present in the walls of the digestive organs, arteries, veins, in the diaphragm (which separates the two cavities of the body: the thorax and abdomen), and in some other internal organs. 


The digestive system of a dog is very similar to a human one. It ensures the ingestion of food and it's transformation (by mechanical and chemical acts) to simple substances which the dog's body can absorb and assimilate. It all starts in the mouth, where food is broken down mechanically and also a little chemically (saliva and teeth). Then they pass through the esophageus to the stomach to the small intestine (only 3 meters long but it has a very strong digestion) and to the large intestine where the faeces are made. Then excretion is made through the rectum and then through the anus. A series of added glands produce substances which are used in digestion and they perform various important jobs. The most important one is the liver (which is an organ). 


In the respiratory system, air goes through the nostrils in the snout. In the nasal cavity air is purified, moistened and warmed. Then it passes through pharynx, larynx (where vocal cords for barking are), and then to the trachea. Air is then canalised through the two bronchi and then inside the lung by the bronchioles which are subdivisions of the bronchi. The last part are the alveoli where oxygen diffuses to the blood. 


The urinary system, is composed by the kidneys and the urinary structures. The blood is purified of toxins and excess water by the kidneys. Then the toxic substances are diluted in urine which passes through the ureters to the urinary bladder (deposit for urine). Then it is released through the urethra to the exterior of the body. 


It is composed of the genital glands (gonads) that produce the reproductive cells, the genital conducts that ensure the transport of the sex cells, and the copulation organs that permit the encounter of the gametes. 


The gonads of the female dog are composed of the ovaries, which are under the kidneys. The ovaries produce the eggs. The ovaries are active after the dog is 4 to 6 months old and the process occurs each 6 months more or less (this is normally called "in heat"). After the egg is produce it goes to the oviduct where it waits for a sperm cell to fertilize it. If a zygote is formed then it stays in the uterus for two weeks until it sticks to the wall. The vagina which is quite long in dogs is used as birth and copulation canal. 


The gonads of the male are the testicles which are inside a sac and the scrotum. The sperm cells are produced here. The prostate is the gland that produces the liquid in which sperm cells are carried. Sperm cells go out through the urethra, which is surrounded by the penile bone, like urine does. The penis of the dog has tissue around the urethra which capable of dilatation when extra blood is pumped (for copulation). 


It includes a four chambered heart, arteries, veins, and lymphatic glands and vessels. The circulation of blood provides the dog's body with oxygen and removes carbon dioxide from it. Oxygenated blood and deoxygenated blood circulate separately 


The nervous system is composed of the central nervous system, the peripheral nervous system, and the autonomous nervous system. The central nervous system is composed by the brain and the spinal cord. The peripheral n. s. is made up of the nerve cells and the autonomous one is made up of the sympathetic and the parasympathetic systems. The autonomous nerve system is connected to the spinal cord and the peripheral to the spinal cord or the brain. 


The eye of a dog consists of the cornea, the iris, the lens, the retina, the choroid coat, the sclera, and the optic nerve which sends the image absorbed by the retina to the brain. The ear of the dog is long and curved. It is composed of the same structures as that of a human: the tympanic membrane, the series of tiny bones such as the hammer, the semi circular canals, etc. The touch of the dog is well developed specially in the legs and the tongue. 



Deoxygenated blood from the right atrium, go to the right ventricle. It then goes through the pulmonary arteries which go to the alveoli capillaries where oxygen diffuses into the blood. Then it returns to the heart, to the left atrium, by the pulmonary veins. From the left ventricle, the blood, rich in oxygen, go to the Aorta, the principal artery which divides and divides into arterioles to reach every cell of the dog's body. Blood pressure is maintained in arteries by the smooth muscle surrounding it. Through the capillaries, the oxygen and the carbon dioxide exchange. Blood moves through veins because of the skeletal muscle movement. In veins blood is prevented to go back by opening and closing valves. Through the veins, to the vena cava, and then to the right atrium, the blood reaches the heart again and the cycle is repeated. The lymphatic system protects the organism from dangerous micro-organisms (it produces and contains white blood cells which produce antibodies against intruders) and drains the inter cellular spaces. The heart beat rate of dogs varies depending on the size of the dog and it's training but it is faster than that of a human. 


Air reaches the lungs by the same system as in humans. The diaphragm contracts, decreasing the size of the lung and therefore expelling the air. Then it relaxes and the change of pressure in the lungs compared to that of the outside, forces air in and the lungs inflate. As heart beat, the respiration rate is faster than in humans. 



The dog needs to drink huge amounts of water for it's body needs since a lot of it is lost in excretion, urine, and the evaporation of water in the respiratory canals. The dog drinks by moving it's tongue back and forth and in form of a spoon. Like that it impulses the water into the mouth. The dog keeps the big chunks of food still with it's anterior paws which serve as "hands". To eat solid food, the dog moves it's head into the food source, penetrating the food in it's mouth. The teeth perform an indispensable job in mechanical digestion even though it is not very effective in the dog since the mandible does not move laterally. The secretion of saliva can be produced by the taste or smell stimulus or when the dog perceive that food is present. 


The dog's stomach only contracts when food is present to mix the food intensively. The daily production of gastric juice is between 2 and 3 litres. This mixture of hydrochloric acid and enzymes breakdown proteins and separate the conjunctive tissue of meat. The production of gastric juice happens when food is ingested or when it's eating time in the dog's schedule. 


The chime, the product from the stomach, passes to the small intestine. The pancreatic juice in the intestine contains enzymes that attack fats, proteins, and starch. The volume of the bile from the liver secreted is of 25milliters per kilogram of the dog's weight each day. Bile helps to set favourable conditions for the pancreatic juice to work and it eliminates different wastes. The gastric juice finish the breaking down of food. The products are then absorbed into the blood in the small intestine. Some go to the blood (water, minerals, sugars, amino acids, and some fats) and some go the lymph (fats). In puppies, digestion of maternal milk is restricted to the stomach. A special enzyme which finishes after lactancy is present for the breakdown of milk. 


The wastes pass through the large intestine where the faeces are produced. Then these faeces go through the rectum and then to the anus where they finally leave the body. 


The male reproductory system as well as the female one is controlled by the hormones coming from the hypophysis and the nerve signals coming from the hypothalamus. THE MALE DOG The reproduction organs of the dog function yearlong. One millimetre of sperm contains 100,000 to 200,000 and the volume and concentration of these lower if the dog copulates too much in a row. The testosterone, the sex hormone of the male dog, is vital for the production of sperm cells, the definition of secondary sex characteristics (bigger size and weight, and the bark is of a lower pitch), and sexual behaviour. 


The reproduction system of the female dog is much more complex than that of a male dog. The female dog has estruation each six months more or less. There are four periods in the sexual cycle: the pro estruation, the menstruation, the posestruation, and the anaestruation. The anaestruation is the period in which the dog is not ready for reproduction. In the proestruation, which lasts for 9 days, the follicles grow in the ovaries. The dog's vagina swells and it spills a mucus type substance, later blood. The estruation comes next and it longs for 4 or 8 days. There is no more blood coming from the vagina but the dog is very nervous. Ovulation occurs starts 3 to 5 days after the estruation starts. It lasts 12 to 72 hours. The posestruation occurs if the dog is not pregnant and it lasts for 2 months. The walls of the uterus widen because of the progesterone secreted in the ovary. The anestro lasts 3 months and a half and it is the resting of the female reproductive system. 


Copulation permits the contact of eggs and sperm cells to form zygotes. The penis of the dog gets erect by the filling of erectal tissue with blood and the penile bone. Then it is introduced into the relaxed and lubricated vagina of the dog. Then the penis of the dog expands and the dog's vagina muscles contract trapping the penis in the vagina for a long time. The vagina then produces three ejaculations in the male dog. The middle one with more sperm than the other two. The female dog then liberates the male dog and there is a possibility of offspring production. If the dog gets fertilized then it takes 2 months for the parturition of the puppies. In the parturition, the female dog expels the puppy after it releases the amniotic liquid and the foetal sacs. The female cleans the puppy with its tongue to familiarize it with its mother and motivate its physiological functions and then it licks its vagina carefully. After. The placenta is released and it is eaten by the mother. The period between puppy is between half and one hour. The puppies sucking of the mother's breast is essential for the further production of milk. The puppies' bumps, with their heads, on their mother's breasts is important for the production of hypofisiary hormones in the dog. 


The dog is a ectothermic animal and its body temperature is of 38.5°C. The temperature regulators in young puppies is still imperfect so it needs the heat of the mother or other siblings. The puppy regulates its temperature but it is higher than that of an adult dog. When temperatures are low the dog uses two different systems to prevent heat loss: lower the loss of heat or producing more heat. The dog can lower the loss of heat by constricting the blood vessels of peripheral regions (extremities, ears, skin). This lowers the amount of heat exposed to the environment. The fur also isolate the body from cold. Little muscles in the bases of the hairs make hairs stiffer, increasing the warm air layer between the fur and the skin. The dog also arcs its body or goes near other dogs to prevent heat loss. If this is not enough, the dog uses more energy to keep the body temperature stable. The metabolism works more intensely (specially in the liver) and fat and other energy sources are used. The contrary situation is also possible, too much heat. The dog uses the blood vessels to release extra heat. It dilates them in the peripheral regions to release the extra energy to the surroundings. When heat is too much the dog seeks cool and humid places to rest and release their heat. In other mammals evaporation of sweat coming from sweat glands alleviate the heat but the sweat glands in dogs are few and located in the feet cushions that makes them quite inefficient. As compensation for the scarce sweat released, the dog has a cooling system based in the mouth. The water of the mouth cavity, bronchi, and trachea evaporates with an intense respiratory rate. This cools the body of the dog but water losses are huge. 


The smell is the most developed sense in the dog. A dog is capable of smelling a drop of acetic acid which has been diluted in a thousand litres of water. A drop of this solution is diluted in a thousand litres and the dog is still capable of detecting the acetic acid. The olfactory capability of dogs depend on the type of dog, the race. As comparison to the human smell, an average dog has 147 million smelling units... the human only has 500,000. The nose and the receptors function as the ones of other mammals such as humans. The ear is also quite developed in dogs. Dogs can hear very high pitched sounds such as ultrasonic. The auditive range of dogs is between 20Hz and 60,000Hz. The human's is from 16Hz to 20,000Hz. The functioning of the ear is the same as in a human. The vision capability of the dog is not comparable to that of a human. They can perceive changes of light but their ability to see clear forms is not much. Concerning the colour, not much study has been done on this. Dogs have a special coating in the eye globe that permits the dog to see more light when it is darker. 


Locomotion in dogs are the same as most complex animals. Flexor and extensor muscles work in pairs to move the structures. 



The male dog marks its "hunting" territory by odour signals produced by the anal glands. The smell also attracts females into the territory. When dogs, male and female, urinate at a certain place, that means that that is a common territory. The dog defends it's territory instinctively when an intruder that is not a companion appears. This defence instinct makes it have a guardian quality. 


Dogs that live in groups of dogs have hierarchies, inherited from their wild ancestors. There are two types of dogs: dominant and submissive. This hierarchy not only divides in into two categories, ranks in the canine society also exist. Dogs do not have to live in groups to have these ranks, it is a natural instinct that even exist in dogs living with owners in cities. In the relation with the human owner, the owner is always dominant except in cases where the dog has mental disorders. 


Dogs are polygamists, they don't have a definite couple. When a female dog is in heat, it is nervous and seeks desperately for a male. It releases pheromones in its urine that a male can smell at kilometres of distance. When they meet, the female exposes it's vulva to the male. The sexual act goes after a series of ritual games and a profound smell familiarization. Then comes the copulation. If various dogs follow a female, disputes settle the hierarchy and the most dominant one gets to reproduce. 


Female and male dogs may feel frustrated when they can't find a mating partner. Dogs can get depressed and they may tend to escape from their home. Male dogs relieve themselves by masturbating with an object. Female dogs may get very nervous and develop a hysterical pregnancy in which the dog produces milk as an expression of it's needs to have puppies. 


Days before giving birth, a dog looks for a safe, warm, and comfortable place to place her dogs to be born, a "nest". The dog looks for it's puppies but those who are deformed are killed by the mother herself (natural selection). A mother cuts the umbilical cord and licks it's new born puppies to stimulate physiological functions. If a mother does not do this, then she lacks maternal instinct. A mother is aggressive if she sees that something (an intruder) is menacing its puppies. If the person is familiar, then she is pleased to have them near its puppies. Very few male dogs care for puppies but some do. 



The dog uses it's urine that contains glandular secretions as well as urine. A dog uses this to transmit messages to others and to mark it's territory. To identify themselves, dogs smell each other profoundly especially the snout, the genitals, and the anus. In sexual reproduction smell also plays an important role as I explained above. The dog also can identify a prey by its smell. 


Dogs use to bark to advert others of danger. A howl in the middle of the night is a concentration call. When a dog is in heat a male dog can howl for hours and hours in desperation. The female responds with the same howl. Two dogs that may encounter an bark are just imposing themselves. This makes them have more confidence. The nature of dog sounds can be described as follows: a hoarse and cut bark expresses a menace (a dare to another animal or an advice to an intruder so that it moves out of the territory), a happy bark ( coming to its owner or before going for a walk), a special bark to indicate a prey, etc. Then there are the obvious sounds: cry, lament, howl, screams, and grunts. 


The look of a dog can tell a lot. A fixed look means fear or malevolence. The look of a dog to sheep or cows can control them and place them in order. 


Gestures of a dog to show happiness are the constant movement of the tail. A tail between the legs means fear. A dog in an aggressive posture shapes and moves its body to make it look bigger: it elongates, it's hair gets straight up, etc. To exhibit submission a dog often lies itself on the floor and with its tail between its legs it shows its genitals. When a dog tries to play with another it lies in a sphinx position, or it "dances" around the other dog, or it gently bites the other's tail, etc. 


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