It seems obvious to most that the horse has developed as a highly specialised loco motor machine, adapted perfectly for travel over long distances at moderate rates of speed, accompanied with the additional capability for occasional bursts of high speed.

In fact this aspect of the horse makes it perfect for many of the equine sports enjoyed today. But how does the structure of the horse result in this perfection, and why did the horse evolve this way?

It is probably easier to begin with the latter question, why did the horse evolve into the creature we know today?

Every child in the UK has to take part in biology classes during their school years, and if we look back at what is taught in these lessons we will almost certainly remember the food chain. In it’s simplest form a food chain shows how each living thing gets its food. Plants, known as autotrophs, are always at the bottom of this chain. The excess energy that autotrophs create during photosynthesis is passed on to the animals in the upper links of the chain.

The next link up the chain is the herbivores, those animals that consume the plants. This is where the horse fits it. The horse is an animal that is adapted to eat primarily plant matter (rather than meat). Although such animals are sometimes referred to as being vegetarian, this term is more properly reserved for humans who choose not to eat meat as opposed to animals that are unable to make such choices.

Following the herbivors is the carnivors. A carnivore is an animal with a diet consisting mainly of meat, whether it comes from animals living (predation on herbivors or other carnivors) or dead (scavenging).

This basic study of the food chain shows that the horse, being a herbivore, is a prey animal. It is hunted by carnivors for food. And this is how the horse has evolved over millions of years. His body and mind have adpated to give him as much chance as possible of escaping the claws of his predator.

This adaptation has lead to the development of what we refere to was “the flight response”.

All animals have a built in physiological aspect known as the fight or flight response, it is the body’s primitive, automatic, inborn response that prepares the body to fight or flee from perceived attack, harm or threat to an individuals survival. But the results of stimulation of this response vary dramatically from species to species. The horse being a prey animal has no real means of fighting with any predator, it possesses no claws or fangs, therefore its initial response is to flee. The flight response.

All riders and handlers will, at some point in their life, experince this response first hand but what exactly does it involve? This brings us back to first of our two questions, how does the modern structure of the horse result in this speed?

Well, first it ismportant to understand that the flight response is a nervous response. Therfore the logical place to start is the nervous system.

Nervous System

All living organisms show the ability to respond to stimuli in ways which help that particular organism to survive, and this is thanks to a complicated nervous system. This system is divided into two disticnt sections, the central nervous system (CNS) and the peripheral nervous system (PNS).

The Central Nervous System consists of the brain and spinal cord. It contains literally millions of neurones (nerve cells). If you were to look closely at the brain or spinal cord you would find some areas appear grey whilst other areas appear rather white. These areas are known as white and grey matter. The white matter consists of the axons of the nerve cells, it appears white because it contains a lot of fatty material called myelin. The myelin sheath insulates an axon from its neighbours meaning that nerve cells can conduct electrical messages without interfering with one another. The grey matter consists of cell bodies and the branched dendrites, which effectively connect nerves together. So this area is mainly the cytoplasm of nerve cells, which is why it appears rather grey.

The Peripheral Nervous System consists of the nerves that make up the rest of the horses body.

Sensory neurones feed information to the spinal cord and brain from receptors. Receptors are the parts of the nervous system that sense changes in the internal or external environments, know as stimuli. These stimuli can be in many forms, including pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.

In the sensory centres of the brain or the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to effector organs via motor neurons. The effector organs can then convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc.

Sensory neurones are usually connected to motor neurones by intermediate neurones (sometimes called inter neurones).

Sensory, intermediate and motor nerves have gaps between them called synapses. These are the gaps between the dendrites of one neurone and the cell body of another one. There is no electrical connection between nerve cells, instead when one neurone stimulates another it does so by secreting a chemical into the synapse.

In the case of the flight response your initial response would be to say that the effector organs are the muscles, and you would not be wrong. However there are many other areas and aspects to take into account, and you cannot fully understand that effect of the muscles without first looking at the skeletal system of the horse.

The Skeletal System

The skeletal system includes all bones attached at joints, cartilage between the joints & fibrous ligaments. The system has several roles in the physiology of the horse, these are -

• Support of the soft tissues, providing the framework of the horse’s body
• Protection of delicate internal organs
• Leverage – providing attachment for ligaments, muscles and tendons
• Mineral storage – especially calcium and phosphorus
• Blood cell production takes place in the marrow of many long bones
• Bone consists of the hard outer cortex, encasing an inner area of spongy bone, called the medullary cavity.  Within this cavity is the marrow, where new blood cells are made.
• A thin membrane called the periosteum covers the entire surface of the bone.

On the basis of shape, bones are classified as either -

• Long bones such as the cannon or radius
• Short bones such as the carpal and tarsal bones
• Flat bones such as the scapula
• Irregular bones such as the vertebrae

Within the skeletal system there are also joints (or articulations). A joint is a point of contact between bones or between cartilage and bones. Joints allow movement, and may be classified into the following types - 

• Fibrous joints – these are bones held by fibrous connective tissue, with no joint cavity, which allow little or no movement. An example would be the skull.
• Cartilaginous joints – these are bones held by cartilage, with no joint cavity. The bones are held together tightly, allowing little movement. An example would be the vertebrae.
• Synovial joints – these joints contain a joint or synovial cavity, articular cartilage and a synovial membrane, making them a fully moveable joint. An example would be the fetlock.

The skeletal system also contains many other tissues to help it achieve its roles within the body of the horse. They are:

• Cartilage, which is a tough connective tissue containing collagen and elastic fibres but no blood vessels or nerves. It covers the ends of bones at some joints.
• Ligaments, which are dense fibrous tissues that connect bone to bone.  Horses also have some ligaments known as “check ligaments”, which connect some bones & tendons. These form part of the horses “stay mechanism”, they lock the limbs and joints so that the horse can relax and sleep in a standing position.
• Tendons that connect muscle to bone and help control the movement of bones and joints. In the horse there are no muscles below the knee or hock, instead tendons connected to muscles in the upper limbs transfer all movement of the pastern and foot.

The skeletal system consists of approximately 205 bones divided into the axial skeleton, which includes the skull, vertebrae, sternum and ribs and the appendicular skeleton, which includes the four limbs of the horse. 

When thinking about the flight response in the horse it is probably more logical to concentrate on the appendicular skeleton, for although the axial skeleton will play a role in the movement it is the limbs of the horse that propel the animal along.

The appendicular skeleton is subdivided into two sections, the thoracic limb (forelimb) and the pelvic limb (hind limb).

The two limbs are very similar, in fact below the carpus and tarsus (knee and hock) they are exactly the same.

The thoracic limb is composed of four segments, the thoracic girdle, the arm, the forearm and the manus.

The thoracic girdle consists of the scapula, a large, well-developed flat bone, which is not directly attached to the axial skeleton due to the lack of a clavicle within the horse’s body. However, occasionally a small tendinous band called the clavicular intersection may be found embedded in the brachiocephalicus muscle at the point of the shoulder.

From a conformation point of view, if we require speed in a horse, the scapula should be well sloping. This means that the angle from the spine of the scapula clockwise to the horizontal plane line should be no more than 450. This allows the scapular to swing the forelimb forward with greater reach, thus lengthening the horse’s stride and increasing his ground coverage, increasing his speed over the ground.

The arm consists of the humerus. A short thick bone, who’s main role is to bare the weight of the horses body down the fore limb. It should form an angle of about 550 with the horizontal plane.

The forearm consists of the radius and ulna.

The radius is the much larger of the two bones within the forearm, extending in a vertical direction from the elbow to the carpus. Ideally the radius should be long in relation to the manus, making the carpus appear very close to the ground. This allows a greater length of stride through the forearm, thus increasing the horse’s speed.

The ulna is a reduced long bone, situated caudal to the radius, and becomes partially fused in the adult horse.

The manus consists of the carpus, cannon bone and digits.

The carpus consists of eight carpal bones arranged into two rows, the proximal and the distal. If you were to look at the left carpus of the horse, and work round anticlockwise from the medial aspect, the carpal bones would appear as follows.

Proximal Row
Radial	Intermediate	Ulnar	Accessory
Distal Row
First	Second	Third	Fourth

The cannon bone, sometimes referred to as the third or large metacarpal bone, is the only fully developed metacarpal bone within the horse’s body. It should rest vertically between the carpus and the fetlock, and appear short in length compared to the radius.

The other two metacarpal bones, the second and forth metacarpals, are more commonly known as the small metacarpal or splint bones.

The digits of the manus consist of the phalanges and sesamoid bones.

The proximal phalanx, more commonly known as the long pastern bone, is situated between the cannon and middle phalanx. It is a long bone and forms an angle of about 550 with the horizontal plane in well-formed limbs. If the angle of the bone is more than this it can lead to a shortened stride length, reducing the horses speed over the ground.

The middle phalanx, also known as the second phalanx or short pastern bone, is located between the proximal and distal phalanges. Its direction should correspond identically to that of the proximal phalanx.

The distal phalanx, also known as the third phalanx, pedal or coffin bone, is entirely enclosed by the hoof, to which it conforms.

The proximal sesamoid bones are located at the distal end of the cannon bone, and are closely attached to the proximal phalanx by strong ligaments. And each conforms to the shape of a three-sided pyramid.

The distal sesamoid, also know as the navicular bone, is rather shuttle shaped and is located at the junction between the middle and distal phalanges.

The pelvic limb, like the thoracic limb, is composed of four segments, the pelvic girdle, thigh, leg and pes.

The pelvic girdle consists of the hipbone, the sacral vertebrae and the first three, or more, caudal vertebrae.

The hipbone, also known as the pelvis or os coxae, is further subdivided into three sections, the ilium, ischium and pubis. These three sections come together to form the acetabulum, a large cavity which articulates with the head of the femur.

The thigh consists of the femur and patella.

The femur is the largest long bone found in the horse’s body. It articulates proximally with the acetabulum and distally with the tibia and fibula.

The patella is a large sesamoid bone that articulates with the distal end of the femur, to form part of the stifle joint.

The leg consists on the tibia and fibula.

The tibia is a long bone, which extends obliquely, distally and caudally from the stifle to the tarsus.

The fibula is a much-reduced long bone that articulates caudally with the tibia.

The pes is similar to the manus of the thoracic limb. It consists of the tarsus, phalanges and sesamoid bones.

The tarsus is composed of seven bones, arranged into the proximal and distal rows.

Proximal Row
Tibial		Central		Fibular
Distal Row
First	Second		Third		Fourth

The phalanges and sesamoid bones of the pelvic limb are identical to those of the thoracic limb, excluding the fact that the angle between the phalanges and the horizontal plane is five degrees greater.

Muscular System

Now that the skeletal system has been explored we can look at the muscular system and how this acts to move the bones of the limbs.

Again the muscles will be divided into the muscles of the thoracic limb and the muscles of the pelvic limb.

The thoracic limb.

The muscles of the thoracic girdle consist of those muscles that attach the thoracic limb with the head neck and trunk of the horse, forming the synsarcosis. The synsarcosis is regarded as consisting of two divisions, the dorsal and ventral.

The dorsal vision consists of two layers.

The first layer is made up of the trapezius, which is a large flat triangular muscle, divided into the cervical trapezius and the thoracic trapezius.

The trapezius acts as a whole to elevate the shoulder. The cervical trapezius drawing the scapula cranially and dorsally, while the thoracic trapezius draws it caudally and dorsally.

The second layer is made up of the rhomboideus, which acts to draw the scapula cranially and dorsally.

The ventral division consists of the:

• Brachiocephalicus, which acts to draw the thoracic limb cranially extending the shoulder joint, and the pectorales superficiales muscles, which are divided into three separate muscles. The pectorlis decendens, which act to adduct and advance the thoracic limb, the pectoralis transversus, which act to adduct and retract the thoracic limb, and the pectoralis ascendens, which also act to adduct and retract the thoracic limb.
• The muscles of the shoulder arise on the scapula and end on the arm of the thoracic limb. They may be divided into two groups.

The lateral group consists of the:

• Deltoideus, which acts to flex the shoulder joint and abduct the arm.
• Supraspinatus, which acts to extend the shoulder joint and prevent dislocation.
• Infraspinatus, which acts to abduct the arm and rotate it laterally.
• Teres minor, which acts to flex the shoulder joint and abduct the limb.
• The medial group consists of the:
• Subscapularis, which acts to adduct the humerus.
• Teres major, which acts to flex the shoulder joint and adduct the limb.
• Coracobrachialis, which acts to flex the shoulder joint and adduct the limb.
• The muscles of the arm consist of five muscles, which are grouped around the humerus.
• The biceps brachii, which act to flex the shoulder joint and fix the shoulder and elbow so the horse can rest while standing.
• The brachialis, which acts to flex the elbow joint.
• The tensor fasciae antebrachii, which acts to tense the fascia of the foreleg and extend the elbow joint.
• The triceps brachii, which acts to extend the elbow joint and flex the shoulder joint.
• The lateral head, which acts to extend the elbow joint.
• The medial head, which acts to extend the elbow joint.
• The anconeus, which acts to extend the elbow joint and raise the capsule of the joint to prevent it being pinched during extension.
• The fasciae and muscles of the forearm and manus.
• A fascia is a sheet or band of fibrous connective tissue enveloping, separating, or binding together muscles, organs, and other soft structures of the body.

The extensor muscles of the forearm and manus consist of the:

• Extensor carpi radialis, which acts to extend and fix the carpus and flex the elbow.
• Extensor digitorum communis, which acts to extend the digit and carpus, and flex the shoulder joint.
• Extensor digitalis lateralis, which acts to extend the digit and carpus.
• Extensor carpi obliquus, which acts to extend the carple joint.
• The flexor muscles of the forearm and manus consist of the:
• Flexor carpi radialis, which acts to flex the carpus and extend the elbow.
• Flexor carpi ulnaris, which acts to flex the carpus and extend the elbow.
• Ulnaris lateralis, which acts to flex the carpus and extend the elbow.
• Flexor digitorum superficialis, which acts to flex the digit and carpus and extend the elbow.
• Flexor digitorum profundus, which acts to flex the digit and carpus and extend the elbow.
• The pelvic limb.

The sublumbar muscles are not confined to the sublumbar region, instead they extent beyond it both cranially and caudally. Their main function is to flex the hip joint, two of the muscles, however, do no have this action. The psoas minor and the quadratus lumbornum act to flex the pelvis on the loins and achieve lateral flexion of the loins.

The lateral muscles of the hip and thigh consist of the:

• Tensor fasciae latae, which acts to tense the fascia lata, flex the hip joint and extend the stifle joint.
• Gluteus superficialis, which acts to abduct the limb, flex the hip and tense the gluteal fascia.
• Gluteus medius, which acts to extend the hip joint and abduct the limb. Also, by it’s connection to the longissimus lumbornum a muscular mass is formed which is one of the chief factors in kicking, rearing and propulsion.
• Gluteus profundus, which acts to abduct the thigh and rotate it medially.
• Biceps femoris, which’s actions are quite complex as it consists of three sections, and acts on all the joints of the limb except those of the digit. However, the general function is to extend the limb, as in kicking rearing and propulsion of the body, as well as abducting the limb.
• Semitendonosis, which acts to extend the hip joint and tarsus, acting with the biceps femoris and the semimembranosus in propulsion of the trunk, rearing and kicking. It also flexes the stifle and rotates the leg medially.
• Semimembranosus, which acts to extend the hip joint and abduct the limb.

The medial muscles of the thigh consist of three layers.

• The first layer consists of the:
• Sartorius, which acts to flex the hip joint an adduct the limb.
• Gracilius, which acts to adduct the limb.

The second layer consists of the:

• Pectineus, which acts to adduct the limb and flex the hip joint.
• Adductor, which acts the adduct the limb, flex the hip joint and rotate the femur medially.
• Semimembranosus, which extends across the second layer as well as the first.

The third layer consists of the:

• Quadratus femoris, which acts to extend the hip joint and adduct the thigh.
• Obturatorius externus, which acts to adduct the thigh and rotate it laterally.
• Obturatorius internus, which acts to rotate the femur laterally.
• Gemelli, which acts to rotate the femur laterally.

The cranial muscles of the thigh consist of the:

• Sartorius, which acts to flex the hip joint an adduct the limb
• Quadriceps femoris, which acts to extend the stifle joint and flex the hip joint.
• Vastus lateralis, which acts to extend the stifle joint.
• Vastus medialis, which acts to extend the stifle joint.
• Vastus intermedius, which acts to extend the stifle joint, and raise the femoropatellar capsule during extension of the joint.

The muscles of the leg and foot are divided into two groups.

The craniolateral group consist of the:

• Extensor digitorum longus, which acts to extend the digit and flex the tarsus, as well as assisting in fixing the stifle joint.
• Extensor digitorum lateralis, which acts to assist the extensor digitorum longus.
• Fibularis, which acts to flex the stifle joint while the tarsus is still flexed.
• Tibialis cranialis, which acts to flex the tarsus.

The caudal group consists of the:

• Triceps surae, a term used to collectively describe the gastrocnemius and the soleus, which work to extend the tarsus and flex the stifle joint. These to actions cannot, however, occur simultaneously.
• Proximal part of the flexor digitorum superficialis, which acts to flex the digit and extend the tarsus. However, due to the exceedingly small amount of muscular tissue used in the action it is regarded as a mechanical effect of other muscles acting on the stifle joint.
• The belly of the flexor digitorum longus, which acts to flex the digit and extend the tarsus.
• Popliteus, which acts to flex the femorotibial joint and rotate the leg medially.

So it can easily be seen that the muscles of the thoracic and pelvic limb provide the main source of propulsion within the horse’s body. They do so by flexing and extending joints in an order that first build up tension and energy before quickly releasing it distally. They usually do this in antagonistic pairs, while one muscle will contract to flex a joint the opposite will contact to extend the joint. But how do muscles contract?

The answer is ATP hydrolysis, a direct consequence of the interaction between myosin and actin proteins. ATP hydrolysis is the reaction by which chemical energy that has been stored and transported in the high-energy phosphoanhydridic bonds in ATP (Adenosine triphosphate) is released. It has the chemical formula

ATP + H2O = ADP + Inorganic Phosphate

Muscle fibres are huge cells formed from many separate, smaller cells. The bulk of the cytoplasm in this huge cell is made up of myofibrils, which are often as long as the muscle cells themselves. These myofibrils contract instantly if ATP and Ca2+ are added to them, meaning that it is these single myofibrils, which are the force generators in the muscle cells.

Within the myofibrils there is a striated sarcomer, about 2.5 microns long. They are separated by Z-discs. Two Z-discs bind the sarcomer in the direction of contraction. Thin filaments made of Actin are attached to each of these discs and extend towards each other inside the sarcomer. In the picture shown below there is a dark band visible between the Z-discs. This is made up of the thick myosin filaments, which overlap partially with the thin Actin filaments.

When myofibrils contract the thin and thick filaments move past each other. Each sarcomer unit of the myofibrils shortens proportionally to the muscle contraction.

Upon contraction it is the thin filaments that shorten, not the thick filaments.

The actin filament has a negative charge at the centre of the sarcomer and a positive charge at either end, the myosin heads walk from the negative centre of the sarcomere to the positive ends on the Z-discs. During this movement, ATP is hydrolysed and subsequent dissociation of the tightly bound products (ADP and P) produce an ordered series of changes in the conformation of myosin, moving the actin filaments along the thick filaments, resulting in muscle contraction.

ADP can then be further hydrolysised to give energy or AMP (Adenosine monophosphate), and another orthophosphate (Pi).

ATP hydrolysis is the final link between the energy derived from food and useful work such as muscle contraction. Therefore it tells us that there are also many other systems within the horse’s body that play an important part in allowing it to perform the flight response.

The synthesis of ATP occurs in cell mitochondria by cellular respiration. This is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The resulting energy is then trapped within the cell in the form of ATP.

To allow this process to occur three particular systems within are needed.

The Digestive System

Although it may not seem initially important to the flight response, once it is understood that food molecules are required for the production of ATP, the importance of the digestive system becomes clear.

If the horse were unable to ingest and digest food then he would be unable to produce energy, and not only would he be unable to respond to danger he would most certainly die.

The Respiratory System

Oxygen is another important ingredient in the process of respiration.

C6H12O6 + 6O2 -> 6CO2 + 6H2O + 38 ATP

More commonly known as:

Glucose + Oxygen -> Carbon Dioxide + ATP

Therefore it is crucial to get oxygen into the horse’s body, and this is where the respiration system comes in.

The Cardiovascular System

It is all fine and well that the respiratory and digestive systems provide the glucose and oxygen for respiration, but these components need to transported from where they enter the body to where they are needed within the body. This is one of the roles of the cardiovascular system.

The main components of the cardiovascular system are the heart, the blood, and the blood vessels.

The heart is a highly muscular organ responsible for pumping the blood through the blood vessels by repeated, rhythmic contractions.

The blood technically a tissue that is composed of a liquid called blood plasma, and blood cells which are suspended within the plasma.

There are various kinds of blood vessels but they can be disticntly seperated into two groups.

The ateries have a thicker lumen and are more muscular than veins. This is because they transport the oxygenated blood around the body.

There is ofcourse one exception to this.

The circulatory system includes the pulmonary circulation loop. This loop extends from the heart to the lungs and back to the heart. Within this loop is the pulmonary artery, which carried deoxygenated blood from the heart to the lungs.

Arteries can be subdivided into ateries and arterioles. The arterioles are a smaller version of the arteries that lead onto the capilleries.

Capillaries are the smallest of a body’s blood vessels and connect arterioles and venules. Their small size enables the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between the blood and surrounding tissues

Veins have a thin lumen and less muscle than arteries, as they carry deoxygenated blood.. They aslo contain venous values that prevent the backflow of blood, as they do not possess the same pressure as the arteries to prevent it.

As with the arteries there is one exception to this within the pulmonary circulation loop. The pulmonary vein carries oxygenated blood from the lungs to the heart.

Veins can be subdidvided into veins and venules. The venules are the veins equivalent of the arteiroles, and connect the capillaries and veins.

Oxygen diffuses from the alveolar air in the lungs into the blood because the small size of the capillaries creates a high pressure within the blood vessels moving the blood at a high speed, maintaining a lower concentration of oxygen molecules within the blood in comparison to the lungs. It then forms a chemical combination with the haemoglobin in red blood cells. These red blood cells then transport the oxygen around the body to where it is needed, they will then exchange the oxygen for carbon dioxide, thus providing the cells with vital ingredients and removing waste products.

In times of high oxygen requirement, such as the flight response, the horse has developed a technique known as splenic contraction.

This is where the horse has stored many oxygenated red blood cells within the spleen.

Upon the appearance of a stimulus that promotes the flight response adrenalin within the endocrine system will promote the spleen to contract. Thus releasing all the stored red blood cells back into the circulatory system. This immediately increases the amount of oxygen available for respiration and thus increases the amount of ATP that can be produced, allowing the muscles of the horse to work harder.

Glucose, unlike oxygen, is not carried attached to the blood cells. Instead it is carried within the blood plasma.

The Lymphatic System

The lymphatic system is a complex network comprising of lymph organs, lymph node, lymphatic tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the cardiovascular system.

Lymph nodes act as filters, with an internal honeycomb like structure of lymphatic tissue. They are filled with lymphocytes that collect and destroy bacteria and viruses. When the body is fighting an infection, lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes, known as lymphangitus.

Lymphatic Tissue is a specialized form of reticular connective tissue in the lymphatic system. This tissue type makes up the spleen, the thymus, and the tonsils, as well as areas which are all associated with mucous membranes of the gastro-intestinal tract, within the digestive system.

Lymph vessels are thin walled, valved structures that carry lymph fluid. As part of the lymphatic system, lymph vessels are complimentary with the cardiovascular system. In contrast to the cardiovascular system, which carries blood under pressure to the entire body, lymph fluid is not under pressure and is propelled in a passive fashion, assisted by the valves. Fluid that leaks from the cardiovascular system is returned to general circulation via lymphatic vessels.

Lymphatic capillaries are tiny thin-walled blood vessels that are closed at one end and are located in the spaces between cells throughout the body. The main purpose of these vessels is to drain excess tissue fluids from around the cell ready to be filtered and returned to the venous circulation.

The lymphatic system has three interrelated functions:

• The removal of extracellular fluids from body tissues.
• The absorption of fatty acids and subsequent transport of fat to the cardiovascular system.
• The production of immune cells such as lymphocytes and monocytes.

So overall the lymphatic system provides a vital role in supporting and maintaining the other systems within the horse’s body. This allows those other systems to operate at their maximum capacity (providing there are no other defects) and thus aids the flight response.

The Example

Now that we have explored the flight response on a macroscopic and microscopic level it may be helpful just to put it together as a simple example.

For this example we shall use a plastic bag as the stimuli. Something that most riders have been on the receiving end of. The horse is happily walking along when out of the corner of his eye he spots something in the bushes up ahead.

Sensory neurons send electronic messages to the brain.

His evolutionary instincts as a prey animal invoke the flight response.

A motor output is distributed by the motor neurons to effector organs.

Meanwhile cellular respiration occurs, using glucose and oxygen provided by digestive, respiratory and cardiovascular systems, to produce ATP.

The production of this process can be increased by splenic contraction.

ATP is then used in ATP hydrolysis to contract the muscles.

The muscles within the limbs contract in antagonistic pairs to propel the horse’s body forward supported by the bones of the skeletal system.

Thus resulting in the horse running away from the plastic bag.