Cover Page

Contents

Preface

Part I The systems of the horse

Chapter 1 The cell

The cell – building block of life

Major cell organelles

Tissue organisation

Cell division

Chapter 2 The skeleton

Points of the horse

Bone

The skeleton (Fig. 2.5)

The forelimb (Fig. 2.11)

The hindlimb (Fig. 2.12)

Diseases of the bone

Joints

Developmental orthopaedic disease (DOD)

Chapter 3 Muscles

Cardiac muscle (Fig. 3.1)

Smooth muscle (Fig. 3.2)

Skeletal muscle (Fig. 3.3)

Muscle contraction

Muscle anatomy

Muscles of the neck and shoulder (Tables 3.1, 3.2)

The shoulder

Muscles of the forearm

Muscles of the trunk (Table 3.3)

Muscles of the hindlimb (Table 3.4)

The importance of muscle in equine performance

Muscle fibre recruitment

Distribution of muscle fibres

Fatigue

Glycogen depletion

Lactic acid build-up

Chapter 4 The lower leg

The tendons and ligaments of the lower leg (Fig. 4.2)

Blood supply to the lower leg (Fig. 4.4)

The nerves of the lower leg

Tendon and ligament injury and healing

The hoof

The functions of the foot

Hoof care

The balanced foot

Infection in the foot

Chapter 5 The respiratory system

Function

Anatomy

The physiology of respiration

Regulation of breathing

Air, gaseous exchange and blood transport

Respiratory disease

Chapter 6 The circulatory system

The heart

Blood vessels (Fig. 6.7)

Heart abnormalities in the horse

The blood

The lymphatic system

Chapter 7 The control systems

The nervous system

The endocrine system

Common problems associated with the nervous system

Chapter 8 The senses

Sight

Hearing

Taste and smell

Chapter 9 The skin

The common integument

Structure of the skin (Fig. 9.1)

Sensation

Melanoma

The hair and coat

Temperature regulation

The skin as an indicator of health and disease

Care of the horse’s skin and coat

Skin and coat colour

Chapter 10 The urinary system

The kidneys

The formation of urine

The regulation of body water

Acid/base balance

Urea

Micturition or urination

Conditions of the urinary system

Chapter 11 The teeth and ageing

The structure of the tooth (Figs 11.2 and 11.3)

Wear and tear (Fig. 11.4)

Ageing (Fig. 11.6)

Care of the teeth

Chapter 12 The digestive system

Nutrients

The digestive system

Mouth

Stomach

Small intestine

Bile

The pancreas

Digestive enzymes

Large intestine

Summary of digestion (Table 12.6)

The liver

Common problems associated with the gut

Chapter 13 Reproduction

Reproductive anatomy of the mare

The oestrous cycle

Reproductive anatomy of the stallion

Fertilisation (Fig. 13.14)

Pregnancy

Preparation for birth

Applied reproduction (1): artificial insemination

Applied reproduction (2): embryo transfer

Applied reproduction (3): cloning

Part II Science in action

Chapter 14 Conformation and soundness

Conformation

Developing the ridden horse

Chapter 15 Movement and lameness

Centre of gravity

The stay apparatus (Fig. 15.3)

Moving the front legs

Moving the hind legs

The diagnosis of lameness

Chapter 16 The invaders

Bacteria

Viruses

Fungi

The immune system

Parasites of the horse

Common ailments and digestive disorders

Chapter 17 Wounds and wound healing

Wound healing

Types of wound

First aid

The use of lasers in the treatment of wounds

Chapter 18 Fitness and feeding

Preliminary work

Development work

Fast work

Interval training

The effects of training on the horse’s body

The circulatory system

Getting the Novice 1-day event horse fit

Interval training to Advanced levels

Feeding the competition horse

Low starch diets

Feeding oil

Glycaemic index

Electrolytes

Prohibited substances in feed

Chapter 19 Genetics and heredity

Genetics

Heredity

Index

Title Page

Preface

Equine science is becoming an increasingly popular subject among both students and horse owners, and as such this book has been comprehensively revised and updated to include new information on all aspects of it.

A comprehensive understanding of this important subject enables those studying or involved with horses to understand the key scientific principles relating to the way the horse’s body works and the changes that may occur during fitness and training, health and disease. A good understanding of equine science results in higher standards of management and welfare for horses, particularly those used for competition.

Equine Science includes detailed information on all systems of the horse and how they work together. It also includes sections on important practical aspects such as wound healing, disease-causing agents, conformation, soundness, fitness and feeding. New comprehensive chapters on the cell and equine genetics have been added to provide information on inheritance in horses and its application to the horse breeding industry.

Equine Science will be of value to students studying for equine degree programmes, Higher National Diplomas, Advanced National Certificate and BHS Stage IV. It will also prove useful to equine professionals and knowledgeable horse owners.

Part I

The systems of the horse

Chapter 1

The cell

The cell – building block of life

A highly complex animal like the horse is composed of billions of cells grouped into various tissues. Different tissues are combined to form organs. In the horse’s body different tasks are carried out by several organs working together. The organs make up a system, for example the digestive system. The body is like a tower block housing a complex organisation and in order to function efficiently it requires a great deal of cooperation, specialisation and mutual interdependence.

All living things are made up of microscopic building blocks or cells (Fig. 1.1). In the horse the total functioning of the body involves the interaction of an estimated 400 trillion cells – the functional units of life. Internally the cell is divided into compartments called cell organelles. Organelles are well defined and clearly identifiable structures within the cell which have a specific structure and function. The interaction of these organelles within the cell contributes to what we call life. Life is an organic means of converting one form of energy to another, in the process a number of acts are performed which are called the vital phenomena, or the attributes of life. All life must fulfil four vital phenomena:

Fig. 1.1 A typical mammalian cell.

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As life forms become more complex other criteria such as movement become important. Others include the transmission of stimuli to other cells and organisation, the grouping of cells into tissues, the building of organs and their integration into a coherent whole.

Functions of all cells include:

Cells are self-contained units with a great variety of structure and function. There are basically two types of cell, prokaryotic and eukaryotic.

Prokaryotic cells

These are found in lower forms of life such as bacteria in which the genetic material is free within the cell and there is no nucleus. There are no membrane-bound organelles such as mitochondria. Mitosis and meiosis are absent.

Eukaryotic cells

These cells include plant and animal cells. The cells have a nucleus and a highly organised cytoplasm containing cell organelles with various functions. Mitosis and meiosis take place in eukaryotic cells.

The cellular structure described below is that of a eukaryotic animal cell such as found in animals (mammals) including horses.

The two most common units used to describe the size of cell organelles are:

Major cell organelles

Cell membrane

The word membrane means ‘very thin layer’, an accurate description of the term. The cell membrane forms the outermost limits of the cell and separates it from other cells. Cell membrane also surrounds cell organelles but the structure remains basically the same and is known as a unit membrane.

The cell membrane allows different substances to pass through at varying rates and is therefore termed partially permeable. It encloses the cytoplasm, a jelly-like mass into which is suspended cell organelles.

The cell membrane consists of layers of lipid molecules with some protein molecules interspersed. It is actually made up of two layers of phospholipid molecules with proteins being embedded in each layer, some spanning from one side of the membrane to the other, others only at the surface of one membrane. These proteins are important for transport of substances across the membrane into and out of the cell. Some proteins also protrude from the cell membrane surface and these are involved in cell recognition. The phospholipids and proteins form a mosaic tile-like arrangement and this is known as a fluid mosaic structure (Fig. 1.2).

Fig. 1.2 Fluid mosaic structure of a cell membrane.

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Nucleus

This is the largest organelle in the cell, lying centrally and surrounded by cytoplasm. The nucleus is the ‘brain’ of the cell containing genetic material responsible for the transmission of hereditary characteristics from cell to cell and from parent to offspring. The nucleus contains DNA (deoxyribonucleic acid). Nucleic acid contains all the necessary information to make a new copy of the cell and to control the cell’s activities. In the dividing cell DNA molecules condense into visible chromosomes. At other times the nucleus appears grainy as DNA material is dispersed within the nucleus as chromatin. The nucleus may have one or more nucleoli which produce RNA (ribonucleic acid), required for making ribosomes. The nucleus is bound by a nuclear membrane containing pores large enough for larger molecules to pass through, e.g. messenger RNA (mRNA).

Endoplasmic reticulum and ribosomes

Together with ribosomes, the endoplasmic reticulum (ER) is involved with the synthesis of protein within the cell. It is a series of complex channels. The narrow fluid-filled space between the membranes acts as a transport network, moving materials through the cell. The folded membrane creates a large surface area. Much of the outer surface of the ER is dotted with tiny ribosomes and this is known as rough endoplasmic reticulum. Smooth ER has no ribosomes attached.

Ribosomes are approximately 20 nm in diameter and are very small structures. They are responsible for carrying out protein synthesis. A ribosome binds to mRNA and uses the encoded information contained therein to assemble amino acids for protein synthesis. Ribosomes to the ER make proteins for export from the cell whereas free ribosomes make proteins for use by the cell itself.

Golgi body (dictyosome)

The Golgi body consists of stacks of plate-like tubules. Its function is to take proteins and enzymes made by the ER and place them in membrane-bound packages or vesicles. Completed vesicles then ‘pinch’ off. The Golgi body is also involved in the secretion of protein from the cell, such as the release of digestive enzymes into the gut.

Mitochondria

Mitochondria are found in all living plant and animal cells. These cylindrical structures are responsible for the production of energy and are known as the ‘power houses’ of the cell. They are the sites of aerobic respiration, i.e. oxidation of glucose to produce energy. Each mitochondrion has two membranes, an outer one surrounding the structure and a highly folded inner one. These folds are known as cristae and have a large surface area for respiration. This is where transfer of energy to form ATP (adenosine triphosphate) occurs. ATP is the energy currency of all cells.

Lysosomes

Although similar in shape to mitochondria, lysosomes have a quite different function. Lysosomes are vesicles containing digestive enzymes. They break down old or surplus cell organelles inside the cell. They may also break down substances brought into the cell by the process of endocytosis. The horse’s body uses this ‘destruction’ process to break down excess muscle in the uterus following birth and to reduce milk-producing tissue in the mare’s udder following weaning.

Perioxisomes (microbodies)

Perioxisomes are small, spherical bodies approximately 1 µm in diameter. They contain enzymes which break down hydrogen peroxide, a highly toxic by-product produced in the cell. Hydrogen peroxide is turned into oxygen and water, thereby rendering it harmless to the cell. Perioxisomes are found in greater numbers in the more active cells such as liver and muscle cells.

Microtubules

Microtubules are found most commonly in eukaryotic cells. They are slender tubes approximately 24 nm in diameter. Microtubules provide an internal cellular skeleton also known as a cytoskeleton. They also aid cell division by forming the spindle, and are a major constituent of cilia and flagella.

Microfilaments

These are very fine strands approximately 6 nm in diameter and are made up of the contractile protein actin and sometimes myosin. Microfilaments are involved in movement within cells.

Centrosomes

Also known as centrioles, the centrosomes are clearly visible during cell division when they become the centre of the spindle apparatus that separates the chromosomes of the resulting cells.

Cell function

The individual cells of the horse’s body act, interact and react with each other, they are not independent. In order to function normally there must be a continuous exchange of information which is normally chemically or electrochemically controlled. These chemical compounds may be hormones or enzymes found in the fluid surrounding the cells (interstitial fluid) or directly transferred between adjacent cells.

The transfer or transport of substances takes place via filtration, diffusion, osmosis, Donnan equilibrium and dialysis. Thus:

Each of these reactions is designed to achieve balance between the cell and its environment, in other words the cell is at ease. A cell which is not in balance is ‘diseased’.

Order and organisation are of great importance in ensuring harmony, yet living things are also subject to the biological clock or temporal rhythms. There is a system of cycles that orders the lives of most living things. Seasons of the year affect the horse’s sexual activity, the length of coat and a host of other functions. Understanding and utilising biological rhythms is of prime importance in managing domestic animals in the unnatural and stressful environment in which they are kept.

Specialised cells are grouped together to form tissues.

Tissue organisation

A tissue is composed of a group of similar cells, which form a particular function. Histology is the study of these tissues.

Tissue types include:

Epithelial tissue

This tissue is associated with a lining tissue subject to mechanical damage, such as the skin. Epithelial tissue has the following characteristics:

Types of epithelial tissue are:

Columnar epithelium consists of tall narrow cells often bordered by cilia on the uppermost surface (Fig. 1.3a). These cells are found in the trachea, nasal cavities and fallopian tubes. Cilia may beat with a rhythm helping to move mucus and other substances along the surface of the epithelium.

Squamous epithelium consists of a flattened single layer of thin cells (Fig. 1.3b). These cells are found in areas of the body where rapid diffusion is necessary, such as in Bowman’s capsule in the kidney or alveoli in the lungs.

Compound epithelium consists of many layers of cells. Cells of the lowermost layers are more columnar in structure. Those near the top are often flattened and frequently dead. These may have been impregnated with the protein keratin. Compound epithelium is found in places where wear and tear takes place, such as in the epidermis of the skin, oesophagus and vagina.

Fig. 1.3 Types of epithelial tissue. (a) Columnar epithelium (ciliated). (b) Squamous epithelium (surface view).

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Connective tissue

Connective tissue contains a variety of different types of cells entrenched in an intercellular substance known as the matrix. Connective tissue includes tissues such as bone and cartilage, and even blood.

Cell division

There is a limit to the size an individual cell and therefore tissues and organs can reach. For example, there is a limit to how long the limb of a horse will grow. Cells within the horse’s limb can make different types of tissue, such as bone, muscle, tendon, nervous tissue and so on. Cells have instructions on what to do and when to stop doing it.

Cells divide by processes known as mitosis or meiosis:

The significance of meiosis is that when the egg and sperm join at fertilisation, the result will be cells with the correct number of chromosomes, i.e. diploid, ready for growth of the new offspring.

Mitosis (Fig. 1.4)

The stages are:

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Interphase

This is the period between cell division. Most of the time the nucleus appears grainy as DNA material is dispersed within the nucleus as chromatin. At the beginning of mitosis the chromosomes replicate themselves, forming a carbon copy within the nucleus of each chromosome. DNA replication takes place during interphase. Cells form new cell organelles to supply the daughter cells and build up a store of energy to support the process of cell division.

Prophase

Following replication, chromosomes contract and become visible as double strands. These chromosome strands – the parent cell strand and its carbon copy, the daughter cell strand – are known as chromatids joined together at their centre by a centromere (Fig. 1.5).

Metaphase

The nuclear membrane disappears and a spindle is formed by the centrioles. This spindle spans the width of the cell. The centromeres with the chromatids then attach to the centre of the spindle.

Anaphase

The centromere splits in two, each taking one chromatid along the spindle fibres to the opposite ends of the cell. This takes place by the spindle fibres contracting and requires energy.

Fig. 1.4 Stages of mitosis.

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Telophase

The chromatids have been separated from each other and lie at separate poles of the cell. The cell membrane begins to grow down between the two by invagination. Two daughter cells are formed and the spindle fibres break down and disappear. The nuclear membrane reforms around each of the two nuclei and chromosomes regain their grainy appearance.

Fig. 1.5 Chromosome structure.

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Meiosis(Fig. 1.6)

A process similar to mitosis occurs in the basic stages, but these take place twice, a first and second meiotic division:

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Interphase

It is similar to mitosis. There are five recognised stages of prophase:

Prophase I (leptotene)

Chromosomes appear and the spindle structure is developing. The chromosomes are separate fine threads.

Prophase I (zygotene)

The two homologous pairs of chromosomes get together, side by side. This pairing is known as synapsis and each pair is known as a bivalent.

Prophase I (pachytene)

The bivalents shorten and thicken. Each individual chromosome is now absolutely double, the replication having occurred in interphase. Each bivalent then consists of four chromatids.

Prophase I (diplotene)

This final stage of prophase is known as diakinesis. Diakinesis ends with the disappearance of the nuclear membrane and the appearance of the spindle. The chromatids move apart from each other, remaining in contact at the points known as chiasmata. Chiasmata are the visible signs of crossing over of genes. In a chiasma, two chromatids, one from each of the original chromosomes, have apparently broken at equivalent places. The broken ends of one chromatid have fused with broken ends of the other (Fig. 1.7). Crossing over results in an exchange of genetic material, which results in greater variation in the resultant sex cells.

Fig. 1.6 Stages of meiosis.

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Chiasmata hold homologous chromosomes together before moving on to the spindle, before splitting.

Fig. 1.7 Crossing over occurs in prophase during meiosis.

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Metaphase I

Homologous pairs of chromosomes position themselves at the centre of the spindle.

Anaphase I

Homologous chromosomes move to opposite ends of the spindle as it contracts. These are attached to the spindle by the centromeres. The chromatids do not separate.

Telophase I

Cell membrane invaginates as chromosomes reach the opposite ends of the spindle. This may be followed by a short period of interphase, or cells may immediately go into the second phase of meiosis, separation of the chromatids.

Prophase II

Spindles begin to form.

Metaphase II

Chromosomes arrange themselves on spindle at the centre.

Anaphase II

Chromatids separate by pulling apart and moving to opposite ends of the cell.

Telophase II

The nuclear membranes form and the cell membrane invaginates. The result is two daughter cells, each with half the number of chromosomes of the original parent cell.

Chapter 2

The skeleton

Points of the horse

It is important to be familiar with the surface anatomy of the horse in order to be able to identify the underlying structures (Fig. 2.1).

Fig. 2.1 Points of the horse.

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Bone

Functions of bone

Bone has several functions including:

Classification, development and growth of bone

Bone has two important properties. It is both rigid, giving strength, and elastic, allowing some flexibility, without which bone would be brittle and easily broken. About 30% of an adult horse’s bone consists of living, organic fibrous tissue or collagen. This provides flexibility to bone. The remaining 70% is made up of inorganic bone salts of which the most important is hydroxy-apatite (Ca10(PO4)6(OH)2). Calcium and phosphorus are thus essential in the diet to maintain bone structure, but sodium, magnesium, potassium, chloride, fluoride, bicarbonate and citrate ions are all present in variable amounts. Essentially bone consists of a matrix encrusted with mineral salts which impart strength.

The proportion of organic and inorganic material in bone varies with age. Thus young horses have ‘soft’ bones containing up to 60% fibrous tissue, while old horses develop ‘brittle’ bones with a low fibrous content.

While bone may look inelastic and almost lifeless it is a highly dynamic structure – the entire calcium content of the skeleton is replaced every 200 days. This ability to mobilise minerals allows the skeleton to act as a mineral reservoir in times of need. A lactating mare, for example, can make up the required calcium for milk from her body reserves should her diet lack calcium. No other tissue in the body is capable of as much growth and as much absorption as bone.

Classification of bone

There are two types of bone:

Spongy bone is made up of slender, irregular trabeculae or bars which branch and unite to form a network. Compact bone is solid except for microscopic spaces. Both contain the same histological elements. With few exceptions both spongy and compact types are present in every bone, but the amount and distribution of each type can vary considerably. Spongy bone is associated with red bone marrow and the synthesis of red blood cells. Both types are penetrated by tiny blood and lymph vessels as well as nerves.

Each bone has a characteristic shape which is determined by its function within the skeleton. There are four main types:

Long bones act as supporting columns and levers, for example the cannon bone. They consist of a shaft (diaphysis) and two ends (epiphyses). Compact bone makes up the shaft and is organised in a tube-like form surrounding yellow bone marrow (the medullary cavity). Each end of the long bone is spongy bone covered by a thin shell of compact bone. Long bones are characteristic of limbs and have terminal enlargements which are associated with joints, for example the end of the femur and tibia. There are two reasons for this:

Flat bones, for example the skull, act as protection. In flat bones, two plates of compact bone enclose a middle layer of spongy bone.

Short or cuboidal bones, for example the carpal bones of the knee and the tarsal bones of the hock, act as shock absorbers.

Irregular bones generally have a specialist function, such as the vertebrae. Most irregular bones consist of spongy bone covered by a thin shell of compact bone.

Regardless of their shape the composition of these bones is similar. A cross-section of long bone (Fig. 2.2) shows an outer layer of compact bone for strength and then an inner layer of spongy bone. The compact bone is thickest where stress is the greatest. Surrounding the compact bone is a dense connective tissue membrane called the periosteum which acts as an attachment point for tendons and ligaments. The periosteum is lined with cells called osteoblasts which secrete new bone matrix. The bone is laid down in bands called lamellae supplied with capillaries which run from the arterioles in the Haversian canal. This structure enables the passage of nutrients and waste to and from the bone cells. The canal also carries lymph and nerve fibres. The lamellar structure means that bone can withstand high compressive and tensile forces.

Fig. 2.2 Cross-section through a long bone.

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Bone formation and growth

In the unborn foal there are two main types of bone formation or ossification:

Intramembranous bone formation gives rise to flat bones such as the skull, jaws and pectoral girdle, and involves ossification of the dermis. Mesenchyma cells become differentiated into rows of osteoblasts which begin to lay down bony plates. The osteoblasts also increase in number and lay down bone salts onto the plates resulting in an increase in size.

The best example of endochondral ossification is in the long bones (Fig. 2.3). In the embryo bone begins as cartilage, then osteoblasts invade the cartilage and lay down bone matrix. Mineral salts are then deposited in the matrix, a process called calcification, resulting in bone. By the time the foal is born, calcification is complete in most bones.

Once the bone is formed it grows; growth involves an increase in diameter as well as in length. The increase in length takes place at two narrow bands of cartilage called the epiphyseal growth plates. Cartilage grows continuously on the side of the growth plate nearest the end of the bone, meanwhile the cartilage on the shaft side of the growth plate is invaded by osteoblasts and converted to bone. Gradually the osteoblasts ‘catch up’ on the cartilage so that when the bone is the correct length the growth plate ‘closes’. The growth plates close at different times:

The closure of the plates is affected by the sex hormones. Colts and fillies grow at a similar rate until they reach puberty when testosterone, the male sex hormone, stops the closure of the plates, so that colts continue to grow. Oestrogen promotes the closure of the plates and the growth rate of fillies slows down. Anabolic steroids tend to hasten the closure of the plates and if given to the young horse will stunt the growth of the skeleton while enhancing muscle development.

Fig. 2.3 Growth and development of a long bone.

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Growth-related problems or developmental orthopaedic diseases can occur when the correct nutrients for bone growth are not supplied in the correct amounts and proportions. Growth must take place so that the horse’s athletic ability is not compromised. Using horses before the closure of the plates can also lead to growth-related problems. The distal radial plates do not close until well into the racing career of the flat racehorse. The cartilage of the open epiphysis is very sensitive to jarring which can give rise to large lumpy swellings immediately above the knee.

The increase in diameter is brought about by osteoblasts which line the periosteum (Fig. 2.4). In simple terms the osteoblasts lay down new layers of bone over the old in a similar way to the growth of a tree which gives rise to rings that can be seen when the tree is felled. Simultaneously the old bone lining the marrow cavity is eaten away by osteoclasts, cells which reabsorb bone. This means that the bone marrow cavity will enlarge but the wall of the shaft does not become too thick and heavy.

Fig. 2.4 Activity of the epiphyseal plate.

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Adaptation of bone to stress

Throughout the horse’s life bone undergoes a remodelling process which is a balance between the breakdown of old bone and the formation of new bone. Remodelling occurs to allow the bone to act as a mineral store and also to let the bone adapt to stress such as exercise.

Remodelling starts when the foal is about 3-months-old, when the newly formed bone begins to rearrange itself into Haversian systems. These systems consist of a series of vertically aligned tubules through which the blood vessels travel. If the Haversian systems are formed too quickly with too little mineral content, the bone becomes porous and not as strong as dense bone. It is essential that there are adequate and balanced amounts of calcium and phosphorus, and other important trace minerals in the diet to allow effective remodelling to take place.

Mineral storage in bone

Bone acts as an important store of calcium and phosphorus which can be readily called upon, for example, during pregnancy or lactation.

Blood cell production

The soft red bone marrow found in the ribs, sternum and long bones of young foals produces red blood cells. As the animal matures the red bone marrow in the long bones is replaced by yellow bone marrow and the red cell production is taken over mainly by the spleen.

The skeleton (Fig. 2.5)

The skeleton consists of bones, cartilage and joints, and can be divided into two parts:

The axial skeleton

The spine

The spine provides longitudinal support for the body and the necessary strength for suspending the enormous weight of the gut. It is relatively rigid and incompressible compared with the flexible spine of cats, for example. The vertebrae themselves are wholly incompressible; bending is allowed by the joints between the vertebrae while the cartilaginous discs between each vertebra allow slight compression. The spine can be divided into five regions:

The vertebrae make up a long bony chain housing and protecting the spinal cord. At each vertebra a pair of spinal nerves branch off from the spinal cord to penetrate every part of the body. Each vertebra has the same basic shape:

The spinal, lateral and articular processes allow for the attachment of muscles and ligaments, and the body and arch protect the spinal cord.

Cervical vertebrae (Fig. 2.6)

The horse’s neck consists of seven cervical vertebrae; the first is the atlas which consists of a short tube with large wings, it articulates with the skull at the occiput, allowing the horse’s head to nod. The wings of the atlas can be felt on either side of the horse’s neck below the poll and behind the jawbone. The second is the axis united to the atlas by a tooth-like projection, the odontoid process which allows the head to move from side to side. The long, strong ligament of the neck, the nuchal ligament, attaches to the axis; it helps hold up the horse’s very heavy head and neck, and allows the head and neck to be raised and lowered. The joints between the other cervical vertebrae enable the horse to bend its neck sideways and to arch its neck. The curves formed by the vertebrae are deep in the neck and do not follow the crest.

Fig. 2.5 The equine skeleton

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Fig. 2.6 Atlas (left) and axis (right) vertebrae.

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Thoracic vertebrae (Fig. 2.7)

The back consists of 18 thoracic vertebrae, five or six lumbar vertebrae, the sacrum and the coccygeal vertebrae. The thoracic vertebrae are typical vertebrae linked by cartilaginous pads called discs; the spinous processes are very large giving the horse its pronounced withers and allowing extensive muscle and ligament attachment. The withers are the highest point of the thoracic spine and are formed by the spinous processes of the third to tenth thoracic vertebrae. The withers are held firmly in place by ligaments between the spines and other muscles and ligaments attached to the spines, including the funicular portion of the nuchal ligament. The way the spinous processes are directed is of great importance to the athletic horse; there are two articulations, one between the discs and the bodies of the vertebrae, and the lateral articulations on each side of the vertebrae. The movement between the horse’s thoracic vertebrae is strictly defined and limited in comparison to many other animals.

Lumbar vertebrae (Fig. 2.8)

The lumbar vertebrae make up the loin region and, as with the thoracic vertebrae, have a strictly defined and very limited degree of movement. In fact, apart from the neck and tail, the horse’s back shows very little movement; some movement only, is seen between the last thoracic and first lumbar vertebrae and between the first three lumbar vertebrae. The degree of movement depends on the thickness of the intervertebral discs, which are firmly attached to the vertebrae, almost like part of the bone that has not yet become calcified or bony. Indeed, as the horse ages it is common to find that the discs do become calcified thus joining the vertebrae together. There may even be further outgrowths of bone acting as bridges across neighbouring vertebrae; two adjacent lumbar vertebrae may be joined by the transverse process on one side and not the other, and this will cause pain until both sides become fused.

Fig. 2.7 Thoracic vertebra.

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Fig. 2.8 Lumbar vertebra.

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Sacrum (Fig. 2.9)

The sacrum is a composite bone made up of five vertebrae, situated beneath the loins in the croup region. The pelvic bones are attached to either side of it by the sacroiliac joint.

Fig. 2.9 Sacrum: (a) dorsolateral view; (b) ventral view.

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Coccygeal vertebrae

There are usually 18 coccygeal or tail vertebrae but the number can vary from 15 to 21 in number and they decrease in size and complexity from first to last.

The total length of the spine is a series of curves, so that it is slightly arched at the upper end of the neck and concave above in the lower third of the neck (Fig. 2.10). At the junction of the neck and the chest there is a marked change in direction, followed by a gentle curve, concave below, through the thoracic and lumbar region, which helps to support the body weight. The dorsal spines of the thoracic vertebrae are held in position by strong ligaments. The horse’s spine is designed like a suspension bridge and these curves give it the strength needed to carry the enormous weight of its gut – up to 200 kg in a 16.2 hh horse – and, in the mare, to also carry the foetus and the associated fluid and membranes. The horse is not designed to carry a rider’s weight on top of its back.

Fig. 2.10 The spine is a series of curves.

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At walk the spine can be seen to move sideways but at faster paces there is increased muscular resistance which minimises any movement. Above the spine the longissimus dorsi muscle and below the spine, the psoas minor muscle, contract to stop the horse flexing its back; when this synchronisation fails, for example if the horse falls, then the back is vulnerable to damage.

The ligaments associated with the vertebrae include the supraspinous ligament, which runs along the top of the spines of the vertebrae and unites the summits of all lumbar and thoracic vertebrae. It divides to go up either side of the neck, becoming the nuchal ligament. The nuchal ligament consists of two parts:

The ribs

Each thoracic vertebra carries a pair of ribs, thus there are 18 pairs of ribs. Eight true ribs are attached to the sternum, 10 pairs of false ribs are connected to each other and form the costal arch.

The sternum

The sternum or breastbone forms the floor of the chest and supports the true ribs. The rear of the sternum is drawn out into the xiphoid cartilage.

The appendicular skeleton

The appendicular skeleton is attached to the axial skeleton by the pelvic girdle and the pectoral girdle. However the horse has no collar bone so that the pectoral girdle is attached only by muscles and ligaments to the spine, ribs and sternum. This means that the forehand of the horse is designed to support the body and absorb concussion, not to propel the horse forwards.

The forelimb (Fig. 2.11)

The forelimb consists of the following:

Fig. 2.11 Forelimb and knee.

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Scapula

The scapula is a triangular, flattened bone which glides back and forth over the rib cage. There is no bony attachment between the scapula and the spine so that the thorax is slung between the two scapulae, allowing the horse freedom of movement and compensating for the lack of flexibility of the spine. The length of the scapula will determine the slope of the shoulder and hence the length of stride of the horse. The scapula is divided lengthways by a prominent ridge called the scapular spine which can be felt through the skin. The supraspinatus muscle lies in front of the ridge and the infraspinatus muscle behind the ridge. The trapezius muscle and deltoid muscle are also attached to the scapular spine.

Humerus

The shoulder joint is formed between the scapula and the humerus. The humerus is one of the strongest bones in the body. The angulation of the humerus allows for shock absorption, and it is also the site of attachment for many muscles.

Radius and ulna

Unlike the human, the radius and ulna of the horse (equivalent to our lower arm) are fused together to prevent any twisting of the horse’s forearm. The ulna has become very small except for the olecranon process which forms the point of the elbow. The elbow joint itself is a hinge (ginglymus) joint which allows movement in one direction only.

The carpus (knee)

The horse’s knee is equivalent to the human wrist and consists of seven or eight small carpal bones; in the upper row are the radial, intermediate and ulnar carpals, with the pisiform bone or accessory carpal bone at the back of the knee. The lower row consists of the first, second, third and fourth carpal bones. The knee is a hinge joint allowing movement in only one direction and the arrangement of the small carpal bones is designed to absorb shock.

The metacarpals

The three metacarpal bones are better known as the cannon-bone and the two splint bones. The splint bones are a legacy from when the horse’s ancestors had several toes, but while they do support the knee they are no longer weight-bearing. The cannon bone is capable of carrying substantial weight, having little spongy or cancellous bone surrounded by solid bone. The amount of ‘bone’ a horse has is the circumference of the leg just below the knee and indicates the weight-carrying capacity of the horse.

The phalanges

The three phalanges are known as the long pastern, short pastern and pedal bone, and are equivalent to the human finger. The tendons from the muscles of the forearm attach to these bones, giving increased leverage and a powerful stride. The joint between the cannon-bone and the long pastern bone is the fetlock joint, and is another hinge joint. It is subjected to large amounts of stress and has a great deal of movement. The pastern joint is between the long and short pastern bones, and has limited movement. The coffin joint is between the short pastern and the pedal bone, and has a good range of movement.

The sesamoid bones

The horse has three sesamoid bones; the proximal sesamoids, known as the ‘sesamoids’ are situated at the back of the fetlock joint while the distal sesamoid or navicular bone is found inside the hoof at the back of the coffin joint. Their role is to act as pulleys enabling the tendons that run over them to exert their pull on the phalanges.

The hindlimb (Fig. 2.12)

The hindlimb has a bony attachment to the spine allowing the propulsive forces to be transmitted to and along the spine to generate movement. The hindlimb consists of:

The pelvic girdle

The pelvic girdle consists of the pelvic bones, the sacrum and the first three coccygeal vertebrae.

The pelvis (Fig. 2.13)

Each half of the pelvis is made up of three flat bones, the ilium, ischium and the pubis, which are fused into one. The upper portion of the pelvis, which is attached to the sacrum is called the ilium. The front of the pelvic floor is the pubis and the rear portion is the ischium. All three bones meet at the acetabulum which articulates with the head of the femur to make the hip joint. The ilium is the largest bone and its outermost angle is seen as the tuber coxae – the point of the hip. Where the ilium attaches to the sacrum is the sacro-iliac joint which is characterised by strong muscle attachments. At the highest point of the hindquarters the two sides of the tuber sacrale form the croup. The point of the buttocks are the thickened ends of the ischium known as the tuber ischii.

The hip joint

The hip joint is deep in the hindquarter of the horse and is most easily seen when the hind leg is flexed. It is the joint between the pelvis and the femur and is capable of a wide range of movement. It acts to protect the internal organs, as a site for muscle attachment and to allow the efficient transfer of force to the spine.

The femur

This very strong bone is designed to act as the medium between the hip joint and the stifle joint and is adapted for the attachment of the muscles of the hindquarter.

The patella

The patella is a sesamoid bone and the equivalent of the human kneecap. It is associated with the stifle – the joint between the femur and the tibia.

Fig. 2.12 Hindlimb and hock.

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The tibia and fibula

The tibia is a long bone running down and back between the stifle and the hock joints. The upper end provides attachment for the muscles acting on the hock and lower limb. The horse’s fibula is so reduced in size as to be practically vestigial.

Fig. 2.13 Pelvis.

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The tarsus (hock)

The hock consists of six or seven short, flat tarsal bones arranged in three rows. In the upper row are the talus and the calcaneus; in the middle row is the central tarsus and below that the fused first and second tarsal and the third tarsal. The fourth tarsal occupies both the middle and lower row. A long bony process, the tuber calcis of the calcaneus, gives rise to the point of hock and guides the Achilles tendon of the gastrocnemius muscle over the hock, allowing tremendous leverage.

The lower limb

Below the hock the arrangement is the same as in the forelimb.

Diseases of the bone

Bones are deeply seated within other tissues and are not richly supplied with blood, which means that bone disease is often overlooked. Diseases of bone fall into five main categories:

Periostitis

Periostitis is inflammation of the surface of the bone and the periosteum. It is usually caused by sprain, blow or infection, and the signs are pain, heat and swelling over the affected area. The periosteum becomes inflamed and the resulting haemorrhage lifts it away from the bone and this stimulates the osteoblasts to make new bone. A tender bony swelling then develops, often accompanied by inflammation of the surrounding soft tissue. As the inflammation subsides the bony lump remodels, becoming smaller and painless. Examples of conditions caused by periostitis are splints, ringbone and sore shins.

Sore shins (Fig. 2.14)

Sore or bucked shins often occur in 2- and 3-year-old races in training. The immature bone is subjected to stress which causes microfractures and haemorrhage under the periosteum at the front of the cannon bone. The front of the cannon bone has a convex outline and the horse may become lame. With correct treatment the horse will recover but the cannon bone may retain a convex outline.

Fig. 2.14 Sore shins. The front of the cannon bone has a convex outline.

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Splints (Fig. 2.15)

The splint bones are attached to the cannon bone by the interosseus ligament. In young horses this ligament is susceptible to strains and tears which result in bleeding and inflammation of the periosteum. New bone is produced which is seen as a bony enlargement or splint usually on the inside of the forelimb. Developing splints often cause lameness but as the splint settles down the swelling becomes smaller and harder and the horse returns to soundness.

Fig. 2.15 Splint formation.

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Ostitis

Ostitis involves inflammation of the bone itself as, for example, in pedal ostitis.

Epiphysitis

Epiphysitis occurs when the growth plate becomes inflamed and is termed a developmental orthopaedic disease (DOD, see p. 34).

Infection

Infection of bone can be very serious requiring immediate veterinary attention. It may require surgery.

Fractures

Fractures can be classified according to the age of the animal and the type and site of the break. Fractures of small bones may not be serious while breaking a large weight-bearing bone may result in the horse having to be destroyed.

Joints

Fig. 2.16