# Anatomy and Physiology of Animals/The Skeleton

!original image by
heschong cc
by{width="400"}

## Objectives

After completing this section, you should know:

-   the functions of the skeleton
-   the basic structure of a vertebrae and the regions of the vertebral
    column
-   the general structure of the skull
-   the difference between 'true ribs' and 'floating ribs
-   the main bones of the fore and hind limbs, and their girdles and be
    able to identify them in a live cat, dog, or rabbit

Fish, frogs, reptiles, birds and mammals are called **vertebrates**, a
name that comes from the bony column of vertebrae (the spine) that
supports the body and head. The rest of the skeleton of all these
animals (except the fish) also has the same basic design with a skull
that houses and protects the brain and sense organs and ribs that
protect the heart and lungs and, in mammals, make breathing possible.
Each of the four limbs is made to the same basic pattern. It is joined
to the spine by means of a flat, broad bone called a **girdle** and
consists of one long upper bone, two long lower bones, several smaller
bones in the wrist or ankle and five digits (see diagrams 6.1 18,19 and
20).

![](Anatomy_and_physiology_of_animals_Mamalian_skeleton.jpg "Anatomy_and_physiology_of_animals_Mamalian_skeleton.jpg")

Diagram 6.1 - The mammalian skeleton

## The Vertebral Column

The vertebral column consists of a series of bones called **vertebrae**
linked together to form a flexible column with the skull at one end and
the tail at the other. Each vertebra consists of a ring of bone with
spines (spinous process) protruding dorsally from it. The spinal cord
passes through the hole in the middle and muscles attach to the spines
making movement of the body possible (see diagram 6.2).

![](Vertebra.JPG "Vertebra.JPG")

Diagram 6.2 - Cross section of a lumbar vertebrae

The shape and size of the vertebrae of mammals vary from the neck to the
tail. In the neck there are **cervical vertebrae** with the two top
ones, the **atlas** and **axis**, being specialized to support the head
and allow it to nod "Yes" and shake "No". **Thoracic vertebrae** in the
chest region have special surfaces against which the ribs move during
breathing. Grazing animals like cows and giraffes that have to support
weighty heads on long necks have extra large spines on their cervical
and thoracic vertebrae for muscles to attach to. **Lumbar vertebrae** in
the loin region are usually large strong vertebrae with prominent spines
for the attachment of the large muscles of the lower back. The **sacral
vertebrae** are usually fused into one solid bone called the **sacrum**
that sits within the **pelvic girdle**. Finally there are a variable
number of small bones in the tail called the **coccygeal vertebrae**
(see diagram 6.3).

![](Anatomy_and_physiology_of_animals_Regions_of_a_vertebral_column.svg "Anatomy_and_physiology_of_animals_Regions_of_a_vertebral_column.svg")

Diagram 6.3 - The regions of the vertebral column dik

## The Skull

The skull of mammals consists of 30 separate bones that grow together
during development to form a solid case protecting the brain and sense
organs. The "box "enclosing and protecting the brain is called the
**cranium** (see diagram 6.4). The bony wall of the cranium encloses the
middle and inner ears, protects the organs of smell in the nasal cavity
and the eyes in sockets known as **orbits**. The teeth are inserted into
the upper and lower jaws (see Chapter 5 for more on teeth) The lower jaw
is known as the **mandible**. It forms a joint with the skull moved by
strong muscles that allow an animal to chew. At the front of the skull
is the nasal cavity, separated from the mouth by a plate of bone called
the **palate**. Behind the nasal cavity and connecting with it are the
**sinuses**. These are air spaces in the bones of the skull which help
keep the skull as light as possible. At the base of the cranium is the
**foramen magnum**, translated as "big hole", through which the spinal
cord passes. On either side of this are two small, smooth rounded knobs
or **condyles** that **articulate** (move against) the first or Atlas
vertebra.

![](Anatomy_and_physiology_of_animals_Dogs_skull.jpg "Anatomy_and_physiology_of_animals_Dogs_skull.jpg")

Diagram 6.4 - A dog's skull

## The Rib

Paired ribs are attached to each thoracic vertebra against which they
move in breathing. Each rib is attached ventrally either to the
**sternum** or to the rib in front by cartilage to form the rib cage
that protects the heart and lungs. In dogs one pair of ribs is not
attached ventrally at all. They are called **floating ribs** (see
diagram 6.5). Birds have a large expanded sternum called the **keel** to
which the flight muscles (the 'breast" meat of a roast chicken) are
attached.

![](Anatomy_and_physiology_of_animals_Ribs.jpg "Anatomy_and_physiology_of_animals_Ribs.jpg")

Diagram 6.5 - The rib

## The Forelimb

The forelimb consists of: **Humerus, radius** and **ulna, carpals,
metacarpals, digits** or **phalanges** (see diagram 6.6). The top of the
humerus moves against (articulates with) the **scapula** at the shoulder
joint. By changing the number, size and shape of the various bones, fore
limbs have evolved to fit different ways of life. They have become wings
for flying in birds and bats, flippers for swimming in whales, seals and
porpoises, fast and efficient limbs for running in horses and arms and
hands for holding and manipulating in primates (see diagram 6.8).

![](Forelimb_dog_corrected.JPG "Forelimb_dog_corrected.JPG")

Diagram 6.6 - Forelimb of a dog

![](Hind_limb_dog_corrected.JPG "Hind_limb_dog_corrected.JPG")

Diagram 6.7. Hindlimb of a dog

![](Anatomy_and_physiology_of_animals_Various_vertebrate_limbs.jpg "Anatomy_and_physiology_of_animals_Various_vertebrate_limbs.jpg")

Diagram 6.8 - Various vertebrate limbs

![](Anatomy_and_physiology_of_animals_Forelimb_of_a_horse.jpg "Anatomy_and_physiology_of_animals_Forelimb_of_a_horse.jpg")

Diagram 6.9 - Forelimb of a horse

In the horse and other equines, the third toe is the only toe remaining
on the front and real limbs. Each toe is made up of a proximal phalange,
a middle phalange, and distal phalange (and some small bones often
referred to as sesamoids. In this image, the proximal phalange is
labeled P3 and the distal phalange is labeled hoof. (which is more
properly the name of the keratin covering that we see in the living
animal).

The legs of the horse are highly adapted to give it great galloping
speed over long distances. The bones of the lower leg and foot are
greatly elongated and the hooves are actually the tips of the third
fingers and toes, the other digits having been lost or reduced (see
diagram 6.9).

## The Hind Limb

The hind limbs have a similar basic pattern to the forelimb. They
consist of: **femur, tibia** and **fibula, tarsals, metatarsals,
digits** or **phalanges** (see diagram 6.7). The top of the femur moves
against (articulates with) the pelvis at the hip joint.

## The Girdles

The girdles pass on the "push" produced by the limbs to the body. The
shoulder girdle or **scapula** is a triangle of bone surrounded by the
muscles of the back but not connected directly to the spine (see diagram
6.1). This arrangement helps it to cushion the body when landing after a
leap and gives the forelimbs the flexibility to manipulate food or
strike at prey. Animals that use their forelimbs for grasping, burrowing
or climbing have a well-developed **clavicle** or collar bone. This
connects the shoulder girdle to the sternum. Animals like sheep, horses
and cows that use their forelimbs only for supporting the body and
locomotion have no clavicle. The **pelvic girdle** or hipbone attaches
the sacrum and the hind legs. It transmits the force of the leg-thrust
in walking or jumping directly to the spine (see diagram 6.10).

![](Anatomy_and_physiology_of_animals_Pelvic_girdle.jpg "Anatomy_and_physiology_of_animals_Pelvic_girdle.jpg")

Diagram 6.10 - The pelvic girdle

## Categories Of Bones

People who study skeletons place the different bones of the skeleton
into groups according to their shape or the way in which they develop.
Thus we have **long bones** like the femur, radius and finger bones,
**short bones** like the ones of the wrist and ankle, **irregular
bones** like the vertebrae and **flat bones** like the shoulder blade
and bones of the skull. Finally there are bones that develop in tissue
separated from the main skeleton. These include **sesamoid bones** which
include bones like the patella or kneecap that develop in tendons and
**visceral bones** that develop in the soft tissue of the penis of the
dog and the cow's heart.

Bird anatomy by Dr.Ankit Kumar Birman

## Bird Skeletons

Although the skeleton of birds is made up of the same bones as that of
mammals, many are highly adapted for flight. The most noticeable
difference is that the bones of the forelimbs are elongated to act as
wings. The large flight muscles make up as much as 1/5th of the body
weight and are attached to an extension of the sternum called the
**keel**. The vertebrae of the lower back are fused to provide the
rigidity needed to produce flying movements. There are also many
adaptations to reduce the weight of the skeleton. For instance birds
have a beak rather than teeth and many of the bones are hollow (see
diagram 6.11).

![](Anatomy_and_physiology_of_animals_Birds_skeleton.jpg "Anatomy_and_physiology_of_animals_Birds_skeleton.jpg")

Diagram 6.11 - A bird's skeleton

## The Structure Of Long Bones

A long bone consists of a central portion or **shaft** and two ends
called **epiphyses** (see diagram 6.12). Long bones move against or
articulate with other bones at joints and their ends have flattened
surfaces and rounded protuberances (condyles) to make this possible. If
you carefully examine a long bone you may also see raised or rough
surfaces. This is where the muscles that move the bones are attached.
You will also see holes (a hole is called a **foramen**) in the bone.
Blood vessels and nerves pass into the bone through these. You may also
be able to see a fine line at each end of the bone. This is called the
**growth plate** or **epiphyseal line** and marks the place where
increase in length of the bone occurred (see diagram 6.16).

![](Anatomy_and_physiology_of_animals_Femur.jpg "Anatomy_and_physiology_of_animals_Femur.jpg")

Diagram 6.12 - A femur

![](Anatomy_and_physiology_of_animals_l-s_section_long_bone.jpg "Anatomy_and_physiology_of_animals_l-s_section_long_bone.jpg")

6.13 - A longitudinal section through a long bone

If you cut a long bone lengthways you will see it consists of a hollow
cylinder (see diagram 6.13). The outer shell is covered by a tough
fibrous sheath to which the tendons are attached. Under this is a layer
of hard, dense **compact bone** (see below). This gives the bone its
strength. The central cavity contains fatty **yellow marrow**, an
important energy store for the body, and the ends are made from
honeycomb-like bony material called **spongy bone** (see box below).
Spongy bone contains **red marrow** where red blood cells are made.

## Compact Bone

Compact bone is not the lifeless material it may appear at first glance.
It is a living dynamic tissue with blood vessels, nerves and living
cells that continually rebuild and reshape the bone structure as a
result of the stresses, bends and breaks it experiences. Compact bone is
composed of microscopic hollow cylinders that run parallel to each other
along the length of the bone. Each of these cylinders is called a
**Haversian system**. Blood vessels and nerves run along the central
canal of each Haversian system. Each system consists of concentric rings
of bone material (the **matrix**) with minute spaces in it that hold the
bone cells. The hard matrix contains crystals of calcium phosphate,
calcium carbonate and magnesium salts with collagen fibres that make the
bone stronger and somewhat flexible. Tiny canals connect the cells with
each other and their blood supply (see diagram 6.14).

![](Anatomy_and_physiology_of_animals_Haversian_system_compact_bone.jpg "Anatomy_and_physiology_of_animals_Haversian_system_compact_bone.jpg")

Diagram 6.14 - Haversian systems of compact bone

## Spongy Bone

Spongy bone gives bones lightness with strength. It consists of an
irregular lattice that looks just like an old fashioned loofah sponge
(see diagram 6.15). It is found on the ends of long bones and makes up
most of the bone tissue of the limb girdles, ribs, sternum, vertebrae
and skull. The spaces contain red marrow, which is where red blood cells
are made and stored.

![](Anatomy_and_physiology_of_animals_Spongy_bone.jpg "Anatomy_and_physiology_of_animals_Spongy_bone.jpg")

Diagram 6.15 - Spongy bone

## Bone Growth

The skeleton starts off in the foetus as either cartilage or fibrous
connective tissue. Before birth and, sometimes for years after it, the
cartilage is gradually replaced by bone. The long bones increase in
length at the ends at an area known as the **epiphyseal plate** where
new cartilage is laid down and then gradually converted to bone. When an
animal is mature, bone growth ceases and the epiphyseal plate converts
into a fine **epiphyseal line** (see diagram 6.16).

![](Anatomy_and_physiology_of_animals_Growing_bone.jpg "Anatomy_and_physiology_of_animals_Growing_bone.jpg")

Diagram 6.16 - A growing bone

## Broken Bones

A fracture or break dramatically demonstrates the dynamic nature of
bone. Soon after the break occurs blood pours into the site and
cartilage is deposited. This starts to connect the broken ends together.
Later spongy bone replaces the cartilage, which is itself replaced by
compact bone. Partial healing to the point where some weight can be put
on the bone can take place in 6 weeks but complete healing may take 3--4
months.

## Joints

Joints are the structures in the skeleton where 2 or more bones meet.
There are several different types of joints. Some are **immovable** once
the animal has reached maturity. Examples of these are those between the
bones of the skull and the midline joint of the pelvic girdle. Some are
**slightly moveable** like the joints between the vertebrae but most
joints allow free movement and have a typical structure with a fluid
filled cavity separating the articulating surfaces (surfaces that move
against each other) of the two bones. This kind of joint is called a
**synovial joint** (see diagram 6.17). The joint is held together by
bundles of white fibrous tissue called **ligaments** and a fibrous
**capsule** encloses the joint. The inner layers of this capsule secrete
the **synovial fluid** that acts as a lubricant. The articulating
surfaces of the bones are covered with **cartilage** that also reduces
friction and some joints, e.g. the knee, have a pad of cartilage between
the surfaces that articulate with each other.

The shape of the articulating bones in a joint and the arrangement of
ligaments determine the kind of movement made by the joint. Some joints
only allow a to and from '*gliding movement**e.g. between the ankle and
wrist bones; the joints at the elbow, knee and fingers are**hinge
joints**and allow movement in two dimensions and the axis
vertebra**pivots**on the atlas vertebra.**Ball and socket joints*\',
like those at the shoulder and hip, allow the greatest range of
movement.

![](Anatomy_and_physiology_of_animals_Synovial_joint.jpg "Anatomy_and_physiology_of_animals_Synovial_joint.jpg")

Diagram 6.17 - A synovial joint

## Common Names Of Joints

Some joints in animals are given common names that tend to be confusing.
For example:

:# The joint between the femur and the tibia on the hind leg is our knee
but the **stifle** in animals.

:# Our ankle joint (between the tarsals and metatarsals) is the **hock**
in animals

:# Our knuckle joint (between the metacarpals or metatarsals and the
phalanges) is the **fetlock** in the horse.

:# The **"knee"** on the horse is equivalent to our wrist (ie on the
front limb between the radius and metacarpals) see diagrams 6.6, 6.7,
6.8, 6.17 and 6.18.

![](Anatomy_and_physiology_of_animals_Common_horse_joints.jpg "Anatomy_and_physiology_of_animals_Common_horse_joints.jpg")

Diagram 6.18 - The names of common joints of a horse

![](Anatomy_and_physiology_of_animals_Common_dog_joints.jpg "Anatomy_and_physiology_of_animals_Common_dog_joints.jpg")

Diagarm 6.19 - The names of common joints of a dog

## Locomotion

Different animals place different parts of the foot or forelimb on the
ground when walking or running.

Humans and bears put the whole surface of the foot on the ground when
they walk. This is known as **plantigrade locomotion**. Dogs and cats
walk on their toes (**digitigrade locomotion**) while horses and pigs
walk on their "toenails" or hoofs. This is called **unguligrade
locomotion** (see diagram 6.20).

:# **Plantigrade locomotion** (on the "palms of the hand) as in humans
and bears

:# **Digitigrade locomotion** (on the "fingers") as in cats and dogs

:# **Unguligrade locomotion** (on the "fingernails") as in horses

![](Anatomy_and_physiology_of_animals_Locomotion.jpg "Anatomy_and_physiology_of_animals_Locomotion.jpg")

Diagram 6.20 - Locomotion

## Summary

-   The skeleton maintains the shape of the body, protects internal
    organs and makes locomotion possible.
-   The **vertebrae** support the body and protect the spinal cord. They
    consist of: **cervical vertebrae** in the neck, **thoracic
    vertebrae** in the chest region which articulate with the ribs,
    **lumbar vertebrae** in the loin region, **sacral vertebrae** fused
    to the pelvis to form the sacrum and **tail** or **coccygeal
    vertebrae**.
-   The **skull** protects the brain and sense organs. The **cranium**
    forms a solid box enclosing the brain. The **mandible** forms the
    jaw.
-   The forelimb consists of the **humerus, radius, ulna, carpals,
    metacarpals** and **phalanges**. It moves against or **articulates**
    with the **scapula** at the shoulder joint.
-   The hindlimb consists of the **femur, patella, tibia, fibula,
    tarsals, metatarsals** and **digits**. It moves against or
    articulates with the **pelvis** at the hip joint.
-   Bones articulate against each other at **joints**.
-   **Compact bone** in the shaft of long bones gives them their
    strength. **Spongy bone** at the ends reduces weight. Bone growth
    occurs at the **growth plate**.

## Worksheet

Use the Skeleton
Worksheet to learn the
main parts of the skeleton.

## Test Yourself

1\. Name the bones which move against (articulate with)\...

:   a\) the humerus
:   b\) the thoracic vertebrae
:   c\) the pelvis

2\. Name the bones in the forelimb

3\. Where is the patella found?

4\. Where are the following joints located?

:   a\) The stifle joint:
:   b\) The hock joint
:   c\) The hip joint:

5\. Attach the following labels to the diagram of the long bone shown
below.

:   a\) compact bone
:   b\) spongy bone
:   c\) growth plate
:   d\) fibrous sheath
:   e\) red marrow
:   f\) blood vessel

![](Section_through_long_bone.JPG "Section_through_long_bone.JPG")

6\. Attach the following labels to the diagram of a joint shown below

:   a\) bone
:   b\) articular cartilage
:   c\) joint cavity
:   d\) capsule
:   e\) ligament
:   f\) synovial fluid.

![](Joint_no_labels.JPG "Joint_no_labels.JPG")

/Test Yourself Answers/

## Websites

-   <http://www.infovisual.info/02/056_en.html> Bird skeleton

A good diagram of the bird skeleton.

-   <http://www.earthlife.net/mammals/skeleton.html> Earth life

A great introduction to the mammalian skeleton. A little above the level
required but it has so much interesting information it\'s worth reading
it.

-   <http://www.klbschool.org.uk/interactive/science/skeleton.htm> The
    human skeleton

Test yourself on the names of the bones of the (human) skeleton.

-   <http://www.shockfamily.net/skeleton/JOINTS.HTML> The joints

Quite a good article on the different kinds of joints with diagrams.

-   <http://en.wikipedia.org/wiki/Bone> Wikipedia

Wikipedia is disappointing where the skeleton is concerned. Most
articles stick entirely to the human skeleton or have far too much
detail. However this one on compact and spongy bone and the growth of
bone is quite good although still much above the level required.

## Glossary

-   Link to
    Glossary

