Dieases

Common Infectious Diseases:

Knowledge is the key to disease prevention
Infectious diseases are those diseases that horses can get from each other, or via a vector, such as a 

mosquito, which may transmit the disease from horse to horse. Horse owners can vaccinate their horses against many of these diseases, or their veterinarian may administer the vaccinations.

Some vaccines are considered "core" vaccines that cover diseases that all horses need to be immunized against. Some horses may need to be immunized only when there is significant risk that they will be exposed to the disease in their natural environment, or if they may be exposed when traveling, showing, or competing. In addition, horses must be vaccinated at different life stages. Most vaccines need to be given annually.

 Tetanus (lockjaw)

Tetanus is caused by a bacterial toxin normally found in the soil and in the feces of horses. The bacteria that produce the tetanus toxin need a decreased oxygen supply to multiply, so any area where there is a deep puncture wound or where a wound has healed over (such as the navel stump of a newborn foal) is an area where tetanus can thrive. Symptoms of tetanus include a protrusion of the third eyelid and stiff neck, progressing to overall muscle stiffness causing a 'sawhorse' stance. Tetanus is often fatal, but a yearly vaccine can prevent it, and the vaccine is a good idea because small cuts can go unnoticed and become infected.

Equine Encephalomyelitis (sleeping sickness)
This is a disease that affects the nervous system, and can be caused by equine encephalomyelitis viruses (Eastern, Western, and Venezuelan), which are carried by mosquitoes. Signs include depression and a high fever, followed by a period when the horse appears blind, nervous and uncoordinated, with muscle tremors, and eventually, complete paralysis. Proper vaccination and good mosquito control are important to help prevent this disease.

Equine Influenza
This viral disease is spread by inhalation of drops of infective material. Signs include a dry, hacking cough, sudden onset of fever, watery nasal discharge, weakness, loss of appetite and depression. Infection with equine influenza is rarely fatal but can cause problems such as emphysema, pneumonia or bronchitis.

Equine Herpesvirus (rhinopneumonitis, rhino, viral abortion)
There are 2 types of equine herpesvirus: EHV-1, which causes respiratory disease (fever, cough, nasal discharge), reproductive problems (abortion, stillbirth), and neurological problems (hindlimb weakness, difficulty walking, sometimes paralysis); and EHV-4, which is usually limited to respiratory problems. Once a horse has been infected with EHV-1 or EHV-4, he will always be a carrier, and may shed the virus during times of stress.

West Nile Virus
Horses get WNV by being bitten by an infected mosquito; some horses do not show any signs and recover on their own, but in some horses the infection affects the central nervous system and causes signs including fever, weakness or paralysis of the hind limbs, impaired vision, lack of coordination, head pressing, convulsions, inability to swallow, and coma.

Rabies
This is a viral infection of the central nervous system, and although it is not common in horses, rabies can be transmitted to horses by the bite of an infected animal such as a skunk, raccoon, fox, dog or bat. Rabies can be transmitted to people. We recommend that you check with your veterinarian regarding recommendations for rabies vaccination for your horse.

Strangles 
This contagious respiratory disease is caused by a bacterial infection. Signs include a fever, thick, yellow, nasal discharge and swollen, abscessed lymph nodes under the jaws. The infection is spread by infected material from nasal discharge or abscesses contaminating stalls, feed troughs, pastures, etc. Young horses are the most susceptible to strangles.

Potomac Horse Fever

This disease is a bacterial infection of the blood and tissues. It is much more common in spring, summer and early fall and is only found in certain areas of the country. Signs include a fever, depression, decreased gut sounds, and a profuse, watery diarrhea that can lead to laminitis, colic, dehydration, shock, and death.

Horse Parts and Markings:

If you hang around a stable for any length of time, you'll notice that horse people have a language all their own. This language — which sounds like a foreign tongue to the uninitiated — is what horse people use to describe the intricate details of the horse's body.

Checking out the parts

Nature made horses to be virtual running machines that can reach speeds of nearly 40 miles per hour. The equine body is an impeccably designed combination of muscle and bone in an elegant and graceful package.

People who spend time around horses not only begin to appreciate equine anatomy but also come to understand it. Horse people talk about their horses' bodies the way mechanics talk about cars. In the equine world, if you want to keep up with such conversations, you must know the lingo and the blueprint. Here are some parts of the horse you need to know (see Figure 1 for more.):

• Withers: The area on the horse's back just after the neck but above the shoulders

• Fetlock: The horse's ankle

• Forelock: The hair between the horse's ears that falls onto the forehead

• Hocks: The elbow-like joint of the horse's back legs

• Muzzle: The area of the horse's head that includes the mouth and nostrils

Figure 1: The parts of the horse work together to build a virtual running machine.

Have you heard the expression, No hoof, no horse? Well, it's true. Without healthy hooves, horses can't function well. Becoming familiar with the parts of the horse's hoof (see Figure 2) gives you intimate knowledge of this most important part of the equine body. This knowledge helps you take better care of your horse's tootsies, too.

Figure 2: Knowing the parts of the hoof is essential in caring for horses.

Looking at markings

Leg and facial markings are great for helping to identify individual horses. Each marking has a name, and each name is universal among equine aficionados. Figure 3 shows the most common horse facial markings. Keep in mind that the following patterns often have subtle variations.

• Bald: White that starts above the forehead, goes to the muzzle, and extends beyond the bridge of the nose to the side of the face

• Blaze: Wide white area that runs along the bridge of the nose

• Snip: White spot located on the muzzle, between or just below the nostrils

• Star: White spot on the forehead

• Stripe: Narrow white stripe down the center of the face, on the bridge of the nose

Figure 3: Facial markings can help you identify individual horses.

Figure 4 shows typical white leg markings on horses. They include

• Coronet: A small white band just above the hoof

• Half cannon: A white marking that extends from the edge of the hoof halfway up the middle of the leg

• Half pastern: A white marking that extends from the edge of the hoof halfway up the pastern

• Sock: A white marking that extends from edge of the hoof two-thirds of the way up the leg

• Stocking: A white marking that extends from edge of the hoof to the knee or hock

Figure 4: You can find a variety of white leg markings on horses.

Basic Horse Body Parts You Need to Know

Knowing the basic horse body parts opens a whole new world "language." If you don't know the basics, you can quickly find yourself lost and overwhelmed in a typical barn conversation with other students or riders.Barn talk aside, having a grasp of these basic parts of the horse will help you to communicate more effectively with your instructor, trainer, and other equine professionals such as farriers, veterinarians, and equine massage therapists.

Face - The face and head are the most sensitive of horse body parts.

When you first start with horses be aware that some horses can startle quickly or may not like their face touched. More explanation on the details of the face can be found below.

Neck- Joins the head to the trunk

He should be able to bend his neck equally well to either side. Often though just like us, they have an easier time bending to one side.

They are also able to flex the neck and "arch" it.

Chest- The chest is the front of the horse and houses the powerful pectoral muscles

Shoulder- Where the shoulder blade is found, and is connected to one of the front leg bones.

Horses don't have a collar bone like humans do. That means their shoulders are attached to the body only by connective tissue, not through any bony connection.

Elbow- The elbow is a joint found near the top of the front leg on the toward the back side of the leg

Knee- The knee is a joint on the front leg joining what we would consider the upper and lower part of the leg.

Most everything found below the knees is tendons, not muscles. An injury below the knee will generally heal slower because of this.

Ankle- The ankle is also known as the fetlock joint.

It is the next joint down from the knee on the front legs and the hocks on the back legs.

Pastern- This is a joint between the hoof and the ankle on both the front and back legs

Hoof- The hoof is below the pastern on all four legs

The hoof is arguably one of the most important horse body parts. In structure it is somewhat comparable to our fingernails. Proper hoof care and shoeing can mean the difference between a sound and unsound (lame) horse. 

Withers- The withers are an important part of the horse and are the tallest point of the back.

Horses are measured using the withers as a guiding point.

Back- The top part of the body from the withers back towards the croup.

The strong back is where you are seated to ride. There are many muscles found there. The saddle sits on the back near the withers.

Loin, Croup, and Buttocks- These areas on the back of the horse comprise the "powerhouse." 

Barrel- The barrel is the big round area of the horse's body below the back where the ribs are located.

Some horses are referred to as wide or wide barreled because the circumference of the ribs and muscle on both sides of the barrel is very large. This can make saddle fitting a challenge.

Flank- Found on both sides of the body where the hind legs join into the body.

The flanks can be very sensitive body parts and horses can be reactive when touched there

Stifle- This rear leg joint is comparable to our knees in structure

Some horses get a "locking stifle." That is a condition where the joint "catches" temporarily.

Hock- The hocks are found on each rear leg and when seen they look to jut towards the back of the horse.

The hock is prone to injury with overuse from riding as well as from trauma such as receiving a kick from his pasture buddy.

Parts of the Head and Face

8

Ears - The ears are found at the top of the head and can rotate around to hear sounds in different directions

Horses have excellent hearing. They can also become "ear shy" from improper handling while haltering, clipping, and bridling.

• Always handle the ears gently

Poll- The poll is the area right between and behind the ears.

It is considered the highest point of the horse body parts, as the ears are not counted. When they flex at the poll in response to the bit it is considered a good thing.

Eyes- Horses see differently than we do. Go to this page to see how their vision effects handling and riding

Nostril and Nose - The nose is the part where a horse smells and breathes.

A colored or consistent discharge from the nose can signal infection

Muzzle - The muzzle is the front part of the nose and includes the nostrils

Horses can "curl up" the top lip in order to get a better smell of something. They often do this "flehman" when they think something tastes or smells funny or different.

Jaw- The jaw or cheekbone has a circular appearance

Throatlatch- The skinnier area of the neck right behind the cheekbone

Most bridles have a skinny strap that is called a "throatlatch" that fastens in this area. The horse's windpipe passes through the throat latch area.

PARTS OF A HORSE 

1 - Ear, 2 - Forelock, 3 - Temple, 4 - Eye, 5 - Nose, 6 - Nostril, 7 - Muzzle, 8 - Lips, 9 - Chin, 10 - Chin groove, 11- Branches of jaw, 12 - Cheek, 13 - Occipital crest, 14 - Poll, 15 - Crest, 16 - Neck, 17 - Throat latch, 18 - Jugular groove,19 - Point of shoulder, 20 - Chest, 21 - Shoulder, 22 - Upper arm, 23 - Forearm, 24 - Knee, 25 - Cannon, 26 - Fetlock joint, 27 - Pastern, 28 - Coronet, 29 - Hoof, 30 - Withers, 31 - Back, 32 - Loin, 33 - Point of hip, 34 - Dock of tail, 35 - Croup, 36 - Rips, 37 - Flank, 38 - Brisket, 39 - Belly, 40 - Thigh, haunch, 41 - Buttock, 42 - Point of buttock, 43 - Stifle,44 - Gaskin, 45 - Hock, 46 - Suspensory ligament, 47 - Tendon, 48 - Chestnut, 49 - Elbow, 50 - Heel, 51 - Trapezium, 52- Tail,  

• back: the area where the saddle sits, beginning at the end of the withers, extending to the last thoracic vertebrae (colloquially includes the loin or "coupling," though technically incorrect usage) barrel: the body of the horse,[1][2] enclosing the rib cage and the major internal organs buttock: the part of the hindquarters behind the thighs and below the root of the tail cannon or cannon bone: the area between the knee or hock and the fetlock joint, sometimes called the "shin" of the horse, though technically it is the metacarpal III chestnut: a callosity on the inside of each leg chin groove: the part of the horse's head behind the lower lip and chin, the area that dips down slightly on the lower jaw; area where the curb chain of certain bits is fastened coupling: see "Loin" below coronet or coronary band: the ring of soft tissue just above the horny hoof that blends into the skin of the leg crest: the upper portion of the neck where the mane grows croup: the topline of the hindquarters, beginning at the hip, extending proximate to the sacral vertebrae and stopping at the dock of the tail (where the coccygeal vertebrae begin); sometimes called "rump" dock: the living part of the tail,[3] consisting of the coccygeal vertebrae, muscles and ligaments. Sometimes used colloquially to refer to the root of the tail, below. elbow: The joint of the front leg at the point where the belly of the horse meets the leg. Homologous to the elbow in humans ergot: a callosity on the back of the fetlock face: the area between the forehead and the tip of the upper lip fetlock: sometimes called the "ankle" of the horse, though it is not the same skeletal structure as an ankle in humans; known to anatomists as the metacarpophalangeal (front) or metatarsophalangeal (hind) joint; homologous to the "ball" of the foot or the metacarpophalangeal joints of the fingers in humans flank: where the hind legs and the barrel meet, specifically the area right behind the rib cage and in front of the stifle joint forearm: the area of the front leg between the knee and elbow, consisting of the fused radius and ulna, and all the tissue around these bones; anatomically, the antebrachium. forehead: the area between the poll, the eyes and the arch of the nose forelock: the continuation of the mane, which hangs from between the ears down onto the forehead of the horse frog: the highly elastic wedge-shaped mass on the underside of the hoof, which normally makes contact with the ground every stride, and supports both the locomotion and circulation of the horse gaskin: the large muscle on the hind leg, just above the hock, below the stifle, homologous to the calf of a human girth or heartgirth: the area right behind the elbow of the horse, where the girth of the saddle would go; this area should be where the barrel is at its greatest diameter in a properly-conditioned horse that is not pregnant or obese hindquarters: the large, muscular area of the hind legs, above the stifle and behind the barrel hock: the tarsus of the horse (hindlimb equivalent to the human ankle and heel), the large joint on the hind leg hoof: the foot of the horse; the hoof wall is the tough outside covering of the hoof that comes into contact with the ground and is, in many respects, a much larger and stronger version of the human fingernail jugular groove: the line of indentation on the lower portion of the neck, can be seen from either side, just above the windpipe; beneath this area run the jugular vein, the carotid artery and part of the sympathetic trunk knee: the carpus of the horse (equivalent to the human wrist), the large joint in the front legs, above the cannon bone loin: the area right behind the saddle, going from the last rib to the croup, anatomically approximate to the lumbar spine mane: long and relatively coarse hair growing from the dorsal ridge of the neck muzzle: the chin, mouth, and nostrils of the face pastern: the connection between the coronet and the fetlock, made up of the middle and proximal phalanx poll: commonly refers to the poll joint at the beginning of the neck, immediately behind the ears, a slight depression at the joint where the atlas (C1) meets the occipital crest; anatomically, the occipital crest itself is the "poll" root of the tail or root of the dock: the point where the tail is "set on" (attached) to the rump;[3] Sometimes also called the "dock" shoulder: made up of the scapula and associated muscles, runs from the withers to the point of shoulder (the joint at the front of the chest, i.e. the glenoid); the angle of the shoulder has a great effect on the horse's movement and jumping ability, and is an important aspect of equine conformation splints: bones found on each of the legs, on either side of the cannon bone (8 total); partially vestigial, these bones support the corresponding carpal bones in the forelimb, and the corresponding tarsal bones in the hindlimb;[4] anatomically referred to asMetacarpal/Metatarsal II (on the medial aspect (inside)) and IV (on the lateral aspect (outside)) stifle: corresponds to the knee of a human, consists of the articulation between femur and tibia, as well as the articulation between patella and femur tail: the long hairs which grow from the dock; may also include the dock[3] throatlatch[5][6][7][8][9][10] (also, throttle, throatlash[citation needed], throat[11] ): the point at which the windpipe meets the head at the underside of the jaw,[3] corresponding to where the eponymous part of a bridle goes.[12] withers: the highest point of the thoracic vertebrae, the point just above the tops of the shoulder blades, seen best with horse standing square and head slightly lowered; the height of the horse is measured at the withers. Digestive system: Horses and other Equids evolved as grazing animals, adapted to eating small amounts of the same kind of food all day long. In the wild, the horse adapted to eating prairie grasses in semi-arid regions and traveling significant distances each day in order to obtain adequate nutrition.[13] Therefore, the digestive system of a horse is about 100 feet (30 m) long, and most of this is intestines. Mouth Digestion begins in the mouth, which is also called the "oral cavity." It is made up of the teeth, the hard palate, the soft palate, the tongue and related muscles, the cheeks and the lips. Horses also have three pairs of salivary glands, the parotid (largest salivary gland and located near the poll), submaxillary (located in the jaw), and sublingual (located under the tongue). Horses select pieces of forage and pick up finer foods, such as grain, with their sensitive, prehensile lips. The front teeth of the horse, called incisors, clip forage, and food is then pushed back in the mouth by thetongue, and ground up for swallowing by the premolars and molars.[14] Esophagus The esophagus is about 4 to 5 feet (1.2 to 1.5 m) in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot vomit.[14] The esophagus is also the area of the digestive tract where horses may suffer from choke. Stomach Horses have a relatively small stomach for their size, and this limits the amount of feed a horse can take in at one time. The average sized horse (800 to 1,200 pounds (360 to 540 kg)) has a stomach with a capacity of around 4 US gallons (15 L), and works best when it contains about 2 US gallons (7.6 L). Because the stomach empties when 2/3 full, whether stomach enzymes have completed their processing of the food or not, and doing so prevents full digestion and proper utilization of feed, continuous foraging or several small feedings per day are preferable to one or two large ones.[14] The horse stomach consists of a non-glandular proximal region (saccus cecus), divided by a distinct border, the margo plicatus, from the glandular distal stomach.[15 In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin allows for the further breakdown of proteins into amino acid chains.[14] Other enzymes include resin and lipase. Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down into chyme.[citation needed] It then leaves the stomach through the pyloric valve, which controls the flow of food out of stomach.[citation needed] Small intestine The horse’s small intestine is 50 to 70 feet (15 to 21 m) long and holds 10 to 12 US gallons (38 to 45 L). This is the major digestive organ, and where most nutrients are absorbed.[16] It has three parts, the duodenum, jejunum and ileum. The majority of digestion occurs in the duodenum while the majority of absorption occurs in the jejunum. Bile from the liver aids in digesting fats in the duodenum combined with enzymes from the pancreas and small intestine . Horses do not have a gall bladder, so bile flows constantly.[14] Most food is digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamines A, D, and E. Any remaining liquids and roughage move into the large intestine. Large intestine[edit source | editbeta] Cecum[edit source | editbeta] The cecum is the first section of the large intestine. It is also known as the "water gut" or "hind gut." It is a cul-de-sac pouch,[16] about 4 feet (1.2 m) long that holds 7 to 8 US gallons (26 to 30 L). It contains bacteria that digest cellulose plant fiber through fermentation.[17]These bacteria feed upon digestive chyme, and also produce certain fat-soluble vitamins which are absorbed by the horse.[14] The reason horses must have their diets changed slowly is so the bacteria in the cecum are able to modify and adapt to the different chemical structure of new feedstuffs.[16] Too abrupt a change in diet can cause colic, as the new food is not properly digested. Colon[edit source | editbeta] The large colon, small colon, and rectum make up the remainder of the large intestine. The large colon is 10 to 12 feet (3.0 to 3.7 m) long and holds up to 20 US gallons (76 L) of semi-liquid matter. It is made up of the right ventral (lower) colon, the left ventral colon, the left dorsal (upper) colon, the right dorsal colon, and the transverse colon, in that order.[14] Three flexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left ventral and left dorsal colon; the diaphragmatic flexure, between left dorsal and right dorsal colon. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type of horse colic called an impaction.[16][17] The small colon is 10 to 12 feet (3.0 to 3.7 m) in length and holds only 5 US gallons (19 L) of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where fecal balls are formed. The rectum is about 1 foot (30 cm) long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus.[14] Mare The reproductive system of the mare is responsible for controlling gestation, birth, lactation, as well as the estrous cycle and mating behavior of the mare. It lies ventral to the 4th or 5th lumbar vertebrae, although its position within the mare can vary depending on the movement of the intestines and distention of the bladder. The mare has two ovaries, usually 7 to 8 centimetres (2.8 to 3.1 in) in length and 3 to 4 centimetres (1.2 to 1.6 in) thick, that generally tend to decrease in size as the mare ages. In equine ovaries, unlike in humans, the vascular tissue is cortical to follicular tissue, so ovulation can only occur at an ovulation fossa near the infundibulum. The ovaries connect to the fallopian tubes (oviducts), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of cilia, which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20 to 25 centimetres (7.9 to 9.8 in) in length. These horns attach to the body of the uterus (18 to 20 centimetres (7.1 to 7.9 in) long). The equine uterus is bipartite, meaning the two uterine horns fuse into a relatively large uterine body (resembling a shortened bicornuate uterus or a stretched simplexuterus). Caudal to the uterus is the cervix, about 5 to 7 centimetres (2.0 to 2.8 in) long, which separates the uterus from the vagina. Usually 3.5 to 4 centimetres (1.4 to 1.6 in) in diameter with longitudinal folds on the interior surface, it can expand to allow the passage of the foal. The vagina of the mare is 15 to 20 centimetres (5.9 to 7.9 in) long, and is quite elastic, allowing it to expand. The vulva is the external opening of the vagina, and consists of the clitoris and two labia. It lies ventral to the rectum. The mare has two mammary glands, which are smaller in maiden mares. They have two ducts each, which open externally. Stallion The reproductive system of the stallion is responsible for the sexual behavior and secondary sex characteristics (such as a large crest) of the stallion. The external genitalia include theurethra; the testes, which average 8 to 12 centimetres (3.1 to 4.7 in) long; the penis, which, when housed within the prepuce, is 50 centimetres (20 in) long and 2.5 to 6 centimetres (0.98 to 2.4 in) in diameter with the distal end 15 to 20 centimetres (5.9 to 7.9 in) and whenerect, increases by 3 to 4 times. The internal genitalia accessory sex glands are thevesicular glands, prostate gland, and bulbourethral glands, which contribute fluid to thesemen at ejaculation, but are not strictly necessary for fertility[18] Teeth A horse's teeth include incisors, premolars, molars, and sometimes canine teeth. A horse's incisors, premolars, and molars, once fully developed, continue to erupt throughout its lifetime as the grinding surface is worn down through chewing. Because of this pattern of wear, a rough estimate of a horse's age can be made from an examination of the teeth. Abnormal wear of the teeth, caused by conformational defects, abnormal behaviors, or improper diets, can cause serious health issues and can even result in the death of the horse. Feet/Hooves 1- Heel perioplium, 2-Bulb, 3-Frog, 4-Frog cleft, 5-Lateral groove, 6-Heel, 7-Bar, 8-Seat-of-corn, 9-Pigmented walls 10-Water line, 11-White line, 12-Apex of the frog, 13-Sole, 14-Toe, 15-How to measure hoof width (blue dotted line), 16-Quarter, 17-How to measure length (blue dotted line) The hoof of the horse encases the second and third phalanx of the lower limbs, analogous to the fingertip or toe tip of a human. In essence, a horse travels on its "tiptoes." The hoof wall is a much larger, thicker and stronger version of the human fingernail or toenail, made up of similar materials, primarily keratin, a very strong protein molecule. The horse's hoof contains a high proportion of sulfur-containing amino acids which contribute to its resilience and toughness. Vascular fold-like structures called laminae suspend the distal phalanx from the hoof wall. Skeletal system[edit source | editbeta] The skeleton of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses have 205 bones, which are divided into the appendicular skeleton (the legs) and the axial skeleton (the skull, vertebral column, sternum, and ribs). Both pelvic and thoracic limbs contain the same number of bones,20 bones per limb. Bones are connected to muscles via tendons and other bones via ligaments. Bones are also used to store minerals, and are the site of red blood cell formation. Ligaments and tendons[edit source | editbeta] Ligament Ligaments attach bone to bone or bone to tendon, and are vital in stabilizing joints as well as supporting structures. They are made up of fibrous material that is generally quite strong. Due to their relatively poor blood supply, ligament injuries generally take a long time to heal. Tendons: Tendons are cords of connective tissue attaching muscle to bone, cartilage or other tendons. They are a major contributor to shock absorption, are necessary for support of the horse’s body, and translate the force generated by muscles into movement. Tendons are classified as flexors (flex a joint) or extensors (extend a joint). However, some tendons will flex multiple joints while extending another (the flexor tendons of the hind limb, for example, will flex the fetlock, pastern, and coffin joint, but extend the hock joint). In this case, the tendons (and associated muscles) are named for their most distal action (digital flexion). Tendons form in the embryo from fibroblasts which become more tightly packed as the tendon grows. As tendons develop they lay down collagen, which is the main structural protein of connective tissue. As tendons pass near bony prominences, they are protected by a fluid filled synovial structure, either a tendon sheath or a sac called a bursa. Tendons are easily damaged if placed under too much strain, which can result in a painful, and possibly career-ending, injury. Tendinitisis most commonly seen in high performance horses that gallop or jump. When a tendon is damaged the healing process is slow because tendons have a poor blood supply, reducing the availability of nutrients and oxygen to the tendon. Once a tendon is damaged the tendon will always be weaker, because the collagen fibres tend to line up in random arrangements instead of the stronger linear pattern. Scar tissue within the tendon decreases the overall elasticity in the damaged section of the tendon as well, causing an increase in strain on adjacent uninjured tissue. Muscular system When a muscle contracts, it pulls a tendon, which acts on the horse's bones to move them. Muscles are commonly arranged in pairs so that they oppose each other (they are "antagonists"), with one flexing the joint (a flexor muscle) and the other extending it (extensor muscle). Therefore, one muscle of the pair must be relaxed in order for the other muscle in the pair to contract and bend the joint properly. A muscle is made up of several muscle bundles, which in turn are made up of muscle fibers. Muscle fibers have myofibrils, which are able to contract due to actin and myosin. A muscle together with its tendon and bony attachments form an extensor or flexor unit. Respiratory system : The horse's respiratory system consists of the nostrils, pharynx, larynx, trachea, diaphragm, and lungs. Additionally, the nasolacrimal duct and sinuses are connected to the nasal passage. The horse's respiratory system not only allows the animal to breathe, but also is important in the horse's sense of smell (olfactory ability) as well as in communicating. The soft palate blocks off the pharynx from the mouth (oral cavity) of the horse, except when swallowing. This helps prevent the horse from inhaling food, but also means that a horse cannot use its mouth to breathe when in respiratory distress—a horse can only breathe through its nostrils. For this same reason, horses also cannot pant as a method of thermoregulation. The genus Equus also has a unique part of the respiratory system called theguttural pouch, which is thought to equalize air pressure on the tympanic membrane. Located between the mandibles but below theocciput, it fills with air when the horse swallows or exhales. Circulatory system The horse's circulatory system includes the four-chambered heart, averaging 8.5 lb (3.9 kg) in weight, as well as the blood and blood vessels. Its main purpose is to circulate blood throughout the body to deliver oxygen and nutrients to tissues, and to remove waste from these tissues. The hoof (including the frog - the V shaped part on the bottom of the horses hoof) is a very important part of the circulatory system. As the horse puts weight onto the hoof, the hoof wall is pushed outwards and the frog compressed, driving blood out of the frog, the digital pad, and the laminae of the hoof. When weight is removed from the hoof, the release of pressure pulls blood back down into the foot again. This effectively creates an auxiliary blood-pumping system at the end of each leg. Some of this effect is lost when a horse is shod (eliminating the expansion and contraction of the hoof wall and raising the frog higher from the ground).[19] The eye A horse's eye The horse has the largest eye of all land mammals.[20] Eye size in mammals is significantly correlated to maximum running speed as well as to body size, in accordance with Leuckart's Law; animals capable of fast locomotion require large eyes.[21] The eye of the horse is set to the side of its skull, consistent with that of a prey animal.[22] The horse has a wide field ofmonocular vision, as well as good visual acuity. Horses have two-color, or dichromatic vision, which is somewhat like red-green color blindness in humans.[23] Because the horse's vision is closely tied to behavior, the horse's visual abilities are often taken into account when handling and training the animal. Hearing[edit source | editbeta] The hearing of horses is good,[24] superior to that of humans, and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head.[25] Often, the eye of the horse is looking in the same direction as the ear is directed.