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The skeleton, joints and types of muscle
Aimee Hailstone
1
Contents
The Structure of the bone .................................................................................................................... 2
Introduction ...................................................................................................................................... 2
The Structure of the bone ................................................................................................................ 3
The Equine Skeleton ............................................................................................................................. 5
The hind limbs ................................................................................................................................... 6
Forelimb ............................................................................................................................................ 7
Types of bone ..................................................................................................................................... 10
Flat bones........................................................................................................................................ 10
Irregular bones ................................................................................................................................ 10
Long bones ...................................................................................................................................... 10
Short bones ..................................................................................................................................... 10
Pneumatic bones ............................................................................................................................ 11
Sesamoid bones .............................................................................................................................. 11
Synovial Joints ..................................................................................................................................... 11
Identify the three main groups of muscles ......................................................................................... 15
Skeletal Muscle ............................................................................................................................... 15
Smooth Muscle ............................................................................................................................... 15
Cardiac Muscle ................................................................................................................................ 15
Bibliography ........................................................................................................................................ 16
Websites ......................................................................................................................................... 16
Textbooks ........................................................................................................................................ 16
2
The Structure of the bone
Introduction
The bone is a dense, living tissue with nerves and blood vessels which contain proteins and
minerals. They are ridged organs and constitute part of the endoskeleton of vertebrates.
Their main roles are to support and protect various organs of the body, produce red and white
blood cells and also store minerals.
These proteins and minerals give the bone its hard and strong structure, approximately 60% of
the weight of the bone is minerals mainly calcium and phosphate the remainder consists of
water and matrix.
A healthy bone is very reliant on the amount of available calcium in the diet; if the minerals are
removed for example it affects the bone by becoming soft and fragile and therefore losing its
strong composition.
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The Structure of the bone
If you analyse a cross-section of a bone (Figure 1), the thin outer
layer consists of a dense connective tissue called Periosteum. The
periosteum provides blood to the bone and is a point for muscular
attachment, it also protects the bone, and contains blood vessels
which nourish the underlying bone. Underneath the periosteum is a
layer of hard compact bone, this hard dense outer layer of a bone is
made up of compact bone tissue and this tissue is what gives a bone
its smooth, white and solid appearance.
The compact bone (Figure 2) is made up
of connective rings of osteocytes called
oesteons also known as a Haversian
system. All compact bones contain many
haversian systems, they line up closely
and this is what creates its dense
structure.
The Haversian canal is a small component of the
bone structure, however it is extremely important,
as without its function the bone would be unable
to retain or hold blood vessels or nerve fibres,
leading to the decay and destruction of the
compact bone.
Figure 3 shows the Haversian system in detail. The
system includes lamellae, osteocytes, lacunae,
canaliculi and a haversian canal. The Haversian
system contains concentric circles called lamellae.
Within the lamellae are gaps known as lacunae this is where osteocytes are kept. Located in
Figure 2- Compact Bone
Figure 1 - Bone composition
Figure 3 - Haversian system
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Figure 4 - A detailed cross section of a bone
the centre of the haversian system is a central canal known as a haversian canal , each contain
blood vessels, connective tissues nerve fibres and lymphatic vessels which provides nutrients to
the living bone tissues.
A bone consists of an organic matrix with mineral salts inside. These salts are in the form of tiny
sub microscopic crystals and they give the bone its property of extreme hardness. The organic
matrix contains densely packed collagen fibres which help to make the bone strong. Both the
organic matrix and mineral salts are produced by cells called osteoblasts. In the development of
a limb bone for example, osteoblasts will arrange themselves in rings around series of
haversian canals. Due to the arrangement of the osteoblasts the matrix is laid down in a series
of layers (lamellae) and therefore enriching the haversian canals. Every osteoblast will
eventually end up in a space (lacunae) in which narrow channels known as canaliculi pass
through the lamella. Once in the lamanae the osteoblasts will stop secreting matrix material
and are then called osteocytes.
The hollow centre of the diaphysis is known as the
medullary cavity, and is filled with red bone marrow
for blood cell production. In a mature long bone the
medullar cavity has a filling of yellow bone marrow
which is used as a storage site for fat. The inside of
the medullary cavity is lined with connective tissue
called the endosteum. Located on the outside of the
epiphysis is the articular cartilage. The smooth, shiny
surface of the cartilage helps to decrease any friction
within a joint.
5
The Equine Skeleton
Key:
The equine skeletal system (Figure 5) is an amazing structure, which has numerous
functions that are vital to the horse’s wellbeing. It supports the mass of the horses’ body,
including the muscles and organs, whilst protecting any internal organs from force or
impact; examples include the skull protecting the brain, ribs protect the lungs and heart,
the pelvis protects and supports the digestive and reproductive organs and the spinal
column protects the spine. This body frame is also designed so that it can remain strong
but also allow locomotion. It must also be able to protect the horse from the stresses of
hard work such as galloping at high speed.
1. Nasal Bone 8. Cervical vertebrae 15 Ribs – 18 22. Stifle joint 29.Fetlock joint 36. Tibia
2. Maxillary bone 9. Scapula 16. Pelvis 23. Patella 30. Coffin joint 37. Tarsal bones
3. Mandible 10. Thoracic vertebrae 17. Hip joint 24. Elbow joint 31. Accessory carpal bone 38. Split bone
4. Orbit 11. Lumbar vertebrae 18. Femur 25. Ulna 32. Splint bone 39. Cannon bone
5. Frontal bone 12. Sacral vertebrae 19. Humerus 26. Radius 33. Sesamoid bone 40. Pastern join
6. Atlas 13. Coccygeal vertebrae 20. Sternum 27. Carpus 34. First phalanx 41. Fibula
7. Axis 14. Shoulder joint 21. Olecranon 28, Cannon (metacarpal) bone 35. Pedal bone 42. Navicular bone
Figure 5 - The equine skeleton
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The equine skeleton is often split into two groups. The first section is called the Axial
skeleton and starts at the head, and includes all of the vertebrae, the cervical vertebrae,
the thoracic vertebrae, and the ribs which connect through cartilage to the breast bone or
the sternum. The horse has 18 ribs on either side, 8 are “true” ribs and attach to both the
vertebrae above them and the sternum below and the remaining 10 are referred to as
“false ribs” which connect to sternum by cartilage.
The axial skeleton consists of 54 bones in its vertebral and is made up of:
7 cervical vertebrae (the neck)
18 thoracic vertebrae (chest)
6 lumbar vertebrae (loins)
5 vertebrae which are fused together which forms a single bone (sacrum)
18 coccygeal vertebrae – this can vary with different breeds. (tail)
The second division of the skeleton of the horse is the appendicular skeleton which
consists of:-
Forelimbs Hindlimbs
Scapula Pelvis
Humerus Femur
Radius/Ulna Tibia/Fibula
Carpals Tarsals
Metacarpals Metatarsals
Phalanges Phalanges
The hind limbs
The hind limbs of a horse start with the pelvis and consist of three sets of bones called the
ischium, ilium and the pubis. Once the horse reaches its full maturity these three bones
fuse together to form one bone. From the pelvis, the first long bone of the hind limb is the
femur. This is a very large and powerful bone in the horse it runs downwards and forwards
to meet the patella and the tibia. The patella, a horse’s knee cap, is different from that of a
human as it has three ligaments which attach it to the tibia; the human equivalent only has
one such ligament. A hook, situated on the inside and bottom end of the femur, on the
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hind limb cups the patella and the medial patella ligament, which in turn prevent the leg
from bending. This helps to stabilize the joint and forms part of the reciprocal apparatus,
the horse then has the ability to sleep whilst standing or rest with minimal muscular effort.
The stifle joint is the equivalent to the human knee joint, consisting of two ligaments and
two cartilages. The tibia is the main leg bone of the second thigh, and the fibula is a very
thin and short bone which attaches to it. The tibia connects down to the tarsal bones
(hock) and the calcaneus bone that forms the hock tip and connects to the calcaneus
tendon, which is the equivalent to the Achilles tendon in the human. This tendon is crucial
for the locomotion of the horse. Following the calcaneus is the talus, which is a rounded
and smooth shaped bone and then further down we come to the central tarsal bones. Just
like the carpal bones in the forelimb we have the second and third tarsal bones and on the
lateral side the forth tarsal bone. Similar again to the forelimb follows the metatarsal
bones. The second metatarsal bone is also known as the splint bone, the third is called the
cannon bone and the and on the lateral side is the fourth metatarsal bone (splint bone)
Connecting to the cannon bone is the two sesamoids, lateral (outside of the horse) and the
medial sesamoid (inside of the horse). Finally we come to the long and short pastern bone
and then the coffin bone (also known as the 1st, 2nd and 3rd phalanx.)
Forelimb
This diagram below shows how the two shoulder
blades are attached to the chest wall by muscles and
ligaments.
The forelimb consists of the following bones from
the shoulder down to the knee.
Figure 6 - Structure of shoulder
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Horses have a shoulder blade (scapula) just like the human; however they do not process a
collar bone. They do not have any bony attachment of the front limb to the rest of the
body. The way this limb stays attached to the horses body is through muscles, tendons,
ligaments and other connective tissues. The scapula is a broad shaped triangular bone
which is placed on the outer side of the ribs. This bone has a limited amount of movement
due to its strong muscular attachment, but can move forwards and backwards against the
rib cage. It is also able to tilt side to side very slightly (this movement would be seen with
dressage horse when asked to do half passes
The humerus (the arm) is located between the point of the shoulder and the elbow joint; it
is a strong yet short bone which is covered in a thick layer of muscles. The humerus runs
downwards and backwards, articulating with the radius bone. The radius is a longer bone
with a slight bend to the front with a rounded shaft. The bone runs the whole length of the
forearm. The ulna is located at the top end of the radius bone and forms part of the elbow
joint. This is fused in the horse.
The carpal bones which are the equivalent to the human wrist consist of two layers of
bones. The top row begins with the radial carpal bone, the intermediate carpal bone, the
ulnar carpal bone and the accessary carpal bone. The second layer of bone, starting on the
Figure 7 - Carpal bones – forelimb posterior view
Figure 8 - Front view of forelimb Figure 9 - Side view of forelimb
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inside is the second third and fourth carpal bone. Further down the limb is the cannon
bone also known as the third metacarpal bone, on the inside is the second metacarpal
bone (the splint bone) and the lateral side is the fourth metacarpal bone. Going south
down the leg we come to the sesamoid bones, the medial and the lateral. Finally is the
long, short and coffin bone, also known as the first, second and third phalanx.
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Types of bone
The equine skeleton consists of a number of different types of bones. They are called, flat
bones, long bones, short bones, irregular bones and pneumatic bones.
Flat bones
These consist of a thin layer of compact bone, separated by spongy bone. A classic example
of a flat bone would be the scapula (horses shoulder), cranium, pelvis and the ribs. These
bones are strong and their main function is to protect the vital organs of the body and
providing a base for muscular attachment.
Irregular bones
They include an outer layer of compact bone with an inner layer consisting mostly of
cancellous bone. Irregular bones are described by their name; they come in a variety of
irregular shapes. Examples include the vertebrae bones located across the back, cervical
vertebras of the neck and the thoracic vertebrae’s (of the back) other examples of irregular
bones include the sacrum (end portion of the horses spine) and coccygeal vertebras (tail)
They can serve my purposes such as the protection of nervous tissue and providing anchor
points for skeletal muscle. They consist of cancellous tissues inside a thin layer of compact
bone.
Long bones
These bones include some of the longest bones in the horse’s body, such as the tibia and
fibula (fibula fuses to the tibia, the tibia runs from the stifle to the hock.), humerus (lies
between the scapula and the radius), radius (located by the horses shoulder joint attaching
to the scapula) and the ulna. They are longer than they are wide, and they are crucial for
skeletal mobility. The outside of the bone contains connective tissues called the
periosteum with the next layer being the compact bone. Deeper inside the bone contains a
layer of cancellous bone, containing yellow marrow (mature adult horses) (red marrow in
young horses)
Short bones
They can be defined as being similar in width as they are in length, consisting of a thin layer
of compact bone with an inner layer of cancellous bone including a large quantity of bone
marrow. The short bone provides stability with little movement. Examples of such a bone
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would be the tarsals (located by the tibia and the fibula bones), carpals (bones located in
the knee) and phalanges bones (fetlock joint, short pastern bones, coffin bone)
Pneumatic bones
These bones are also irregular in shape, however shouldn’t be confused with irregular
bones. Pneumatic bones allow large air space, making them very light weight. The equines
sinuses located are good examples of a pneumatic bone.
Sesamoid bones
These bones are located in where a tendon passes over a joint. Its function is to reduce
wear and tear occurring to the tendon, and it helps increase the leverage if the muscle and
its tendon over a joint. Examples of these bones can be found in the patella (stifle),
proximal sesamoid bones located around the fetlock and the accessory carpal bones.
Synovial Joints
Joints can be classified by the tissue which connects the bone to the joint. They are three
different types called, Fibrous, cartilaginous and synovial joints.
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Fibrous joints are joints which are connected by fibrous ligaments and are a fixed joint, and
therefore unable to move examples of this joint would be the bones of the skull and the
pelvis.
Cartilaginous joints are joined by fibrocartilage or hyaline cartilage (Hylaine cartilage has a
slippery texture and is strong when it is compressed, however when stretched it does not
retain the same strength. Fibrocartilage is tough and strong when compressed and
stretched.) They allow a certain amount of movement but are unlike the synovial joint
which is a free moving joint.
There are also two types of cartilaginous joints which include:
1. Synchondrosis – found in epiphyseal plates during growth development during
growth development in young horses
2. Symphysis – these are strong and allow slight movement.
All moving joints in a horse are synovial joints and are designed to absorb concussion
whilst still allowing movement. The joint starts with two bone endings (Ephysis). These
bone endings are covered with a coating of articular cartilage which is smooth and hard
wearing; this allows the movement of the joints whilst preventing the bones from grinding
together. The fibrous joint capsule surrounds the joint and provides it stability, keeping the
synovial fluid secure and in place. Each capsule also contains an inners ling called the
synovial membrane; its function is to provide synovial fluid to the joints in order to
lubricate them. Synovial fluid is also important for supplying nutrition and removing
metabolic waste.
13
Figure 15 – Plane or gliding joint
Figure 10 - Saddle joint Figure 11 - Pivot Joint Figure 12 - Hinge Joint Figure 13 - Condylar Joint
Figure 16 - Ellipsoidal joint Figure 14 – Ball and socket joint
There are seven types of synovial joints:
The saddle joint (Figure 10) permits all types of movement such as flexion, abduction and
circumduction, this joint is unable to rotate. The saddle joint is a biaxial joint which fits into
an identically shaped socket on the other bone.
The pivot joint (Figure 11) allows one bone to spin on another bone and has the ability to
rotate in either direction an example of such joint is the atlas and the axis.
The hinge joint (Figure 12) offers easy movement, however they only provide movement in
one plane this is due to strong collateral ligaments. There is no twisting, sliding or side to
side movement. Where two bones meet, one end of the bone is a concave shape and the
other is a cylinder shape which allows a snug fit. An example of a hinge joint is the fetlock.
Condylar joints (Figure 13) are able to move about one another but are unable to rotate.
They have two knuckle shaped endings which fit into a cup shaped surface. An example
would be the twin mandibular joints, (the jaw is quite flexible however we are unable to
rotate it)
The ball and socket joint (Error! Reference source not found.14) allows a rotational
movement and a back and forth movement in all planes. It includes a rounded sphere head
bone which fits into cup shaped cavity. An example of a ball and socket joint is the hip.
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Plane or gliding joints (Figure 15) do not allow much distance in movement, they move in a
gliding or sliding movement. These joints are also unable to rotate. An example of this joint
is the articular surfaces of the cervical vertebrae.
An ellipsoid joint (Figure 16) has a similar type of movement as the ball and socket joint, it
allows bending and extending, rocking from side to side. It can move in two planes without
rotation, an example of this joint is the radio carpal joint.
15
Identify the three main groups of muscles
Skeletal Muscle
This muscle is attached to the bones, and plays a large role in skeletal movements, they are
also responsible for stabilises joints and generating heat, this heat is vital for maintaining
normal body temperature. Skeletal muscles cover the skeleton and are either attached by
strong tendons or connected directly to the bone. These muscles are voluntary which
basically means that they are consciously controlled by the horse.
Smooth Muscle
Smooth muscle is found in the walls of hollow organs such as the
intestines and stomach. They are subconscious muscles which
means they work automatically. Some of the roles of the smooth
muscle are that contracts the intestines which in turn push food
through the body, muscular walls in the intestine also contract to
remove urine from the body. The muscular walls of your intestines
contract to push food through your body. Muscles in your bladder wall contract to expel
urine from your body
Cardiac Muscle
Cardiac muscle is only found in the walls of the heart, and
is an involuntary muscle. Cardiac muscle never gets tired
and constantly contracts and relaxes squeezing blood out
of the heart and filling it again without ever pausing to rest.
Figure 17 - Skeletal Muscle
Figure 18 - Smooth Muscle
Figure 19 - Cardiac Muscle
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Bibliography
Websites
1. http://www.highlands.edu/academics/divisions/scipe/biology/faculty/decker/a
p1bonetissue.ppt
2. http://www.wisegeek.com/what-is-compact-bone.htm
3. http://www.wisegeek.com/what-is-the-haversian-system.htm
4. . http://visual.merriam-webster.com/human-
being/anatomy/skeleton/structure-long-bone.php
5. www.equestrianandhorse.com
Textbooks
1. S Hastie, B. Vet. Med.MRCVS and J Sharples: Horselopaedia, A complete guide
to Horse Care.
2. Islay Auty FBHS, The BHS complete manual of Stable Management
3. M Roberts, M Reiss, G Monger: Biology, Principles and Processes
4. J Sutton: Biology, Macmillan Foundations
5. P. Stewart Hastie MRCVS, The comprehensive reference: The BHS Veterinary
Manual
6. Guyton, A,C.and Hall J.E. Textbook of medical physiology (9th ed.)
(Philadelphia:saunders, 1996)
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