The skeleton, joints and types of muscle

Aimee Hailstone

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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

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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.

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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.

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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.

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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)