Basic equine anatomy

If you own a horse it is useful to have a basic understanding of the horse’s anatomy. Having this basic knowledge will enable you to spot problems or diseases your horse may have at a much earlier stage, and will enable you to communicate effectively with your vet.

The equine skeleton is the framework of the horse’s body as well as providing protection for the horse’s vital organs, such as the heart, lung and intestines.
The skeleton system consists of the bones which are held together by ligaments at the junctions of bones to form a joint. Ligaments are bands of tough connective tissue that provide joint support, but not much movement; they also have a limited blood supply which is why they can take so long to heal following an injury. Cartilage is found on the end of bones in the joints which act as shock absorbers and reduce friction.
Joints are the junctions at which any two bones meet, and are protected by a synovial lining that secretes synovial fluid. This fluid is very important; it reduces friction by lubricating the joint, it absorbs shock and it also provides oxygen and nutrition for the joint.
Traumatic injuries involving a joint should be treated as an emergency; if the joint has been penetrated infection can quickly set in, leading to inflammation and lameness. Other common, but not serious conditions, include bog spavin, windgalls and throughpin. Degenerative joint diseases (DJD) and developmental orthopaedic diseases (DOD) are more serious conditions which can cause lameness and potentially loss of use.

On average, the horse’s skeleton consists of 205 bones:

  • The horses skull consists of 34 bones
  • On average the spine consists of 53 bones, but these can vary depending on breed:
    • 7 cervical vertebrae (neck)
    • 18 thoracic vertebrae (connected to the ribs)
    • 5 lumbar vertebrae
    • 5 sacral vertebrae (these are fused)
    • 18 coccygeal vertebrae (the tail)
  • The ribcage consists of 18 pairs of ribs which curve around the internal organs to meet at the sternum (breast bone)
  • The forelimbs consist of:
    • The scapula (shoulder blade)
    • Humerus
    • Radius
    • 8 carpal bones (the knee)
    • Cannon and splint bones
    • Long and short pastern
    • Coffin bone (in the foot)
  • The hindlimbs consist of:
    • The ilium, ischium and pubis (the pelvis)
    • Femur
    • Tibia and fibula
    • 7 tarsal bones (the hock)
    • Cannon and splint bones
    • Long and short pastern
    • Coffin bone (in the foot)

There are three types of muscle which are all different in terms of structure, function and regulation of contraction:

  • Smooth muscles are involuntary, i.e. they work without conscious effort, and can tense and relax. Smooth muscles are found in many places, such as the bladder, gastrointestinal tract, respiratory tract, kidneys, etc.
  • Cardiac muscles are also involuntary and are the foundation of the heart.
  • Skeletal muscles are voluntary, i.e. they work with conscious effort, and are responsible for movement.

All skeletal muscles are attached to tendons which anchor the muscles to bone. Tendons are very strong, however they are not elastic and can become strained or damaged. This is particularly true of tendons on the legs where there are no muscles to act as “shock absorbers”, so the tendons below the hock or knee can easily be damaged.
Muscles and/or tendons around a joint are cushioned by a synovial bursa, this is a small sac filled with synovial fluid that is found around most major joints. The bursa enables muscles and tendons to glide over bony and ligamentous surfaces. Traumatic injuries involving a bursa should be treated as an emergency; if the bursa has been penetrated infection can quickly set in, leading to inflammation and lameness. Common, but not serious conditions involving the bursa include capped elbow, capped hock and carpal hygroma. Navicular bursitis is a more serious condition of the foot which is often caused by a penetrating injury to the frog and can be devastating to the horse if gone unnoticed and untreated.

The cardiovascular system

Consisting of the heart, arteries, veins, capillaries and blood. The heart pumps the blood around the body through arteries and capillaries, and returns the blood through the veins. The blood supplies oxygen and nutrition to the horse’s muscles and other internal organs. The most common cardiovascular problems in horses include irregular heart rhythms and murmurs (abnormal sounds when listening to the heart).

The lymphatic system

This is part of the immune system and consists of small vessels that contain lymphatic fluid and lymph nodes. The lymphatic system’s primary function is to create immune cells. Lymphatic fluid is full of white blood cells, which are the body’s main tools in the immune response; it also removes toxins, such as fat and bacteria, from tissues within the body and moves it to the circulatory system. The lymph nodes become enlarged when the body is fighting bacteria. Enlargement of the lymph nodes under the lower jaw is an indication of an infection in the head, e.g. respiratory viral or bacterial infection, tooth root infection.

The endocrine system

A system of glands that produces hormones; these hormones regulate the body including mood, growth/development, reproduction, tissue function and metabolism.

The gastrointestinal system

Consisting of the mouth, pharynx, oesophagus, stomach, small intestine, caecum, large intestine, large/small colon, rectum and anus. This is where the body consumes, breaks down food to provide nutrition for the rest of the body, and then excretes the waste from body functions. Diseases of the gastrointestinal system can result in combinations of colic (abdominal pain), weight loss, reduced appetite and diarrhoea.

The nervous system

Controls all of the other body systems, and is the most complex system in the body. It consists of the brain, spinal cord, and sensory and motor nerves. The central nervous system (CNS) is the centre of all nervous control. The peripheral nervous system controls nerves located outside of the CNS, i.e. from body parts to the CNS and vice versa; this system also controls the involuntary systems, such as the cardiovascular and respiratory systems, as well as the horse’s “fight or flight” reactions, and resting, sleeping, digestion and elimination. Problems with the nervous system result in a wide range of signs including coordination difficulties, altered behaviour and fits (seizures).

The reproductive system

In the mare this is responsible for controlling the oestrous cycle, mating behaviour, gestation, birth and lactation. It consists of the ovaries, fallopian tubes (oviducts), uterus, cervix, vagina and vulva. The mare also has two mammary glands that have two ducts each, which open externally.
In the stallion this is responsible for controlling sexual behaviour. It consists of the testes (within the scrotum), penis (within the sheath/prepuce) and urethra. Internal sex glands (gonads) consist of the vesicular glands, prostate gland and bulbourethral glands, all of which contribute fluid to the semen at ejaculation.

The respiratory system

Enables the body to get oxygen into other body systems which is necessary for the body to stay alive. It also helps to feed the muscles and excrete toxins. The respiratory system consists of the nose, mouth, larynx, trachea (windpipe), bronchial tubes, lungs and diaphragm. Respiratory disease can cause signs ranging from a sudden onset struggling to breathe, coughing, nasal discharge and reduced athletic performance.

The urinary system

Responsible for maintaining fluid balance within the body and for excreting waste in the form of urine. The urinary system consists of the kidneys, ureters, bladder and urethra. The kidney’s biggest job is to filter the blood and remove waste products. This waste is sent to the bladder via the ureters (muscular tubes) and excreted via the urethra which is a tube that travels through the penis in the male horse, and exits at the bottom of the vulva in the mare. Urinary problems can result in signs including increased drinking and urination, difficulty urinating, abnormal coloured urine, colic and depression.

The skin

Acts as a protective barrier, maintains body temperature, senses the outside environment and helps the immune system to protect from disease. The skin is constantly changing and consists of three layers: the epidermis, dermis and subcutaneous layers. The epidermis is the top layer of skin which protects the lower layers and is shed approximately every 2 weeks. The dermis is the middle layer which contains pain and touch receptors. The subcutaneous layer is the deepest layer which helps conserve the body’s heat and protects the internal organs from injury by acting as a shock absorber. Skin diseases can result in itching, scabs, crusts, lumps and bumps.

The eyes

The eyes of a horse are the largest of any land animal, and twice the size of human eyes. They are positioned at the side of the head so their range of sight is almost 360° with just a small blind-spot behind them, and a triangular blind-spot for the first few feet directly in front of them. Horses do see colour, although not to the same extent as humans. The eyes include the eyeball, surrounding muscles and nerves, and eyelids. Common eye problems include corneal ulcers and uveitis (inflammation of the eye).