Equine Biological Systems

Equine biological systems

Pack1

The skeleton

Pack Code: EBS1

About this pack

Objectives

When you have completed this pack you should able to describe the main components and functions of the horse skeleton.

The pack will help you to:

•Identify the main bones of the horse skeleton and their functions.

The pack is also relevant to the level 3 unit Understand the Principles of Animal Biology, and in particular to:

•Outcome 3: Know the structure and function of animal skeletal systems.

It is also relevant to the level 3 unit on Understand Animal Anatomy and Physiology, and in particular to:

•Outcome 4: Understand how an animal’s body structure and systems are adapted to its environment.

Links to other packs

This is one of a series of learning packs, each tackling an aspect of equine biology. They are:

•EBS1: The skeleton
•EBS2: Joints
•EBS3: Circulation
•EBS4: Respiration
•EBS5: The digestive system of the horse
•EBS6: The reproductive system
•EBS7: Oestrus and hormones
•EBS8: Gestation and birth
•EBS9: How animal cells work
•EBS10: How animal cells divide
•EBS11: Connective and epithelial tissue / •EBS12: Muscle and nervous tissue
•EBS13: The horse’s sensory organs
•EBS14: Introduction to genetics
•EBS15: Meiosis
•EBS16: Inheritance
•EBS17: Selective breeding
•EBS18: Breeding technology
•EBS19: Managing breeding
•EBS20: The lymphatic system
•EBS21: The endocrine system
•EBS22: The nervous system

Overview of the skeleton

This pack describes the general skeletal system for horses.

Functions of the skeleton

•support soft tissue, and maintain the animal's body shape

•protect the delicate organs e.g. the ribcage protects the heart and lungs

•attach muscles and tendons

•enable movement through joints, e.g. to move a limb forward

•provide a site for forming blood cells (in the bone marrow)

•store calcium and phosphate in bones until they are needed by the body.

Types of bones

Bones come in four basic shapes, each of which has particular roles to play.

Long bones:for movement and strength – these include most of the limb bones e.g. thigh bone

Short bones:for limited movement and strength – cube shaped bones e.g. carpals (wrist bones in humans)

Flat bones:for protection – these include skull bones, shoulder blades and pelvis

Irregular bonesfor muscle attachment – all the bones which are not long, short or flat e.g. vertebrae, kneecap.

Review quiz 1

Are the following statements true or false?TrueFalse

a)Irregular bones protect delicate organs. 

b)Long bones provide strength.

c)Short bones provide strength.

d)Flat bones function mainly as points for muscle attachment

Answers are given at the end of this pack.

Two parts of the skeleton

The skeleton includes all the bones of the animal. It is divided into two parts: the central axial skeleton to which the appendicular skeletonis attached. The following diagram shows the main bones we consider during the course of this pack.

Activity

This activity will help you start to familiarise yourself with the bones of an animal – which could be a pet. You may find it helpful work on it with other students. You don't need to learn the names of the bones at this stage.

For an animal you work with:

•identify the main bones shown on the word diagram on the animal

•where possible feel the bones to get an idea of their location and shape.

The axial skeleton

The axial skeleton includes the skull, backbone, ribcage and tail.

Skull

The skull is the complex set of 37-38 bones in the head of the animal. Most of the bones are connected by immovable joints. The only freely moveable joint is that of the lower jaw (mandible). This diagram shows the skull of a horse.

Note that the skulls of different animals have different adaptations – while the horse – a herbivore – does not have large canines and moves its jaws in a circular way to grind food, predators have strong canine teeth and bite with a slicing movement. Whales have evolved rather different skulls.

There are three main groups of skull bones:

1Bones of the cranium – these bones surround and protect the brain. Internal bones are not visible from the outside, whereas external bones can be recognised on the animal.

2Bones of the ear – these bones are all hidden.

3Bones of the face. These bones give shape to the face. External bones of the face include:

•incisive bones which house the upper incisors

•nasal bones which form the bridge of the nose. The length of the animal's face is reflected in the length of these bones, which are long in the horse.

•maxillary bones which make up the upper jaw (maxilla) and hard palate, and house the upper teeth other than the incisors

•the mandible (lower jaw) which houses the lower teeth.

Teeth

Different types of teeth and their functions are described in Pack 5 The digestive system of the horse.

Backbone (vertebral column)

The main functions of the backbone are to:

•provide support to the limbs

•protect the spinal cord (by forming a channel called the 'vertebral canal')

The backbone is made up of a series of irregular shaped bones called vertebrae which extend from the skull to the tail. It is divided into five regions. This table compares the number of vertebrae in each region for horses with other farm animals .

horse / cattle / goat / pig / sheep
Cervical: / neck region / 7 / 7 / 7 / 7 / 7
Thoracic: / above the chest / 18 / 13 / 13 / 14-15 / 13
Lumbar: / above the abdomen / 6 / 6 / 7 / 6-7 / 6-7
Sacral: / above the pelvis / 5 / 5 / 5 / 4 / 4
Coccygeal: / the tail / 15-21 / 18-20 / 16-18 / 20-23 / 16-18

Vertebrae do not have individual names, instead they are numbered within each region from head to tail, so C7 is the seventh cervical vertebrae, and L2 the second lumbar vertebra.

In different animals there have been adaptations of this basic design. While the giraffe has the same number of neck or cervical bones as other mammals – seven – the bones have become much larger as the giraffe’s neck has grown longer. There are different explanations for this – some people argue that the giraffe’s neck is for reaching high branches, others that they are used for fighting.

Vertebrae

A typical vertebra has a body, an arch and a group of processes (protrusions). Protrusions act as sites for muscle attachment and help form joints between adjacent vertebrae. When the holes in the vertebrae are lined up they form the spinal canal which houses and protects the spinal cord. Intervertebral discs lie between the vertebrae and help to absorb shock.

The vertebrae throughout the backbone are fairly similar, with the following exceptions:

•the atlas (the first cervical vertebra) is a simple ring of bone with two processes which allows for nodding movement of the head

•the axis (the second cervical vertebrae) has a process which fits into the axis, allowing for side to side movement of the head.

•the sacral vertebrae are fused to form the sacrum – a single structure joined to the pelvis.

Chest (thoracic cage)

The bones of the chest include vertebrae and:

•The breastbone (sternum) which forms the front of the thorax.

•The ribs – most horses have 36 ribs, though some breeds have 37 or 38. Joints between the vertebrae and ribs, and cartilage between the ribs and the breastbone allow the ribcage to expand and contract as the animal breathes.

Review quiz 2

•1 Match the following regions with vertebrae

• Neck:coccygeal

• Chest:sacral

• Abdomen:cervical vertebrae

• Pelvis:lumbar

• Tail:thoracic

•2 Match the following common and technical names for bones in the axial skeleton:

• upper jawmaxilla

• lower jawsternum

• breastbonemandible

3 Are the following statements true or false?TrueFalse

a) teeth are housed in the cranium

b) the lower jaw is the only bone which moves in the head

c) the ribs are fixed rigidly to the spine

d) the axis allows for side to side movement of the head

Answers are given at the end of this pack.

Aquatic mammals

The skeletons of whales and dolphins have evolved to become rather different from other mammals. The backbone is long – a dolphin has 73 vertebrae – though the vertebrae do not extend into the tail which is made up of cartilage. Whale bones are spongier than other mammals as the animals are supported by water.

The forelimbs have become flippers – though interestingly the bones are remarkably similar to those in humans. However there are no external hind limbs.

The appendicular skeleton

The appendicular skeleton includes the bones of the fore and hind limbs.

Forelimb

The forelimb includes the bones from the shoulder blade down through the limb to the phalanges. Unlike humans there is no collar bone to link the forelimb to the backbone, instead the attachment is achieved through muscles and tendons.

This diagram shows the forelimb of a horse. The bones are:

•Scapula (shoulder blade) – this includes the socket part of the shoulder joint.

•Humerus – this is the long bone of the upper arm, and includes the ball part of the shoulder joint.

•Radius – the main weight-bearing bone of the forearm, which works with the ulna.

•Ulna – the humerus and ulna lie parallel to each other and meet at the elbow joint.

•Carpals – this is two rows of bones, which is called the knee in horses. In humans they form the wrist.

•Metacarpals – in humans these bones form the palm of the hand. Horses have only one carpal bone.

•Phalanges – in humans the five fingers (or digits) each have three phalanges. Horses have one digit with three phalanges.

Different animal species have again evolved different arrangements of the forelimb. In digging animals like moles the arms are stout, the hands are broad and claws are strong.

In bats the forelimb has evolved into a wing. The finger bones are much longer than in many mammals, and are more flexible.

Hindlimb

The hindlimb is directly connected to the backbone through the pelvis. This avoids the need for a sling of muscles and tendons used to connect the forelimb to the backbone, thus leaving room in the abdominal cavity for the reproductive, digestive and urinary structures.

This diagram shows the hind limb of a horse. The bones are:

•Pelvis (not shown here – see skeleton diagram at end of pack) – the pelvis is a girdle attached to the backbone at the sacroiliac joint. It provides the socket for the hip joint. The newborn animal must pass through the space in the middle of the female pelvic girdle at birth.

•Femur (thigh bone) – the top end of this long bone forms the ball part of the ball and socket hip joint. The lower end forms the stifle joint with the patella and tibia.

•Patella (kneecap) – the kneecap is the largest irregular (sesmoid) bone in the body, and forms the front of the stifle joint where it protects the tendon.

•Tibia (shinbone) – this is the main weight bearing bone in the lower leg. It meets the femur in the stifle joint.

Note that in some animals such as dogs and cats there is a thin bone parallel to the tibia called the fibula. In cattle only the ends of the bone are present. It serves as a muscle attachment site.

•Tarsals ('hock' in horses; ankle in humans) – the hock comprises two rows of short bones similar to the carpal bones in the forelimb. The calcaneal tuberosity projects up and back forming the point of the hock, and is equivalent to our heel. It is the attachment point for the calf muscle.

•Metatarsals - similar to the metacarpals of the forelimb. In humans these form the sole of the foot.

•Phalanges - as for the forelimb.

Activity

Once again, the skeletal design of different animals varies to reflect how they move. The diagrams here show the hind legs, from left to right, of the horse, sheep and dog. What differences do you notice between them?

The horse has no muscles below the knee and so the lower leg is held together by a group of tendons (which attach muscle to bone) and ligaments (which attach bone to bone usually forming a joint). There is much strain on the ligaments especially when the horse is in gallop and if too much pressure is applied the ligaments (normally the suspensory ligament) can give way – this is sometimes irreparable.

Activity

Label the parts of the horse skeleton using the following terms:

•cervical vertebrae, carpals, coccygeal, femur, humerus, lumbar vertebrae, metacarpals, metatarsals, pelvis, phalanges, radius, ribs, sacrum, scapula, skull, sternum, tarsual, thoracic vertebrae, tibia.

Distinguish (with colour or other means) the bones of the axial and appendicular skeletons.

•

Check your answers with the word skeleton at the beginning of this pack.

Find a diagram of a skeleton of a different animal – for example, a sheep, dog or rabbit – and note down differences between them. What might explain these differences?

Answers to review quizzes

Review quiz 1

a) False

b) True

c) True

d) False

Review quiz 2

1Neck:cervical vertebrae

Chest:thoracic

Abdomen:lumbar

Pelvis:sacral

Tail:coccygeal

2 upper jawmaxilla

lower jawmandible

breastbonesternum

3a) False b) True c) False d) True

Answers to activity on page 4

You may have noticed that skeletal design differs in different animals. For example, in many animals the metacarpals/metatarsals and phalanges are much more extended than the equivalent hands and feet in humans.

Don't worry if you found the names of bones confusing, or they were difficult to identify - the rest of this pack goes on to look at the skeleton in more detail.