Functional Anatomy

Cells and Tissues

the cell is the basic unit of life
all cells arise from already existing cells through reproduction {cell theory}
cells reproduce by cell division; ordinary cell division is called mitosis; sex cells divide by meiosis
cell activities are carried out by organelles
with the exception of mature red blood cells, all animal cells have a nucleus, which is the cell's center of control
the outer layer of an animal cell is the cell membrane, which is selectively permeable (plant cells have cell walls)
the cytoplasm is the material within the cell, in which the organelles move around
mitochondria are the organelles responsible for the chemical reactions which supply energy to the cell
all organs are made up of tissues, and all tissues are made up of cells
the four types of tissue are
- epithelial tissue
- connective tissue
- muscular tissue
- nervous tissue
epithelial tissue includes the skin, as well as the lining of hollow organs such as the alimentary canal, the bladder, and the uterus; glands are also made up of epithelial tissue
connective tissue includes bones, tendons, ligaments, and sheets of fibrous tissue which protect various organs; blood is also a connective tissue
muscular tissue has the capacity for movement
muscular tissue includes the skeletal muscles, smooth muscles of the alimentary canal, bladder, etc., and the cardiac muscle of the heart
nervous tissue has the capacity to transmit messages
nervous tissue includes the brain, spinal cord, and nerves

The Digestive System

the digestive system is located in the abdominal cavity
the alimentary canal (GI tract) is 100 feet long
the alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, cecum, small colon large colon, rectum, and anus.
digestion begins in the mouth
prehension = the grasping of food
mastication = chewing
mastication includes both mechanical and chemical digestion. The teeth break up the food mechanically, and chemical digestion begins with saliva.
a horse has twenty-four milk teeth at birth
a full mouthed stallion or gelding has forty teeth, including tushes, which are removed in many horses, since they serve no purpose and often interfere with the bit.
mares normally lack canine teeth, or have only vestigial evidence of them, and have thirty-six teeth.
saliva is produced by three sets of salivary glands, which produce approximately ten gallons of saliva every day.
saliva contains the enzyme ptyalin, which converts starch to maltose, a simple sugar.
food passes through the pharynx, the funnel shaped cavity where the respiratory and digestive tracts cross, after it leaves the mouth
the esophagus is a muscular tube about 60 inches, or about five feet, which takes food into the stomach
while food is in the esophagus, it is known as bolus
food in the esophagus is moved by a series of wave-like muscular contractions collectively known as peristalsis.
since peristalsis is a one way movement, it is impossible for a horse to vomit
food enters the stomach through the cardia, which is held closed by a muscle known as the cardiac sphincter.
the stomach has a capacity of only 8 to 17 quarts
water is almost immediately passed out of the stomach
gastric juices are produced by the gastric mucosa
the two main gastric juices are peptic acid, or pepsin, which breaks down proteins, and hydrochloric acid, which dissolves mineral matter.
the stomach doesn't completely empty unless food is withheld for over twenty-four hours.
while in the stomach, food is known as chyme
chyme leaves the stomach through the pyloric valve
the small intestine is approximately 70 feet long, with a capacity of 12 gallons
the small intestine consists of 3 sections:
- the duodenum
- the jejunum
- the ileum
bile is a bluish green fluid secreted by the liver which aids in the emulsification (breakdown) of fats.
since the horse lacks a gall bladder, bile is used as it is produced, and there isn't a stored supply.
pancreatic juice is produced in the pancreas, and works on carbohydrates.
bile and pancreatic juice enter the small intestine through a common duct, and work with other enzymes to digest fats, carbohydrates other than cellulose, proteins and the remaining minerals.
nutrients in the small intestine are absorbed by the villi (finger-like projections in the intestinal wall), mainly in the jejunum
from the small intestine, food travels into the cecum, also called the water gut or blind gut.
the cecum is 4 feet long, with a capacity of between 28 and 32 quarts.
vitamin B12, or cobalamin, is synthesized in the cecum, as are several non-essential amino acids and other B vitamins
material moves more slowly through the cecum than any other section of the digestive tract.
from the cecum, undigested material moves into the remainder of the large intestine, where the rest of the B complex vitamins and amino acids are synthesized.
cellulose is digested with the help of the microscopic organisms living in the large intestines.
the large intestine is approximately 25 feet long, with a capacity of 80 quarts.
accessory organs to the digestive system include the liver, pancreas, teeth and salivary glands.
the largest gland in the body is the liver
the 3 pairs of salivary glands are the parotid, the submaxillary (submandibular), and the sublingual
the submandibular and submaxillary salivary glands are the same gland
the membrane which protects the digestive organs is the peritoneum
the mesentery is a double fold of the peritoneum which supports the small intestine
the mesentery artery supplies blood to the small intestine

The Respiratory System

the respiratory system is responsible for breathing, which is only part of the process of respiration
the process of respiration also includes the exchange of gases in individual tissues
the respiratory system is located in the thoracic cavity, commonly called the chest cavity
the respiratory system may be divided for discussion into the upper respiratory tract and the lower respiratory tract
the upper respiratory tract moves air from the nostrils to the lower respiratory tract
air is taken in through the nostrils to the nasal passages, then through the pharynx into the larynx
the larynx determines how much air enters the trachea and lower respiratory tract
the trachea is commonly called the windpipe
the trachea is 75 to 80 centimeters long
the trachea divides into 2 bronchi, which enter the lungs and further divide into numerous smaller bronchioles, the smallest of which end with alveoli, the sac-like structures where capillaries exchange gases
airways are lined with mucous membranes, and air is filtered by small hairlike projections called cilia
inhaling and exhaling are controlled by the diaphragm, the muscle which separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts, the chest cavity expands and air is forced into the lungs. When the diaphragm relaxes, air is released.
a normal, resting horse has a respiration rate of 10 breaths per minute

The Circulatory System

the circulatory system uses blood to carry oxygen, food, and water to cells throughout the body, as well as to transport waste materials and hormones. The circulation of blood also aids in the regulation of temperature.
the circulatory system consists of the blood, veins, arteries, smaller blood vessels, and the heart.
the muscle which pumps blood throughout the body is the heart
the heart is made up of cardiac muscle tissue
the heart has 4 chambers, known as the right and left ventricles and the right and left auricles.
the wall of tissue which divides the heart in half is known as the septum
the membrane that surrounds the heart is the pericardium
blood returns to the heart through the veins
blood leaves the heart through the arteries
with one exception, the pulmonary vein, the veins contain unoxygenated blood
with one exception, the pulmonary artery, the arteries carry oxygenated blood
blood enters the heart through the vena cava
blood leaves the heart to return to the body through the aorta
the heart has four valves, including the tricuspid valve, which separates the right atrium and right ventricle, and the mitral valve, which separates the left atrium and left ventricle
the horse's normal pulse rate is about 36 beats per minute
bradycardia is an abnormally slow heartrate
tachycardia is an abnormally fast heartrate
blood contains:
- plasma
- red blood cells
- white blood cells
- platelets (which aid in blood clotting)
plasma makes up 40 to 50 percent of the total blood
plasma is 93% water, 6% protein, and 1% mineral salts
red blood cells are also called erythrocytes
red blood cells contain the pigment hemoglobin
red blood cells are the only cells in the body which do not have a nucleus
red blood cells carry oxygen
white blood cells are also called leucocytes
white blood cells are part of the immune system, and destroy bacteria and other foreign matter in the bloodstream
the 5 types of leucocytes are:
- lymphocytes (non-granular)
- monocytes (non-granular)
- esinophils (granular acidic)
- basophils (granular alkaline)
- neutrophils(granular neutral)
when a blood vessel is damaged, the loss of blood is stopped by the process of blood clotting
vitamin K is necessary for blood clotting
when a large artery breaks, blood is forced out in spurts and blood pressure prevents a clot from forming unless first aid treatment is applied
the spleen destroys old blood cells and stores new ones for release when they are needed

The Blood's Path Through the Heart

vena cava --to-- right atrium --to-- right ventricle --to-- pulmonary artery --to-- the lungs --to-- pulmonary vein --to-- left atrium --to-- left ventricle --to-- aorta --to-- the body

The Lymphatic System

the lymphatic system is a secondary system of transport which removes excess water from tissues
lymph fluid is moved throughout the body by the actions of the skeletal muscles
a stabled horse does not have the freedom to move around, and therefore does not use the skeletal muscles, causing the lymph fluid to collect in the lower legs. This is known as "stocking up" or "filled leg" and the condition corrects itself with exercise.

The Urinary System

the urinary, or excretory, system consists of two kidneys, two ureters, the urinary bladder and urethra
the left kidney is slightly farther back than the right kidney
the right kidney is under the last three ribs
the left kidney is opposite the last rib
the ureters connect the kidneys to the urinary bladder
the urethra leads out of the body from the bladder
the urinary system functions to maintain water and electrolyte balance within the body, as well as to excrete waste products such as urea
the kidneys produce urine
the urinary bladder stores urine until it is excreted
adult horses urinate 4 to 6 times a day
mares in heat urinate more frequently

The Nervous System

the nervous system is one of the two main systems of control
the nervous system is made up of nerve cells, or neurons, which carry messages in the form of electro
chemical impulses through the body
the two main divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS)

The Central Nervous System (CNS)

the central nervous system consists of the brain and spinal cord
all higher functions of the nervous system are the responsibility of the CNS, while basic functions are carried out by the PNS
the brain and spinal cord are protected by three membranes, or meninges:
- the dura mater (the outermost)
- the arachnoid mater (the middle)
- the pia mater (the innermost)
the brain is protected by the skull, while the spinal cord is protected by the vertebrae
the brain forms 1% of the adult horse's bodyweight
the three primary segments of the brain are:
- hind brain
- midbrain
- fore-brain
the hind brain is the brain stem, which includes the pons and the cerebellum
the brain stem regulates breathing
the cerebellum controls movement
the pons is involved with emotions and behavior
the midbrain is responsible for sight, smell, and control of hind brain activity (i.e. voluntary control of breathing)
the forebrain contains the pituitary gland
the spinal cord is approximately six to seven feet long
forty-two pairs of spinal nerves connect the spinal cord to the peripheral nerves

The Peripheral Nervous System (PNS)

the peripheral nervous system consists of all the nerves in the body

The Autonomic Nervous System:

is divided into the sympathetic and parasympathetic nervous systems, which are responsible for the regulation of involuntary activity.

Neurons

individual neurons can only conduct impulses in one direction
the gap between one neuron and the next is called the synapse
nervous tissue is classified as either white matter or gray matter
bunches of nerve tissue are called ganglia

The Endocrine System

the endocrine system is the second of the two main systems of control (other is the nervous system)
the endocrine system is responsible for the secretion of hormones
hormones are body regulating chemicals secreted by glands
hormones are carried throughout the body by the bloodstream

Basic Facts on the Primary Glands and Hormones

the pituitary gland is divided into the anterior and posterior pituitary
the anterior pituitary produces:
- FSH (Follicle Stimulating Hormone)
- LH (Leutinizing Hormone)
- Prolactin
- GH (Growth Hormone)
- TSH (Thyroid Stimulating Hormone)
- ACTH
FSH (Follicle Stimulating Hormone) causes the development of the follicle in the ovary
LH (Leutinizing Hormone) causes ovulation
Prolactin causes milk secretion
GH (Growth Hormone) has a general action on metabolism, and aids in growth regulation
TSH (Thyroid Stimulating Hormone) causes the secretion of thyroxine
ACTH causes the adrenal cortex to produce cortisol
the posterior pituitary produces oxytocin
oxytocin causes uterine contractions
the pancreas produces insulin
insulin controls the level of sugar in the blood
the thyroid gland produces thyroxine, which is involved in metabolic control
the two adrenal glands are located in front of each kidney
the adrenal gland consists of the adrenal cortex and the adrenal medulla
the adrenal cortex produces cortisone
cortisone affects the water content of tissues
the adrenal medulla produces adrenaline
adrenaline, the "fight/flight" hormone, increases heartrate and the blood supply to muscles, and also affects sweating
the yellow body of the ovary produces progesterone
progesterone causes the changes of diestrus & pregnancy
the yellow body follicle of the ovary produces estrogen
estrogen causes estrus, or heat, and associated behavior
prostaglandin and PMSG are produced in the uterus
prostaglandin stops the secretion of progesterone
PMSG (pregnant mare's serum gonadotrophin) maintains pregnancy
the pineal gland inhibits the sex drive

Additional Endocrine System Facts

steroid hormones are hormones that have a chemical structure of four carbon rings, and are therefore closely related to one another
the body can change one steroid hormone into another
steroid hormones include progesterone, estrogen, testosterone, and cortisone

PROGESTERONE
- secreted in non-pregnant mares by the yellow body of the ovary
- in pregnant mares progesterone is secreted by the placenta
- a detectable amount in the blood indicates that the mare is not in heat; a high amount indicates that the mare is in the first 120 days of pregnancy
- after the first 120 days of pregnancy, progesterone is not detectable in the bloodstream, although it is present in the reproductive tract
- progesterone causes changes of diestrus and pregnancy, and helps to maintain pregnancy
ESTROGENS
- estrogens are hormones secreted by the lining of the follicles in the ovaries
- the Cuboni test, used to diagnose pregnancy after 120 days, tests for estrogen in the urine
- estrogens include:
  - estradiol
  - estrone sulphate
  - estrone
  - equenin
  - equilinin
- equenin and equilinin are unique to the horse family, and are produced only during pregnancy
- estrogens are responsible for signs of heat, feminine characteristics, and fetal growth
CORTISONE
- cortisone is produced by the outer lining of the adrenal gland (adrenal cortex)
- cortisone stimulates metabolism and prepares an individual for strenuous activity
- cortisone is released when ACTH is secreted by the pituitary
TESTOSTERONE
- testosterone is the male sex hormone secreted by the testes
- testosterone is responsible for male characteristics, libido, and retention of nitrogen in the body
- since testosterone promotes muscle development, it is sometimes injected into racehorses, although it is against Jockey Club rules
FOLLICLE STIMULATING HORMONE
- FSH is secreted by the pituitary and stimulates the growth of follicles in the ovary
- FSH also stimulates secretion of estrogen
LUTENIZING HORMONE
- LH is secreted by the pituitary and stimulates the ovulation
- LH is also secreted by the placenta during pregnancy
PMSG (Pregnant Mare's Serum Gonadotrophin)
- is also called equine Chorionic Gonadotrophin of eCG
- PMSG contains both FSH and LH
- PMSG is produced in the chorion of the placenta in the uterus of pregnant mares
- PMSG maintains pregnancy by protecting the fetus against the mare's immune system
GnRH (Gonadotrophin Releasing Hormone)
- GnRH is produced in the brain and causes the secretion of FSH and LH
peptides are substances of low molecular weight composed of amino acids which are produced throughout the body, tying together the systems of control (i.e. endorphins)

The Skin

the skin in the horse's largest organ
the skin is responsible for protection, excretion, temperature control, synthesization of vitamin D, and the sense of touch
the skin is thickest over the back and thinnest on the face
the skin has three layers:
- the epidermis
- the dermis
- the subcutis
the three layers of the epidermis are:
- the stratum corneum
- the stratum spinosum
- the stratum germinativum
the stratum corneum, the outer layer of the epidermis, is made up of dead cells which are shed as dandruff
new epidermal cells are made in the stratum germinativum, by mitosis, or cell division
hair follicles grow downwards from the epidermis into the dermis
the dermis contains vessels, nerves, and sweat glands
nerves in the skin include nerves which sense pressure, pain, and temperature
the subcutis is made up of fatty connective tissue which loosely connects the skin to muscle
the panniculus muscle runs beneath the subcutis, and is responsible for shivering and twitching away external parasites
the panniculus is sometimes called the fly twitcher
shivering produces heat

The Senses

the five senses are: sight, hearing, smell, taste, and touch
the sense organs are the eyes, ears, nose, tongue, and skin

The Optic System

the horse has both monocular and binocular vision
monocular vision: the ability to look in different directions with each eye
binocular vision: the ability to focus both eyes on a single object
the eye is found in the orbital cavity of the skull
the horse has three eyelids, two external and one internal
the first two eyelids are composed of a sheet of cartilage with skin on the outside and conjunctiva on the inside
the third eyelid is made up of mucous membrane and cartilage
the third eyelid is called the nictating membrane, or the haw
the lacrimal glands secrete tears through tear ducts to keep the eyes moist and clean
the conjunctiva is a thin, pink, moist mucous membrane on the inside of the eyelids which turns into a layer of transparent cells forming part of the cornea in order to cover the front of the eye
the cornea is a thick, tough, and transparent tissue which forms the anterior (front) portion of the eyeball; at the outer edges it is continuous with the sclera
the sclera is the white of the eye
the juncture of the cornea and sclera is called the limbus
the iris is the colored part of the anterior portion of the eye that regulates the amount of light entering the eye
the pupil is the hole in the center of the iris through which light enters the eye
bright light contracts the pupil, while darkness causes it to become enlarged
the fluid within the eye is called vitreous humour
the lens is behind the iris, and may be shortened and lengthened by the muscles which hold it in place
the optic nerve enters the eye at the back, passes through the sclera, and spreads to help form the retina
images in the form of light enter the eye through the pupil and are projected onto the retina by the lens, which brings the images into focus, although upsidedown, on the retina, before sending them to the brain via the optic nerve
the horse has two blind spots, and when facing forward they are directly behind him, and directly in front of his nose; he is unable to see objects within a triangular space in these two spots

The Auditory System

the three parts of the ear are the outer ear, middle ear, and inner ear
a secondary function of the ear is to maintain equilibrium (balance) and inform the brain about the position of the head
the erect cartilaginous portion of the ear is called the outer ear, or pinna
the outer ear is not only a part of the auditory system, but is used in the communication of emotions as well
the horse can move the outer ear in any direction, in order to funnel sounds into the ear from all sides
the horse's ears move independently of one another
the middle ear consists of the auditory canal, which attaches the outer ear to the eardrum and inner ear
the middle ear is connected to the pharynx by the eustachian tube, allowing for the equalization of air pressure
horses and donkeys have a guttural pouch off this tube
when noises, in the form of soundwaves, strike the eardrum, it vibrates and sends the vibrations down a chain of small bones to the inner ear
the three bones in the ear are the malleus, incus, and stapes
the inner ear is sometimes called the labyrinth
the main cavity of the inner ear is the vestibule
the inner ear consists of a series of membranous tubes which are filled with a fluid called endolymph
the inner ear may be divided into the cochlea, which is responsible for hearing, and the semicirular canals which are responsible for balance

The Olfactory System

the olfactory system is responsible for the sense of smell
wild horses used smell to:
- detect and avoid predators
- find distant water holes
- for reproduction
stallions can smell an in heat mare from 1/2 mile away
when a horse raises his upper lip and appears to be smiling, he is making a flehmen face
by making a flehmen face, a horse is able to trap a smell and memorize it
the nasal cavities include the vomeronasel organs and Jacobsen's organs, which detect pheromones
pheromones are animal scent signals
when horses greet one another by blowing into one another's nostrils, they are memorizing one another's scent

The Sense of Taste

the sense of taste is closely related to smell
taste is detected by the taste buds on the tongue
horses can taste the four basic tastes:
- sweet
- salty
- sour
- bitter
more precise tastes result from the interaction of smell
horses have a higher tolerance for bitter tastes than humans do

The MusculoSkeletal System

Bones

bones are made up of bone cells and minerals
a bone cell is called an osteocyte
most of the mineral matter of bone is Calcium Phosphate
very dense bones are called compact bones
the 4 types of bones are:
- long
- short
- flat
- irregular
long bones are characteristically found in the limbs, and include enlargements at each end for attachment of tendons and ligaments
short bones include the pastern bones, coffin bone, etc.
flat bones have a large surface area, like the scapula and sternum
irregular bones include those in the vertebral column, and have a large number of projections
red blood cells are produced in the marrow of the long bones
the growth plate of a long bone is called the epiphysis
the membrane which surrounds and protects a bone is called the periosteum

Tendons and Ligaments

tendons connect muscle to bone
ligaments connect bone to bone
both tendons and ligaments are made up of connective tissue
neither tendons or ligaments have much elasticity

Joints

articulations, or joints, are junctions between bones
the four types of joints are:
- hinge
- ball and socket
- gliding
- and immovable joints, or fixed joints
immovable or fixed joints do not allow any movement
the sutures in the skull are immovable joints, joining the plates of the skull together
a gliding or sliding joint has limited movement, but allows the two bones to slide over one another, such as the vertebrae in the spine
a hinge joint allows movement in one plane only; the joint can bend or extend, but may not move from side to side
an example of a hinge joint is the fetlock
a ball and socket joint allows the widest range of motion, such as that in the shoulder
the synovial capsule encloses a movable joint
the lubricating fluid found in joints is called synovial fluid

Muscles

the three types of muscle are smooth, skeletal, and cardiac
cardiac muscle is found in the heart; smooth muscle, or involuntary muscle, is found in the alimentary canal, bladder, and other organs
skeletal muscle, or voluntary muscle, is found in the musculo
skeletal system
skeletal muscles are made up of muscle fibers
muscle fibers are surrounded by thin membrane sheaths called the sarcolemma
muscles pull, they do not push
all muscles work in one direction, and are therefore paired
the opposite of muscle contraction is relaxation
the extensor muscle causes the joint to extend, or straighten, when it contracts
the flexor muscle causes the joint to bend when it contracts

The Skeleton

the horse's body has approximately 205 bones
the skull forms the cranial cavity, which holds the brain
the eye socket is properly called the orbit
the lower jaw is properly called the mandible
the summit of the occipital crest forms the poll
the first section of the vertebral column consists of the cervical vertebrae, or vertebrae of the neck
the first cervical vertebrae is called the atlas
the second cervical vertebrae is called the axis
there are a total of seven cervical vertebrae
the second section of the vertebral column consists of the thoracic vertebrae (the vertebrae of the back)
the thoracic vertebrae are sometimes called the dorsal vertebrae
there are a total of eighteen thoracic vertebrae
each thoracic vertebrae attaches to a pair of ribs
the withers are formed by the spinous processes of the fifth through eighth thoracic vertebrae
the third section of the vertebral column consists of the lumbar vertebrae
there are six lumbar vertebrae
the sacrum is the fourth section of the vertebral column, and consists of five fused sacral vertebrae
the last section of the vertebral column consists of the coccygeal vertebrae, which make up the tail
there are between 18 and 21 coccygeal vertebrae
there are 18 pairs of ribs (32 total ribs)
the ribs protect the thoracic cavity
each pair of ribs attaches to one of the thoracic vertebrae
the first 8 pairs of ribs also attach to the sternum (or breastbone) and are called "true ribs"
the last 10 pairs of ribs are called "false ribs" because they do not connect to the sternum, and therefore are only attached to the thoracic vertebrae
the breastbone is properly called the sternum, and consists of 8 segments
the foreleg has no skeletal attachment to the horse's trunk, and is attached to the body by muscles at the shoulder
the shoulder blade is properly called the scapula
the bones of the foreleg include:
- the scapula
- the humerous
- the ulna
- the radius
- the 7 carpal bones
- the cannon bone
- the 2 splint bones
- the 2 proximal sesamoids
- the long pastern bone
- the short pastern bone
- the coffin bone
- the navicular bone
the 6 joints of the foreleg are:
- the shoulder
- the elbow
- the knee
- the fetlock
- the pastern
- the coffin joint
the point of the elbow is formed by the ulna (specifically, the olecranian process of the ulna)
the main bone of the forearm is the radius
the carpal bones form the carpus or knee
the 7 carpal bones are:
- pyramidal bone or cuneiform
- semilunar bone or lunate
- radial carpal bone or scaphiod
- trapezoid
- os magnum
- unciform
- accessory carpal bone or pisiform
the horse's knee is analogous to the human's wrist
the cannon bone is properly called the 3rd metacarpus
the 1st and 2nd metacarpal bones are the splint bones
the medial splint is on the inside of the leg and the lateral splint is on the outside of the leg
the proximal sesamoids are the 2 bones on the back of the fetlock
the long pastern bone is properly called the 1st phalanx
the short pastern bone is properly called the 2nd phalanx
the coffin bone is also known as the pedal bone or the 3rd phalanx
the navicular bone is also called the distal sesamoid bone
the two bones entirely within the hoof are the navicular bone and the coffin bone
the short pastern bone is partially within the hoof
the bones of the hind leg include:
- the bones of the pelvis
  - ilium
  - pubis
  - ischium
- the femur
- the patella
- the tibia
- the fibula
- the 7 tarsal bones
- the metatarsal (hind cannon)
- the 2 splint bones
- the 2 proximal sesamoids
- the long pastern bone
- the short pastern bone
- the coffin bone
- the navicular bone
the femur is the largest bone in the body
the patella is the largest sesamoid bone in the horse's body
the horse's patella is analogous to the human's kneecap
the 7 tarsal bones make up the tarsus or hock
the horse's hock is analogous to the human ankle
the cannon bone is known as the metatarsus in the hind leg
the horse has 7 joints in the hind leg:
- the sacro-iliac
- the hip
- the stifle
- the hock
- the fetlock
- the pastern
- the coffin joint
the stifle and hock are dependent joints (they bend together)

Membranes

structure covered	name of membrane
a bone	periosteum
the heart	pericardium
the digestive system	peritonium
a cartilage	perichondrium
the root of a tooth	peridontal membrane
the lungs	pleura