Directional Terms

Directional terms are used to provide additional information about the position of organs and tissues (Table 4.1 and Figure 4.1). They are commonly used during diagnostic imaging.

Anatomical Planes

Anatomical planes are used to describe the location of parts of the animal (Table 4.2 and Figure 4.2).

Root Words, Prefixes and Suffixes

By learning the meanings of root words, prefix and suffix words can be broken down into components to determine their meanings (Table 4.3).

Commonly used root words are shown in Table 4.4. For commonly used prefixes and suffixes please refer to Table 4.5.

Thoracic Cavity

• Houses the heart, the lungs and associated structures
  
• Defined by the thoracic inlet (cranially), diaphragm (caudally), vertebrae (dorsally), sternum (ventrally) and ribs (laterally).
  
• Lined by a serous membrane known as the parietal pleura. Where this covers the lungs, it is known as the pulmonary pleura.
  
• Between the lungs and the parietal pleura is the pleural space
  
  
  
  • – The pleural space has a negative pressure, which allows the lungs to inflate.
    
  • – There is a small amount of fluid here known as pleural fluid, which allows the lungs to move freely against the ribcage [1–3].

Mediastinum

• The space in the anterior chest between the lungs. It contains the thymus, heart, aorta, trachea, oesophagus, a variety of nerves and blood vessels [1–3].

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Directional term	Description
  Cranial (anterior)	Towards the head
  Caudal (posterior)	Towards the tail
  Rostral	Towards the nose
  Dorsal	Towards or near the back
  Ventral	Towards the belly
  Lateral	Away from the midline
  Medial	Towards the midline
  Proximal	Towards the point of attachment
  Distal	Away from the point of attachment
  Dorsal (in relation to the limb)	The front surface of the lower limb
  Palmar	The back or under surface of the lower forelimb area
  Plantar	The back or under surface of the lower hindlimb area
  Ipsilateral	On the same side
  Contralateral	On the opposite side
  Superficial	Nearer the surface
  Deep	Further from the surface

  

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Anatomical plane	Description
  Median plane	A line which divides the body along the mid‐line into right and left halves
  Sagittal plane	Any line parallel to the median plane
  Dorsal plane/Frontal plane	A line parallel to the back of the animal
  Transverse plane	A line perpendicular to the long axis of the animal

  

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Component	Description	Example
  Root	The essence of the word meaning; often relates to an organ, structure or disease	Cardium/Cardi(o) – relates to the heart
  Prefix	Placed at the beginning of a word to alter or modify the meaning	Endocardium – within the heart
  Suffix	Placed at the end of a word to alter or modify the meaning	Carditis – inflammation of the heart
  Prefix + Root + Suffix – Endocarditis – inflammation within the heart

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Root word	Description	Example
  Athro(o)	Joint; articulation	Arthritis – inflammation of a joint
  Cardio(o)	Heart	Cardiology – the study of the heart and its function
  Chondro	Cartilage	Chondrocyte – cartilage cell
  Cyst(o)	Bladder	Cystotomy – incision into the bladder
  Dermat(o)	Skin	Dermatitis – inflammation of the skin
  Gloss(o)	Tongue	Hypoglossal – situated below the tongue
  Haemat(o)/Haem(o)	Blood	Haemorrhage – bleeding from a ruptured vessel
  Hepat(o)	Liver	Hepatocyte – liver cell
  Hist(o)	Tissue	Histology – the study of tissues
  Mamm(o)	Breast; mammary gland	Mammogram – radiograph of a mammary gland
  Metra/Metro	Uterus	Endometrium – lining of the uterus
  Myo‐	Muscle	Myositis – inflammation of a voluntary muscle
  Neur(o)	Nerve	Neuralgia – pain in a nerve
  Opthalm(o)	Eye	Ophthalmoscope – instrument used to examine the eye
  Orchi	Testis (testicle)	Orchitis – inflammation of a testis
  Oste(o)	Bone	Osteomyelitis – inflammation of a bone
  Pneum(o)	Air or gas; lung	Pneumonia – inflammation of the lung tissue
  ‐pnoea	Respiration; breathing	Dyspnoea – difficulty in breathing
  Ren‐	Kidney	Renal artery – the artery that supplies the kidney with blood
  Rhin(o)	Nose	Rhinitis – inflammation of the mucous membrane of the nose
  Vas(o)	Vessel; duct	Vasoconstriction – a decrease in the diameter of a blood vessel

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  	Prefix/Suffix	Meaning	Example
  1	Ab‐	Away	Abduction – moving a limb away from the midline Abrasion – scraping something away, e.g. graze Abnormal – deviating from the norm
  2	Ad‐	Towards	Adduction – moving a limb towards the midline Administer – application of a drug/therapy Admittance – allow entry
  3	Ex‐	Out from	Exocytosis – transport of substances out of a cell Exophthalmos – anterior displacement of the eye Exocrine gland – secrete substances onto a surface
  4	Ecto‐	Outside	Ectoparasite – parasite that lives on the outside of the host Ectocytic – outside a cell Ectocornea – the outer layer of the cornea
  5	Endo‐	Within	Endoscopy – viewing internal parts of the body Endothelium – layer of tissue lining the inside of blood vessels and organs Endotracheal – within the trachea
  6	Epi‐	Upon	Epidermis – the superficial layer of skin Epiphysis – the end part of a long bone Epimysium – sheath of tissue surrounding a muscle
  7	Hypo‐	Under/Low	Hypoxia – low oxygen levels Hypothermia – low body temperature Hypotension – low blood pressure
  8	Hyper‐	Above/high	Hyperthermia – abnormally high body temperature Hypertension – abnormally high blood pressure Hypercapnia – abnormally high levels of carbon dioxide in the blood
  9	Brady‐	Slow	Bradycardia – slow heart rate Bradypnoea – slow respiratory rate Bradykinesia – slow movement
  10	Tachy‐	Fast	Tachycardia – fast heart rate Tachypnoea – fast respiratory rate Tachyphylaxis – rapid decrease in medication response
  11	A/an	Lack of	Anaemia – insufficient red blood cells/haemoglobin in the blood to transport oxygen Anorexia – lack of appetite Apnoea – cessation of breathing
  12	Inter‐	Between	Intercellular – occurring between cells Interstitial – space between structures Intervertebral – between the vertebrae
  13	Intra‐	Within	Intracellular – occurring within a cell Intravascular – within a blood vessel Intraarticular – within a joint
  14	Peri‐	Around	Perivascular – around blood vessels Pericardium – serous membrane surrounding the heart Perimysium – sheath of connective tissue surrounding a bundle of muscle fibres
  15	Post‐	After	Postoperative – the period following surgery
  16	Uni/mono‐	One	Monorchid – having only one testicle Uniparous – producing only a single offspring Monogastric – having a stomach with a single compartment
  17	Bi/di‐	Two	Dichromatic – vision where only two of the three primary colours can be seen Biceps – a muscle comprising two heads
  18	Tri‐	Three	Triceps – a muscle comprising three heads
  19	Quad/tetra‐	Four	Tetralogy of Fallot – heart defect characterised by four specific defects occurring together Quadriceps – a muscle comprising four heads
  20	Poly‐	Many	Polyuria – excess urination Polyphagia – excess hunger Polypharmacy – use of multiple medicines at one time
  21	Oligo‐	Few	Oliguria – reduced urine output Oligodontia – less than the normal number of teeth
  22	Macr‐	Large	Macroscopic – visible to the eye
  23	Micro‐	Small	Microscopic – not visible to the eye Microscope – instrument used to visualise things that are too small to be seen by the eye
  24	Hepato‐	Liver	Hepatocyte – liver cell
  25	Derm‐	Skin	Dermatitis – irritation of the skin Dermatology – study of the skin
  26	Pneumo‐	Lung	Pneumothorax – air escaping the lungs into the thorax Pneumonia – infection of the lung(s) Pneumopathy – any disease of the lungs
  27	Gastro‐	Stomach	Gastroscopy – Viewing the inside of the stomach
  28	Cyst‐	Bladder	Cystoscopy – looking inside the bladder Cystitis – inflammation of the bladder Cystotomy – cutting into the bladder to remove intact urinary calculi
  29	Nephro/reno‐	Kidney	Nephrosplenic ligament – connects the kidney to the spleen Nephrotoxicity – dysfunction of the kidney caused by toxic chemicals Nephritis – inflammation of the kidney
  30	Cardio‐	Heart	Cardiology – study of the heart Cardiomyopathy – disease affecting the muscle of the heart Cardiogenic shock – condition caused by the inability of the heart to pump blood sufficiently
  31	Entero‐	Intestines	Enteropathy – disease of the intestines Enterology – study of the intestines
  32	Optho‐	Eye	Ophthalmology – study of the eyes Ophthalmoscope – instrument for visualising the eye
  33	Thoraco‐	Thorax/chest	Thoracocentesis – puncturing the thorax to remove air/fluid Thoracotomy – surgical procedure to gain access to the pleural space
  34	Abdomino‐	Abdomen	Abdominocentesis – puncturing the abdomen to obtain a fluid sample
  35	Arthro‐	Joint	Arthroscopy – procedure looking into a jointArthrodesis – surgical immobilisation of a joint by fusing the bones
  36	Teno‐	Tendon	Tenocyte – mature tendon cell Tenoblast – immature tendon cell
  37	Cyto‐	Cell	Cytology – study of cells Cytotoxic – toxic to living cells Cytostatic – inhibits cell growth and multiplication
  38	Haem‐	Blood	Haematology – study of blood Haematoma – collection of blood outside of the vessels Haemopoiesis – production of blood cells
  39	Erythro‐	Red blood cell	Erythropenia – low red blood cell count Erythropoiesis – production of red blood cells Erythropoietin – hormone that stimulates red blood cell production
  40	Leuco‐	White	Leukocyte – white blood cell Leukopenia – low white blood cell count
  41	Thrombo‐	Blood clot	Thrombocyte – cell fragment responsible for clot formation (platelets) Thrombocytopenia – decreased number of platelets in the blood Thrombosis – formation of a clot in a vessel
  42	Pyo‐	Pus	Pyometra – pus in the uterus Pyothorax – pus in the thorax Pyonephritis – pus in the renal pelvis
  43	Hydro‐	Water	Hydrotherapy – use of water in treatment Hydrolysis – breaking a substance down using water Hydrocele – collection of fluid in the scrotum
  44	Chrondro‐	Cartilage	Chondrocyte – cell responsible for cartilage formation Chondropathy – disease of cartilage
  45	Laryn‐	Larynx	Laryngeal hemiplegia – paralysis of one or both arytenoid cartilages of the larynx Laryngeal nerve – nerve supplying the larynx
  46	Myo‐	Muscle	Myocardium – muscle of the heart Myopathy – disease affecting voluntary control of muscle Myology – study of muscles
  47	Osteo‐	Bone	Osteocyte – bone cell Osteology – study of bones Osteomyelitis – painful inflammation of bone
  48	Dys‐	Difficult	Dyspnoea – difficulty breathing Dysphagia – difficulty swallowing/eating Dysuria – difficulty urinating
  49	Mal‐	Bad/poor	Malabsorption – poor absorption of nutrients from food Malfunction – functioning badly Malpractice – poor practice
  50	Iso‐	Same	Isometric – equal dimensions Isotonic solution – has the same water/solute concentration as cells Isotonic contraction – tension remains the same in a muscle and the muscle shortens
  51	Patho‐	Disease	Pathology – the study of disease Pathogen – disease agent Pathogenesis – development of disease
  52	Neo‐	New	Neonate – new‐born Neoplasia – new, abnormal growth Neonatology – medical specialism focussing on new‐borns
  53	‐pathy	Disease	Myopathy – disease affecting the control of muscles Neuropathy – nerve damage that can cause pain, weakness or numbness Cardiomyopathy – disease of the heart muscle
  54	‐scopy	View	Arthroscopy – procedure looking in a joint Gastroscopy – procedure looking in the stomach Laparoscopy – procedure looking in the abdomen
  55	‐otomy	Cut into	Laparotomy – incision through the abdominal wall Hysterotomy – incision into the uterus Thoracotomy – incision into the pleural space
  56	‐ectomy	Cut out	Ovariectomy – surgical removal of an ovary/ovaries Neurectomy – surgical removal of all/ part of a nerve Hysterectomy – surgical removal of all/part of the uterus
  57	‐ostomy	Form an opening	Tracheostomy – incision into the trachea to create an opening Urethrostomy – creating a temporary opening for diversion of urine when the urethra is blocked Thoracostomy – incision in the chest wall with maintenance of the opening for drainage
  58	‐plasty	Repair/reform	Vulvoplasty – procedure altering the construction of the vulva, i.e. Caslick’s procedure Hernioplasty – surgical reshaping of a hernia Tendoplasty – reparative surgery on a tendon
  59	‐centesis	Puncture	Abdominocentesis – puncturing the abdomen to obtain a fluid sample Thoracocentesis – puncturing the thorax to remove air/fluid
  60	‐algia	Pain	Neuralgia – pain along the course of a nerve Myalgia – pain in a muscle Arthralgia – joint pain
  61	‐itis	Inflammation	Nephritis – inflammation of the kidney Carditis – inflammation of the heart Arthritis – inflammation of the joints
  62	‐osis	Condition	Cyanosis – blue colouration caused by hypoxia Salmonellosis – infection of the salmonella bacteria Acidosis – build‐up of acid
  63	‐ology	Study	Histology – study of tissues Cytology – study of cells Pathology – study of disease
  64	‐graphy	Record	Radiography – taking X‐rays Ultrasonography – imaging the body with ultrasound waves Scintigraphy – imaging the body using a radioactive substance
  65	‐emia	Of the blood	Anaemia – insufficient RBCs/haemoglobin in the blood to transport oxygen Hypoxaemia – low oxygen levels in the blood Ischemia – restricted blood flow
  66	‐cyte	Cell	Osteocyte – mature bone cell Tenocyte – mature tendon cell Erythrocyte – red blood cell
  67	‐therapy	Course of treatment	Hydrotherapy – use of water as treatment Chemotherapy – use of cytotoxic drugs to treat cancer Cryotherapy – use of extreme cold treatment
  68	‐pnoea	Breathing	Apnoea – cessation of breathing Dyspnoea – difficulty breathing Tachypnoea – fast respiratory rate
  69	‐penia	Deficiency	Leukopenia – low white blood cell count Neutropenia – low neutrophil count in the blood Erythropenia – low red blood cell count
  70	‐oma	Swelling	Haematoma – a swelling of clotted blood Lipoma – benign tumour of fatty tissue Melanoma – a pigmented tumour
  71	‐lysis	Destruction	Haemolysis – destruction of red blood cells Bacteriolysis – rupture of bacterial cells Hydrolysis – breaking a substance down using water
  72	‐phagia	Eating	Dysphagia – difficulty eating/swallowing Polyphagia – excessive eating Aphagia – inability to swallow

  

Abdominal Cavity

• Houses the digestive system and some components of the urogenital system.
  
• Defined by the diaphragm (cranially), pelvic opening (caudally), vertebrae (dorsally) and abdominal muscles (laterally and ventrally). Lined by a serous membrane known as the peritoneum. This produces a small amount of peritoneal fluid, which enables the abdominal organs to move freely against each other [1–3].

Pelvic Cavity

• Anatomically but not physically separated from the abdomen.
  
• Houses the reproductive organs, the bladder and the rectum.
  
• Defined by the pelvic inlet (cranially), pelvic outlet (caudally), pelvic bones (dorsally) and muscles around the pelvic girdle (laterally) [1–3].

Cell Structure

Cells are the individual units that make up tissues, organs and body systems. Cells are microscopic and carry out several basic functions, including taking in nutrients, excreting waste, respiring and reproducing. Depending on their location in the body and their function, some cells have specialised structures, but the basic structure of all cells is the same [1–3] (see Figure 4.4).

Cell membrane – composed of a phospholipid bilayer, the cell membrane is the semi‐permeable outer covering that controls the entry and exit of materials/molecules in and out of the cell.

Nucleus – known as the cell’s control centre, the nucleus contains deoxyribonucleic acid (DNA) in the form of chromosomes. The smaller nucleolus forms part of the nucleus and contains ribonucleic acid (RNA). The nucleolus is also responsible for the manufacturing of ribosomes.

Cytoplasm – the fluid component of the cell that contains the organelles.

Organelles – small structures located in the cytoplasm, to include:

• Centrosome – consists of two centrioles and is involved in cell replication.
  
• Mitochondria – produce energy via aerobic respiration, extract energy from food substances and store it as adenosine triphosphate (ATP), a form that the cell can use.
  
• Ribosomes – responsible for protein synthesis and often attached to the rough endoplasmic reticulum (RER).
  
• Rough endoplasmic reticulum (RER) – responsible for synthesising and transporting proteins along with the attached ribosomes.
  
• Smooth endoplasmic reticulum (SER) – responsible for the synthesis and transport of lipids.
  
• Golgi body/apparatus – flattened membrane sacs that are involved in the production of lysosomes, secretory granules and the plasma membrane. Responsible for the transport and modification of glycoproteins and other substances.
  
• Vacuole ‐ The main function is to store substances, typically either waste or harmful substances, or useful substances the cell will need later on.
  
• Lysosomes – a collection of digestive enzymes in membrane sacs that form part of the defence mechanism of the cell [1, 3].

Cell Division

Cells reproduce by division, a process during which DNA is replicated. There are two types of division that can take place – mitosis and meiosis.

Fluid

Fluid provides the medium in which the body’s biochemical reactions take place. Water is essential for maintaining the body’s internal environment and keeping it in a state of balance, known as homeostasis. The amount of water in the body will vary and is affected by age and how fat or thin the horse is, but it should make up approximately 60–70% of total body weight [1–3].

Each mammalian cell contains approximately 80% water. This is divided into intracellular fluid (ICF) and extracellular fluid (ECF). Intracellular fluid is found inside the cells and accounts for 40% of total body weight. ECF is the fluid which lies outside the cells and accounts for 20% of total body weight. Approximately 75% of ECF is interstitial fluid (fluid found around the cells), approximately 25% is plasma and a very small amount is transcellular fluid (includes lymphatic fluid, synovial fluid and cerebrospinal fluid (CSF) [1–3].

Body water is continually lost from the body and must be replaced to maintain total fluid balance. Water is taken in via drinking and eating, and is lost via urine, faeces, tears, secretions, sweat and respiration. Water that is lost via sweat and from the respiratory tract is termed insensible fluid loss. Fluid loss can lead to dehydration and hypovolaemic shock. Fluid losses will often be increased in sick horses via reflux, diarrhoea and blood loss [1–3].

Electrolytes

Fluid in the body is also made up of minerals that are dissolved in water, known as a solution. Solutions containing free ions are known as electrolytes. Free ions, which conduct electricity, can have a positive or a negative charge. Ions with a positive charge are referred to as cations, and ions with a negative charge are referred to as anions. The number of electrolytes present will increase or decrease the concentration of the solution, the more electrolytes, the greater the concentration.

The cations found in the intracellular fluid are potassium, magnesium and sodium. The anions are phosphate, bicarbonate and chloride. The cations found in the ECF are sodium, potassium, magnesium and calcium. The anions are chloride, bicarbonate and phosphate [1, 3].

2.2 Diffusion and Osmosis

To maintain homeostasis, body water and the associated chemical substances that it contains move around the body. The processes involved in this are diffusion, osmosis and active transport [1, 3].

Diffusion is a passive process that involves the movement of molecules/electrolytes from a solution where they are at a high concentration to a solution where they are at a low concentration and takes place where there is no barrier to the free movement of molecules (Figure 4.7). If the molecules are too big to pass through a cell membrane, then a different process takes place [1, 3].

Osmosis is a passive process that involves the movement of water from a solution of low concentration to one of a high concentration and takes place through a semi‐permeable membrane (Figure 4.7). The pressure with which water molecules are drawn across a semi‐permeable membrane is known as the osmotic pressure. Fluid that has the same osmotic pressure as plasma is known as isotonic. Fluid with a higher osmotic pressure than plasma is known as hypertonic, and fluid with a lower osmotic pressure than plasma is known as hypotonic [1, 3].

If the plasma’s osmotic pressure is high, water moves into the blood to equalise the concentration. If the plasma’s osmotic pressure is low, water moves out of the blood and into the tissue spaces [1, 3].

Active transport is the movement of electrolytes against an osmotic gradient. Using energy, cells transport electrolytes across a cell membrane allowing them to move from a solution of low concentration to one of a high concentration [1, 3].

Epithelial Tissue

Epithelial tissue covers the internal and external surfaces of the body, providing absorption, secretion and protection for the underlying structures. The thicker the epithelium, the more protective it is and further protection may be provided by the presence of keratin. When epithelial tissue covers structures such as the lining of the heart, blood vessels and lymph vessels, it is known as endothelium. Epithelial tissue is classified according to the appearance of the cells (Table 4.6 and Figure 4.8) [1–3].

Connective Tissue

Connective tissue supports and holds the organs and tissues of the body in place (Table 4.7). It also provides a transport system, carrying nutrients to the body and waste products away. The basic structure consists of cells, fibres and a glycosaminoglycan matrix [1–3].

Muscle

Muscle tissue contains muscle cells that are arranged as fibres. Depending on the muscle type, it contracts and relaxes either voluntarily or involuntarily to bring about movement. There are three main types of muscle – smooth muscle, cardiac muscle and skeletal muscle (Table 4.8 and Figure 4.9) [1–3].

Nervous Tissue

Nervous tissue consists of many neurons which are responsible for the transmission of nervous impulses (signals).

Connective tissue type	Location
Blood	Circulating through the blood vessels
Haemopoietic tissue	Long bones
Areolar tissue (loose connective tissue)	All over the body Examples: beneath the skin, around blood vessels and nerves, between and connecting organs, and between muscle bundles
Adipose (fatty) tissue	In the dermis of the skin Around the kidneys
Dense connective tissue	Parallel arrangement fibres – tendons and ligaments Irregularly interwoven fibres – dermis of the skin, capsules of joints, testes, lymph nodes
Cartilage	Hyaline cartilage – between the epiphysis and diaphysis of growing long bones, at the articular surfaces of moving joints, walls of the respiratory tract, ventral ends of the ribs Fibrocartilage – in the intervertebral discs, the pubic symphysis, at the attachment points of ligaments and tendons Elastic cartilage – in the auricle of the ear, external auditory canal, Eustachian tube, epiglottis
Bone	The skeleton

Muscle	Description	Control	Location
Smooth muscle(also called visceral muscle or involuntary muscle)	Unstriated (smooth) appearance Cells are long and spindle shaped, and surrounded by small amounts of connective tissue that binds the cells into sheets or layers The nucleus in each cell lies in its centre	Involuntary control	Walls of blood vessels, digestive tract, respiratory tract, bladder, uterus
Cardiac muscle	Striated (striped) appearance Cells are cylindrical in shape Branch to create a network of fibres which are linked by intercalated disks – these enable nerve impulses to be conveyed across the muscle producing a rapid response to any changes required by the body	Involuntary control	Heart Forms the myocardium
Skeletal muscle	Striated (striped) appearance Cells are long and cylindrical and lie parallel to each other Each fibre has several nuclei which lie on the outer surface	Voluntary control	Attached to the skeleton

Functions of the Skin

• Protection:
  
  
  
  • – Protects underlying structures from physical injury [4].
    
  • – Prevents entry of microorganisms [1].
    
  • – Protection from ultraviolet (UV) radiation [4].
    
  • – Protection against water loss [5].
  
  
• Thermoregulation
  
  
  
  • – Sweat production cools the skin surface via evaporation [2].
    
  • – Piloerection traps a layer of air, acting as insulation [3].
    
  • – Vasoconstriction diverts blood away from the surface in cold temperature [3].
    
  • – Vasodilation promotes heat loss [5].
    
  • – Adipose tissue under the skin acts as insulation [5].
  
  
• Sensation
  
  
  
  • – Skin is a sense organ for temperature, touch, pressure and pain [3].
    
  • – Sensory nerve endings associated with hairs facilitate physical assessment of the environment and can trigger avoidance behaviours such as skin twitching to ward insects away [6].
  
  
• Production
  
  
  
  • – Sebum is produced by sebaceous glands within the skin and is secreted onto the surface of the skin to form a water‐resistant layer [5].
    
  • – Vitamin D is synthesised from 7‐dehydrocholesterol in the skin upon exposure to UV light. Vitamin D is required for the uptake of dietary calcium [1].
  
  
• Storage
  
  
  
  • – Adipose tissue forms both an energy store and an insulating layer against cold weather [5].
  
  
• Communication
  
  
  
  • – Pheromones are produced by specialised skin glands for intraspecific communication [1].

Skin Structure

The skin comprises two true layers: the epidermis and the dermis. The hypodermis lies beneath the skin [2]. The thickness of the skin varies in different regions of the body and is typically thicker in regions more prone to abrasion [5].

Hypodermis

The hypodermis is also known as the subcutis and is not a true layer of skin. It lies below the dermis (Figure 4.10) and contains connective and adipose tissue [3].

Physiology of Bone

Bone is a living tissue that continually changes throughout the horse’s life [1]. Bone comprises an extracellular matrix that contains collagen fibres and the protein ‘osteonectin’, which are combined to form the organic material ‘osteoid’. The osteoid forms the unmineralised aspect of the bone, but as the bone develops, calcium phosphate crystals are deposited within the bone. The crystals are insoluble and cause the bone tissue to become calcified; it is then referred to as ‘mineralised bone’ [3].

The osteoid is synthesised by osteoblasts, which are immature bone cells [1]. As bone becomes calcified, the osteocytes, which are mature bone cells, become trapped in spaces called ‘lacunae’ [3]. The osteocytes are responsible for maintaining bone structure. A third type of bone cell, called osteoclasts, are responsible for breaking down and remodelling bone [1].

Within the matrix of bone material are channels that carry blood vessels and nerves. These are called Haversian canals and each one is surrounded by lamellae, which are concentric cylinders of bone material [3]. Each canal and associated lamellae and lacunae are collectively called a ‘Haversian System’ or ‘osteon’ [1]. The outer surface of a bone is covered by a fibrous membrane called the periosteum [3].

The two types of bone are compact bone and cancellous bone. Compact bone is found in the cortices (outer surfaces) of all bones. The structure of the bone is very dense and regular [1], with Haversian systems tightly packed together [3]. Cancellous bone is also known as ‘spongy bone’ and consists of interconnecting bars of bone called trabeculae with red bone marrow in the spaces [3]. The network of trabeculae means that cancellous bone is not as strong as compact bone [1].

Long Bone Structure

Long bones consist of a shaft and two ends (Figure 4.11), with separate regions that have differing characteristics [2]. The central shaft is known as the diaphysis [1], which has a dense cortex and an inner medulla [2]. Each end of the bone is known as the epiphysis and the region between the epiphyses and diaphysis is called the metaphysis. The metaphysis contains the epiphyseal growth plate, a region that is crucial in bone development [1].

Features of Bones

The shape of bones varies greatly and there are a variety of terms that can be used to describe features such as prominences and depressions. Terms used to describe protuberances include tuberosity, trochanter and tubercle, which each typically act as a site for the attachment of muscles. Bones can also have grooves known as a trochleae (singular: trochlea), which tendons typically pass through or over, thus permitting the tendon to act as a pulley [3]. Condyles are rounded protuberances at the end of a bone [1], which are usually involved in the articulation with another bone. An epicondyle is a lateral projection on a bone, situated above the condyle [3]. Foramina (singular: foramen) are holes or openings in a bone and a sinus is a hollow cavity within a bone. A depression in a bone is referred to as a fossa, whereas a raised area is referred to as a crest. A process is a thin, elongated projection of bone [1].

Ossification

Ossification is the process by which bones are formed. There are two types of ossification: intramembranous ossification and endochondral ossification [3].

Intramembranous Ossification

Bone formed by intramembranous ossification does not have a cartilage precursor. Instead, bone cells are situated between two membranes [2] and the osteoblasts lay down bone material, replacing the fibrous connective tissue [1]. This is the process by which the flat bones of the skull are formed [3].

Endochondral Ossification

This type of ossification requires a cartilage model [5]. The model is present within the embryo and comprises hyaline cartilage, which is gradually replaced [2]; a process which continues after birth. Long bones develop by endochondral ossification [3]. The stages of endochondral ossification are outlined below:

1. A hyaline cartilage model develops within the embryo [3].
   
2. Primary ossification centres appear in the diaphysis, where osteoblasts lay down new bone material that replaces the cartilage and extends towards the end of the bone [3].
   
3. Secondary ossification centres appear in the epiphyses of the bone [3].
   
4. The primary and secondary ossification centres meet at the epiphyseal growth plate [1], which is a narrow band of persisting cartilage [3] that is radiolucent when radiographed.
   
5. On the epiphysis side of the growth plate, new cartilage is produced, which results in the lengthening of the bone at each end [1].
   
6. Eventually, the cartilage on the side of the plate nearest to the diaphysis is replaced with bone material laid down by osteoblasts [1].
   
7. The interior of the diaphysis is remodelled by osteoclasts; the medullary cavity is created during this process [1].
   
8. When the horse has reached its final size, the cartilage cells stop dividing and all remaining cartilage is ossified [1] and the epiphyseal plate is described as ‘closed’ [3].

Classification of Bone

Bones can be classified into the following shapes.

Joints

A joint occurs where two or more bones join together or articulate. Different joints types facilitate different degress of movement. Joints are classified as follows [1, 2]:

• Synovial (Figure 4.12a). – These joints have a wide range of movement are are also known as diarthroses. Synovial joints are formed when a synovial membrane connects two bones and there is a space between the ends of the bones or joint cavity that is filled with synovial fluid. The ends of the bone are covered in articular (hyaline) cartilage and this allows low‐friction articulation between opposing bones. Synovial fluid is a viscous substance that is made from the fluid component of plasma and hyaluronic acid. Synovial fluid acts as a lubricant between the cartilage of the two bones ends and nutrients diffuse from the synovial fluid into the articular cartilage. The synovial membrane lines the joint and produces the synovial fluid. A joint capsule made of dense connective tissue surrounds the whole joint. Some joints contain a pad of fibrocartilage called a meniscus such as the stifle joint, which contains two menisci that are firmly attached by ligaments to the tibial plateau.
  
• Cartilaginous (Figure 4.12b) – These joints have little or no movement. These joints can present as synchondoses, which are joints between the epiphyses and diaphysis in growing animals, or as symphyses, which are joints between the mandible bones of the lower jaw and the pubic bones of the pelvis
  
• Fibrous (Figure 4.12c) – These joints have little or no movement. Fibrous joints as present as sutures in the skull, or syndesmoses between two areas of bone.

The range of movement of a joint can be described using the following terms [1]:

• Flexion – bending the limb by decreasing the angle of the joint
  
• Extension – straightening the limb by increasing the angle of the joint
  
• Adduction – moving the limb distal to the joint towards the midline/body
  
• Abduction – moving the limb distal to the joint away from the midline/body

  

  
  
• Gliding – flat surfaces moving over each other e.g. in the carpus
  
• Rotation – movement shown by a pivot joint
  
• Circumduction – moving one end of a bone (usually the end distal to the joint) in a circular motion
  
• Protraction – lengthening the limb by moving the distal limb away from the body
  
• Retraction – shortening the limb by moving the distal limb towards the body

The Skeleton

Horses have approximately 205 bones. The skeleton has several functions in addition to acting as a framework for the horse. These include facilitation of movement, protection of soft organs and tissues, haemopoiesis and storage of minerals [1]. Figure 4.13 shows the skeleton of the horse.

The equine skeleton can be divided into two sections: the axial skeleton and the appendicular skeleton [7].

The Axial Skeleton

The axial skeleton comprises the skull, vertebrae, ribs and sternum [7].

The Skull

The main function of the skull is to protect the brain, inner ear, the eye and the nasal passages. The bones of the skull are joined by fibrous joints known as sutures [7], which give the skull a rigid structure. The region of the skull in which the brain is housed is called the cranium. There is one moveable joint in the skull (the temporomandibular joint) which facilitates chewing [3]. The bones of the skull are displayed in Figure 4.14.

The majority of the dorsal and lateral walls of the cranium are formed by the parietal bones. Situated below the parietal bones, on the caudolateral surface of the skull are the temporal bones. One each temporal bone is a rounded prominence at the ventral aspect. This is called the tympanic bulla and contains the structures of the middle ear. The floor of the cranial cavity is formed by the sphenoid bone, which has many foramina, permitting the passage of blood vessels and nerves [3].

The occipital bone is situated at the caudal aspect of the skull [3]. There is a hole in the occipital bone through which the spinal cord passes called the foramen magnum [2]. A pair of bony prominences called the occipital condyles are located on either side of the foramen magnum. The condyles articulate with the atlas, the first cervical vertebra. Jugular processes are situated on either side of the occipital condyles and serve as attachment sites for muscles [3]. The equine skull has a prominent occipital ridge that the nuchal ligament attaches to [1].

The forehead is formed by the frontal bone, which also forms the front aspect of the cranium. The roof of the nasal cavity is formed by the nasal bone. The nasal septum, a cartilaginous plate, divides the nasal cavity into left and right halves. Each side is filled with nasal turbinates (also referred to as conchae), which are fine, delicate scrolls of bone, covered in ciliated mucous epithelium. At the caudal aspect of the nasal cavity, the ethmoid bone forms the boundary between the nasal cavity and the cranial cavity. The cribriform plate is located in the centre of the ethmoid bone, through which olfactory nerves pass, transmitting impulses from the mucosa in the nasal cavity to the olfactory bulbs within the brain [3].

The maxilla forms the upper jaw, and the lower jaw is formed by the mandible [7]. The mandible is divided into the horizontal and the vertical ramus. The vertical ramus has two processes: the condylar process and the coronoid process [2]. The condylar process contributes to the temporomandibular joint, where the mandible articulates with the rest of the skull. The coronoid process projects into the temporal fossa and is a site of muscle attachment for the temporalis muscle [3]. The lower cheek teeth are housed in the horizontal ramus of each mandible and each side is joined at the mandibular symphysis [2], which is a cartilaginous joint. On the lateral aspects of each mandible is a depression called the masseteric fossa, where the masseter muscle is situated [3]. The upper incisors are housed in the incisive bone, the most rostral bone of the skull [3] which, in conjunction with the maxilla and palatine bone forms the hard palate and base of the nasal cavity [7].

Situated in the intermandibular space is the hyoid apparatus. This structure consists of a series of bones and cartilage, joined in a trapeze‐shaped manner. The larynx and tongue are suspended from the skull by the hyoid apparatus, permitting movement of these structures when swallowing. The hyoid apparatus articulates with the skull via a cartilaginous joint in the temporal region [3].

The zygomatic bone projects laterally from the skull to form the ‘cheekbone’. The prominent nature of this bone serves to protect the eye. The orbit, commonly referred to as the eye socket, is formed by aspects of several bones. At the base of the orbit is the lacrimal bone. Tears drain from the eye, through the lacrimal bone, into the nose [3].

The Teeth

Embedded within the maxilla, mandible and incisive bone are the teeth, adapted in equids for breaking down fibre. Horses have different types of teeth, suited to different jobs. The first type are the incisor teeth, situated at the rostral end of the oral cavity. The upper incisors are embedded in the incisive bone and the lower incisors are embedded in the mandible. They are referred to as central, lateral and corner incisors in accordance with their position (the central incisors are the most medial and the corners are the most lateral, with the lateral incisors positioned between the two). They are responsible for prehending food and cutting grass [3].

Caudal to the incisors are the canine teeth, also known as ‘tushes’. Canine teeth erupt in the gap (diastema) between the incisors and cheek teeth; these often fail to erupt in mares, but males have four canines, which erupt at approximately five years of age. The lower canines are situated more rostrally than the upper canines, meaning that no contact occurs between these teeth (Figure 4.15) [2]. The premolars and molars are collectively referred to as the ‘cheek teeth’. These are flattened to facilitate grinding food into smaller particles. In some horses, vestigial teeth known as ‘wolf teeth’ may develop in front of the pre‐molars [3]. The wolf teeth are known as the first premolar. These are more commonly seen in the upper jaw than the lower jaw [2].

Similarly to other mammals, horses have two sets of teeth. The first set, which are smaller and seen in younger animals are referred to as deciduous teeth. The deciduous teeth are replaced by permanent teeth [1], a process which occurs from the age of two‐and‐a‐half and is not complete until the horse is five [4]. Horses do not have deciduous molars or canine teeth. The timing of the eruption is as follows:

• Deciduous central incisors: by four weeks [4].
  
• Deciduous lateral incisors: between one and three months [4].
  
• Deciduous corner incisors: between eight and nine months [4].
  
• Permanent central incisors: at two‐and‐a‐half years and in wear at three years [4].
  
• Permanent lateral incisors: at three‐and‐a‐half years and in wear by four years [4].
  
• Permanent corner incisors: at four‐and‐a‐half years and in wear by five years [4].
  
• Deciduous premolars: by three months [4].
  
• Permanent premolars: the first erupts at around two‐and‐a‐half years, and the others at around three‐and‐a‐half years [4].
  
• Permanent molars: the first erupts at approximately 9 months, followed by the second at 18 months. The final molar erupts at four‐and‐a‐half years [4].
  
• Permanent canines: five years [4].

Dental formulae indicate the number of each tooth type present. The dental formulae for a full set of deciduous and permanent teeth are as follows:

• Deciduous teeth in a male or female: (i3/3, pm3/3) = 24 teeth.
  
• Permanent teeth in a male: (I3/3, C1/1, PM3–4/3, M3/3) × 2 = 40 (without wolf teeth) or 42 (with wolf teeth).
  
• Permanent teeth in a female: (I3/3, C0/0, PM3–4/3, M3/3) × 2 = 36 (without wolf teeth) or 38 (with wolf teeth).

Each tooth is held in place in a socket, otherwise known as an alveolus. The outer part of the crown comprises enamel, while the outer aspect of the root comprises cementum. Dentine forms the inner aspect of each tooth, and a pulp cavity is present at the centre of each tooth (Figure 4.16). The pulp cavity contains, blood vessels, lymphatic vessels and nerves [1]. Equine teeth have adapted to a diet high in fibre and are referred to as hypsodont (long crowned). These teeth continue to erupt throughout the horse’s life [2], which compensates for wear, which is approximately 2–3 mm per year [7]. A layer of enamel is not present over the occlusal surface (otherwise known as the ‘table’) of the teeth [3].

The Vertebrae

The vertebrae collectively form the vertebral column, which forms a rod to support the body. It also houses and protects the spinal cord, provides sites of insertion for muscles and provides an attachment site for the ribs [3]. The vertebral column is divided into the following regions: cervical, thoracic, lumbar, sacral and coccygeal [1].

Basic Structure of Vertebrae

Vertebra all have the same basic structure with variations in accordance with differing functions [3]. A basic vertebra has a cylindrical body [2] at the ventral aspect. The cranial end of the body is convex, while the caudal end is concave. Above the vertebral body is the neural arch, forming the vertebral foramen. The vertebral foramina collectively form the spinal canal. Above the neural arch is the spinous process; the height of these processes differs in different regions of the vertebral column [3]. Transverse processes are situated on each side of each vertebra [6], the size of which also vary in different regions of the vertebral column. The transverse processes separate the muscles of the vertebral column into the epaxial muscles (situated above the processes) and the hypaxial muscles (situated below the processes) [3]. The vertebrae also have two pairs of articular processes [6], which are the cranial and caudal articular processes. The articular processes form synovial joints with adjacent vertebrae [3].

Adjacent to the body of each vertebra is an intervertebral disc [3], which permits slight compression [4], thereby acting as a shock absorber. The outside of each disc comprises a layer of fibrous connective tissue called the annulus fibrosus. Gelatinous material fills the centre of each disc and is called the nucleus pulposus [3].

Cervical Vertebrae

The horse, like all other mammals, has seven cervical vertebrae located in the neck (C1–C7). The first cervical vertebra is the atlas [4], which possesses large, wing‐shaped processes [3]. The second cervical vertebra is the axis [7], which possesses a prominent cranial process called the dens [3], also known as the odontoid process [6]. The movement of the atlas and axis is not inhibited by spinous or transverse processes, which allows a large range of movement [3]. The joint between the skull and the axis permits the nodding movement of the head [4] and the articulation between the atlas and axis permits the rotation of the atlas around the odontoid process. Unlike the atlas and the axis, C3–C7 follow the basic vertebral structure [3].

Thoracic Vertebrae

The horse has 18 thoracic vertebrae (T1–T18) [7] which possess very prominent spinous processes [4]. The height of each process decreases slightly, moving towards the lumbar region and the withers are formed by T4–T9. The head of each rib articulates with the costal fovea of each corresponding thoracic vertebra, while the tubercle of the rib articulates with the transverse fovea [3].

Lumbar Vertebrae

The horse has six lumbar vertebrae (L1–L6) [4], with the exception of some Arab horses, which may have only five lumbar vertebrae. These vertebrae have smaller spinous processes than the thoracic vertebrae but possess large transverse processes. This restricts lateral movement but contributes to the protection of the kidneys [3].

Sacral Vertebrae

The horse has five sacral vertebrae (S1–S5) that are fused to form the sacrum [8], thus greatly restricting movement. An interosseous ligament joins the pelvis to the sacrum, forming the sacroiliac joint [3].

Coccygeal Vertebrae

The horse has between 15 and 20 coccygeal vertebrae (Cd1–Cd15–20) [5], but Arab horses typically have fewer coccygeal vertebrae. From Cd1 to the final coccygeal vertebra, each vertebra decreases in size and complexity [4]. The first few have very small spinous and transverse processes, which decrease in size further down the tail until they resemble a simple rod‐shaped structure. The small processes permit a greater degree of movement in the tail [3].

Vertebral Formulae

The vertebral formula for the horse and the donkey differs as follows:

• Horse: C7, T18, L6, S5, Cd15–20.
  
• Donkey: C7, T18, L5, S5, Cd15–17.

The Ribs

The horse has 18 pairs of ribs [2], with the exception of Arab horses, which may have only 17 pairs. Each rib has a bony aspect that articulates with the thoracic vertebrae and the ventral part of each rib is made of cartilage, referred to as the costal cartilage. The region where the bony aspect of the rib meets the cartilaginous aspect is the costochondral junction [1]. The first eight pairs of ribs articulate directly with the sternum and are classified as ‘true’ ribs [7] or ‘sternal’ ribs. The remaining 10 pairs of ribs are referred to as ‘false’ or ‘asternal’ [3] and instead articulate with the rib in front, forming the costal arch. The space between each rib is called the ‘intercostal space’ [2].

The Sternum

The sternum supports the true ribs and forms the ventral boundary of the thoracic cavity [4]. The equine sternum is made up of eight bones called sternebrae. The manubrium is the most cranial sternebra and the xiphoid is the most caudal sternebra [1]. Situated between the sternebrae is a disc of cartilage called sternebral cartilage [3].

The Appendicular Skeleton

The appendicular skeleton comprises the bones of the limbs [7].

Bones of the Proximal Forelimb

The most dorsal bone of the forelimb is the scapula, commonly referred to as the shoulder blade [7]. This is a large, flat bone, divided in half on the lateral aspect by the scapular spine. This permits the insertion of the supraspinatus and infraspinatus muscles. At the proximal aspect of the scapula is a wing of cartilage, which also permits muscle attachment [3]. The horse does not possess a clavicle (collarbone); instead, the forelimb is connected to the trunk by a group of muscles referred to as the thoracic sling.

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