# 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


# Anatomy and Physiology of Animals/Muscles

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

## Objectives

After completing this section, you should know:

-   The structure of smooth, cardiac and skeletal muscle and where they
    are found.
-   What the insertion and origin of a muscle is.
-   What flexion and extension of a muscle means.
-   That muscles usually operate as antagonistic pairs.
-   What tendons attach muscles to bones.

## Muscles

Muscles make up the bulk of an animal's body and account for about half
its weight. The meat on the chop or roast is muscle and is composed
mainly of protein. The cells that make up muscle tissue are elongated
and able to contract to a half or even a third of their length when at
rest. There are three different kinds of muscle based on appearance and
function: smooth, cardiac and skeletal muscle.

### Types of Muscle

-   Smooth muscle

Smooth or Involuntary muscle carries out the unconscious routine tasks
of the body such as moving food down the digestive system, keeping the
eyes in focus and adjusting the diameter of blood vessels. The
individual cells are spindle-shaped, being fatter in the middle and
tapering off towards the ends with a nucleus in the centre of the cell.
They are usually found in sheets and are stimulated by the non-conscious
or autonomic nervous system as well as by hormones (see Chapter 3).

-   Cardiac muscle

Cardiac muscle is only found in the wall of the heart. It is composed of
branching fibres that form a three-dimensional network. When examined
under the microscope, a central nucleus and faint stripes or striations
can be seen in the cells. Cardiac muscle cells contract spontaneously
and rhythmically without outside stimulation, but the sinoatrial node
(natural pacemaker) coordinates the heart beat. Nerves and hormones
modify this rhythm (see Chapter 3).

-   Skeletal muscle

Skeletal muscle is the muscle that is attached to and moves the
skeleton, and is under voluntary control. It is composed of elongated
cells or fibres lying parallel to each other. Each cell is unusual in
that it has several nuclei and when examined under the microscope
appears striped or striated. This appearance gives the muscle its names
of striped or striated muscle. Each cell of striated muscle contains
hundreds, or even thousands, of microscopic fibres each one with its own
striped appearance. The stripes are formed by two different sorts of
protein that slide over each other making the cell contract (see diagram
7.1).

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

Diagram 7.1 - A striped muscle cell

### Muscle contraction

Muscle contraction requires energy and muscle cells have numerous
mitochondria. However, only about 15% of the energy released by the
mitochondria is used to fuel muscle contraction. The rest is released as
heat. This is why exercise increases body temperature and makes animals
sweat or pant to rid themselves of this heat as part of
thermoregulation.

What we refer to as a muscle is made up of groups of muscle fibers
surrounded by connective tissue. The connective tissue sheaths join
together at the ends of the muscle to form tough white bands of fiber
called **tendons**. These attach the muscles to the bones. Tendons are
similar in structure to the **ligaments** that attach bones together
across a joint (see diagrams 7.2a and b).

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

Diagram 7.2 a and b - The structure of a muscle

Remember:

**Tendons Tie** muscles to bones

:

    :   and

**Ligaments Link** bones at joints

### Structure of a muscle

A single muscle is fat in the middle and tapers towards the ends. The
middle part, which gets fatter when the muscle contracts, is called the
**belly** of the muscle. If you contract your biceps muscle in your
upper arm you may feel it getting fatter in the middle. You may also
notice that the biceps is attached at its top end to bones in your
shoulder while at the bottom it is attached to bones in your lower arm.
Notice that the bones at only one end move when you contract the biceps.
This end of the muscle is called the **insertion**. The other end of the
muscle, the **origin**, is attached to the bone that moves the least
(see diagram 7.3).

![](Anatomy_and_physiology_of_animals_Antagonistic_muscles,_flexion&tension.jpg "Anatomy_and_physiology_of_animals_Antagonistic_muscles,_flexion&tension.jpg")

Diagram 7.3 - Antagonistic muscles, flexion and extension

### Antagonistic muscles

Skeletal muscles usually work in pairs. When one contracts the other
relaxes and vice versa. Pairs of muscles that work like this are called
**antagonistic muscles**. For example the muscles in the upper forearm
are the biceps and triceps (see diagram 7.3). Together they bend the
elbow. When the biceps contracts (and the triceps relaxes) the lower
forearm is raised and the angle of the joint is reduced. This kind of
movement is called **flexion**. When the triceps is contracted (and the
biceps relaxes), the angle of the elbow increases. The term for this
movement is **extension**.

When you or animals contract skeletal muscle it is a voluntary action.
For example, you make a conscious decision to walk across the room,
raise the spoon to your mouth, or smile. There is however, another way
in which contraction of muscles attached to the skeleton happens that is
not under voluntary control. This is during a **reflex action**, such as
jerking your hand away from the hot stove you have touched by accident.
This is called a **reflex arc** and will be described in detail in
chapter 14-15.

## Summary

-   There are three different kinds of muscle tissue: **smooth muscle**
    in the walls of the gut and blood

:   vessels; **cardiac muscle** in the heart and **skeletal muscle**
    attached to the skeleton.

-   **Tendons** attach skeletal muscles to the skeleton.
-   **Ligaments** link bones together at a joint.
-   Skeletal muscles work in pairs known as **antagonistic pairs.** As
    one contracts the other in the

:   pair relaxes.

-   **Flexion** is the movement that reduces the angle of a joint.
    **Extension** increases the angle

:   of a joint.

## Test Yourself

1\. What kind of muscle tissue:

:   a\) moves bones
:   b\) makes the heart pump blood:
:   c\) pushes food along the intestine:
:   d\) makes your mouth form a smile:
:   e\) makes the hair stand up when cold:
:   f\) makes the diaphragm contract for breathing in:

2\. What structure connects a muscle to a bone?

3\. What is the insertion of a muscle?

4\. Which muscle is antagonistic to the biceps?

5\. Name 3 other antagonistic pairs and tell what they do.

6\. When you bend your knee what movement are you making?

7\. When you straighten your ankle joint what movement happens?

8\. What organelles provide the energy that muscles need?

9\. State the difference between a tendon and a ligament.

10.In the section \"Skeletal Muscle\" there are 2 proteins mentioned.
Name these proteins, state their size difference, and tell what they
actually do to help produce movement.

## Website

-   <http://health.howstuffworks.com/muscle.htm> How muscles work

Description of the three types of muscles and how skeletal muscles work.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/Respiratory System

!original image by Zofia
P cc
by{width="400"}

## Objectives

After completing this section, you should know:

-   why animals need energy and how they make it in cells
-   why animals require oxygen and need to get rid of carbon dioxide
-   what the term gas exchange means
-   the structure of alveoli and how oxygen and carbon dioxide pass
    across their walls
-   how oxygen and carbon dioxide are carried in the blood
-   the route air takes in the respiratory system (i.e. the nose,
    pharynx, larynx, trachea, bronchus,

:   bronchioles, alveoli)

-   the movements of the ribs and diaphragm to bring about inspiration
-   what tidal volume, minute volume and vital capacity are
-   how the rate of breathing is controlled and how this helps regulate
    the acid-base balance of the blood

## Overview

!**Diagram 9.1**: Alveoli with blood
supply
Animals require a supply of energy to survive. This energy is needed to
build large molecules like proteins and glycogen, make the structures in
cells, move chemicals through membranes and around cells, contract
muscles, transmit nerve impulses and keep the body warm. Animals get
their energy from the large molecules that they eat as food. Glucose is
often the energy source but it may also come from other carbohydrates,
as well as fats and protein. The energy is made by the biochemical
process known as **cellular respiration** that takes place in the
**mitochondria** inside every living cell.

The overall reaction can be summarised by the word equation given below.

**Carbohydrate Food (glucose) + Oxygen = Carbon Dioxide + Water +
energy**

As you can see from this equation, the cells need to be supplied with
**oxygen** and **glucose** and the waste product, **carbon dioxide**,
which is poisonous to cells, needs to be removed. The way the digestive
system provides the glucose for cellular respiration will be described
in Chapter 11 (\"The Gut and Digestion\"), but here we are only
concerned with the two gases, oxygen and carbon dioxide, that are
involved in cellular respiration. These gases are carried in the blood
to and from the tissues where they are required or produced.

Oxygen enters the body from the air (or water in fish)and carbon dioxide
is usually eliminated from the same part of the body. This process is
called **gas exchange**. In fish gas exchange occurs in the gills, in
land dwelling vertebrates lungs are the gas exchange organs and frogs
use gills when they are tadpoles and lungs, the mouth and the skin when
adults.

Mammals (and birds) are active and have relatively high body
temperatures so they require large amounts of oxygen to provide
sufficient energy through cellular respiration. In order to take in
enough oxygen and release all the carbon dioxide produced they need a
very large surface area over which gas exchange can take place. The many
minute air sacs or **alveoli** of the lungs provide this. When you look
at these under the microscope they appear rather like bunches of grapes
covered with a dense network of fine capillaries (see diagram 9.1). A
thin layer of water covers the inner surface of each alveolus. There is
only a very small distance -just 2 layers of thin cells - between the
air in the alveoli and the blood in the capillaries. The gases pass
across this gap by **diffusion**.

## Diffusion And Transport Of Oxygen

!**Diagram 9.2**: Cross section of an
alveolus
The air in the alveoli is rich in oxygen while the blood in the
capillaries around the alveoli is deoxygenated. This is because the
haemoglobin in the red blood cells has released all the oxygen it has
been carrying to the cells of the body. Oxygen diffuses from high
concentration to low concentration. It therefore crosses the narrow
barrier between the alveoli and the capillaries to enter the blood and
combine with the haemoglobin in the red blood cells to form
**oxyhaemoglobin**.

The narrow diameter of the capillaries around the alveoli means that the
blood flow is slowed down and that the red cells are squeezed against
the capillary walls. Both of these factors help the oxygen diffuse into
the blood (see diagram 9.2).

When the blood reaches the capillaries of the tissues the oxygen splits
from the haemoglobin molecule. It then diffuses into the tissue fluid
and then into the cells.

## Diffusion And Transport Of Carbon Dioxide

Blood entering the lung capillaries is full of carbon dioxide that it
has collected from the tissues. Most of the carbon dioxide is dissolved
in the plasma either in the form of **sodium bicarbonate** or **carbonic
acid**. A little is transported by the red blood cells. As the blood
enters the lungs the carbon dioxide gas diffuses through the capillary
and alveoli walls into the water film and then into the alveoli. Finally
it is removed from the lungs during breathing out (see diagram 9.2).
(See chapter 8 for more information about how oxygen and carbon dioxide
are carried in the blood).

## The Air Passages

When air is breathed in it passes from the nose to the alveoli of the
lungs down a series of tubes (see diagram 9.3). After entering the nose
the air passes through the **nasal cavity**, which is lined with a moist
membrane that adds warmth and moisture to the air as it passes. The air
then flows through the **pharynx** or throat, a passage that carries
both food and air, to the **larynx** where the voice-box is located.
Here the passages for food and air separate again. Food must pass into
the oesophagus and the air into the windpipe or **trachea**. To prevent
food entering this, a small flap of tissue called the **epiglottis**
closes the opening during swallowing (see chapter 11). A reflex that
inhibits breathing during swallowing also (usually) prevents choking on
food.

The trachea is the tube that ducts the air down the throat. Incomplete
rings of cartilage in its walls help keep it open even when the neck is
bent and head turned. The fact that acrobats and people that tie
themselves in knots doing yoga still keep breathing during the most
contorted manoeuvres shows how effective this arrangement is. The air
passage now divides into the two **bronchi** that take the air to the
right and left lungs before dividing into smaller and smaller
**bronchioles** that spread throughout the lungs to carry air to the
alveoli. Smooth muscles in the walls of the bronchi and bronchioles
adjust the diameter of the air passages.

The tissue lining the respiratory passages produces**mucus** and is
covered with miniature hairs or **cilia.** Any dust that is breathed
into the respiratory system immediately gets entangled in the mucous and
the cilia move it towards the mouth or nose where it can be coughed up
or blown out.

## The Lungs And The Pleural Cavities

!**Diagram 9.3**: The respiratory
system
The lungs fill most of the chest or **thoracic cavity**, which is
completely separated from the abdominal cavity by the **diaphragm**. The
lungs and the spaces in which they lie (called the **pleural cavities)**
are covered with membranes called the **pleura**. There is a thin film
of fluid between the two membranes. This lubricates them as they move
over each other during breathing movements.

### Collapsed Lungs

The pleural cavities are completely airtight with no connection with the
outside and if they are punctured by accident (a broken rib will often
do this), air rushes in and the lung collapses. Separating the two lungs
is a region of tissue that contains the oesophagus, trachea, aorta, vena
cava and lymph nodes. This is called the **mediastinum**. In humans and
sheep it separates the cavity completely so that puncturing one pleural
cavity leads to the collapse of only one lung. In dogs, however, this
separation is incomplete so a puncture results in a complete collapse of
both lungs.

## Breathing

!**Diagram 9.4a**: Inspiration; **Diagram 9.4b**:
Expiration
The process of breathing moves air in and out of the lungs. Sometimes
this process is called **respiration** but it is important not to
confuse it with the chemical process, **cellular respiration**, that
takes place in the mitochondria of cells. Breathing is brought about by
the movement of the diaphragm and the ribs.

### Inspiration

The diaphragm is a thin sheet of muscle that completely separates the
abdominal and thoracic cavities. When at rest it domes up into the
thoracic cavity but during breathing in or **inspiration** it flattens.
At the same time special muscles in the chest wall (external intercostal
muscles) move the ribs forwards and outwards. These movements of both
the diaphragm and the ribs cause the volume of the thorax to increase.
Because the pleural cavities are airtight, the lungs expand to fill this
increased space and air is drawn down the trachea into the lungs (see
diagram 9.4a).

### Expiration

**Expiration** or breathing out consists of the opposite movements. The
ribs move down and in and the diaphragm resumes its domed shape so the
air is expelled (see diagram 9.4b). Expiration is usually passive and no
energy is required (unless you are blowing up a balloon).

### Lung Volumes

!**Diagram 9.5**: Lung
volumes
As you sit here reading this just pay attention to your breathing.
Notice that your in and out breaths are really quite small and gentle
(unless you have just rushed here from somewhere else!). Only a small
amount of the total volume that your lungs hold is breathed in and out
with each breath. This kind of gentle "at rest" breathing is called
**tidal breathing** and the volume breathed in or out (they should be
the same) is the **tidal volume** (see diagram 9.5). Sometimes people
want to measure the volume of air inspired or expired during a minute of
this normal breathing. This is called the **minute volume**. It could be
estimated by measuring the volume of one tidal breath and then
multiplying that by the number of breaths in a minute. Of course it is
possible to take a deep breath and breathe in as far as you can and then
expire as far as possible. The volume of the air expired when a maximum
expiration follows a maximum inspiration is called the **vital
capacity** (see diagram 9.5).

### Composition Of Air

The air animals breathe in consists of 21% oxygen and 0.04% carbon
dioxide. Expelled air consists of 16% oxygen and 4.4% carbon dioxide.
This means that the lungs remove only a quarter of the oxygen contained
in the air. This is why it is possible to give someone (or an animal)
artificial respiration by blowing expired air into their mouth.

Breathing is usually an unconscious activity that takes place whether
you are awake or asleep, although, humans at least, can also control it
consciously. Two regions in the hindbrain called the **medulla
oblongata** and **pons** control the rate of breathing. These are called
**respiratory centres**. They respond to the concentration of carbon
dioxide in the blood. When this concentration rises during a bout of
activity, for example, nerve impulses are automatically sent to the
diaphragm and rib muscles that increase the rate and the depth of
breathing. Increasing the rate of breathing also increases the amount of
oxygen in the blood to meet the needs of this increased activity.

### The Acidity Of The Blood And Breathing

The degree of acidity of the blood (the **acid-base balance)** is
critical for normal functioning of cells and the body as a whole. For
example, blood that is too acidic or alkaline can seriously affect nerve
function causing a coma, muscle spasms, convulsions and even death.
Carbon dioxide carried in the blood makes the blood acidic and the
higher the concentration of carbon dioxide the more acidic it is. This
is obviously dangerous so there are various mechanisms in the body that
bring the acid-base balance back within the normal range. Breathing is
one of these homeostatic mechanisms. By increasing the rate of breathing
the animal increases the amount of dissolved carbon dioxide that is
expelled from the blood. This reduces the acidity of the blood.

### Breathing In Birds

Birds have a unique respiratory system that enables them to respire at
the very high rates necessary for flight. The lungs are relatively solid
structures that do not change shape and size in the same way as
mammalian lungs do. Tubes run through them and connect with a series of
air sacs situated in the thoracic and abdominal body cavities and some
of the bones. Movements of the ribs and breastbone or sternum expand and
compress these air sacs so they act rather like bellows and pump air
through the lungs. The evolution of this extremely efficient system of
breathing has enabled birds to migrate vast distances and fly at
altitudes higher than the summit of Everest.

## Summary

-   Animals need to breathe to supply the cells with **oxygen** and
    remove the waste product **carbon dioxide**.
-   The lungs are situated in the **pleural cavities** of the
    **thorax**.
-   **Gas exchange** occurs in the **alveoli** of the lungs that provide
    a large surface area. Here oxygen diffuses from the alveoli into the
    red blood cells in the capillaries that surround the alveoli. Carbon
    dioxide, at high concentration in the blood, diffuses into the
    alveoli to be breathed out.
-   **Inspiration** occurs when muscle contraction causes the ribs to
    move up and out and the diaphragm to flatten. These movements
    increase the volume of the pleural cavity and draw air down the
    respiratory system into the lungs.
-   The air enters the nasal cavity and passes to the **pharynx** and
    **larynx** where the **epiglottis** closes the opening to the lungs
    during swallowing. the air passes down the trachea kept open by
    rings of cartilage to the **bronchi** and **bronchioles** and then
    to the alveoli.
-   **Expiration** is a passive process requiring no energy as it relies
    on the relaxation of the muscles and recoil of the elastic tissue of
    the lungs.
-   The rate of breathing is determined by the concentration of carbon
    dioxide in the blood. As carbon dioxide makes blood acidic, the rate
    of breathing helps control the **acid/base balance** of the blood.
-   The cells lining the respiratory passages produce mucus which traps
    dust particles, which are wafted into the nose by cilia.

## Worksheet

Work through the Respiratory System
Worksheet to
learn the main structures of the respiratory system and how they
contribute to inspiration and gas exchange.

## Test Yourself

Then use the Test Yourself below to see how much you remember and
understand.

1\. What is meant by the phrase "gas exchange"?

2\. Where does gas exchange take place?

3\. What is the process by which oxygen moves from the alveoli into the
blood?

4\. Why does this process occur?

5\. How does the structure of the alveoli make gas exchange efficient?

6\. How is oxygen carried in the blood?

7\. List the structures that air passes on its way from the nose to the
alveoli:

8\. What is the function of the mucus and cilia lining the respiratory
passages?

9\. How do movements of the ribs and diaphragm bring about inspiration?
Circle the correct statement below.

:   a\) The diaphragm domes up into the thorax and ribs move in and down
:   b\) The diaphragm flattens and ribs move up and out
:   c\) The diaphragm domes up into the thorax and the ribs move up and
    out.
:   d\) The diaphragm flattens and the ribs move in and down

10\. What is the function of the epiglottis?

11\. What controls the rate of breathing?

/Test Yourself Answers/

## Websites

-   <http://www.biotopics.co.uk/humans/resyst.html> Bio topics

A good interactive explanation of breathing and gas exchange in humans
with diagrams to label, animations to watch and questions to answer.

-   <http://www.schoolscience.co.uk/content/4/biology/abpi/asthma/asth3.html>
    School Science

Although this is of the human respiratory system there is a good diagram
that gives the functions of the various parts as you move your mouse
over it. Also an animation of gas exchange and a quiz to test your
understanding of it.

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

Wikipedia on the lungs. Lots of good information on the human
respiratory system with all sorts of links if you are interested.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/Lymphatic System

!original image by Toms
Bauģis cc
by{width="400"}

## Objectives

After completing this section, you should know:

-   the function of the lymphatic system
-   what the terms tissue fluid, lymph, lymphocyte and lymphatic mean
-   how lymph is formed and what is in it
-   the basic structure and function of a lymph node and the position of
    some important lymph nodes in the body
-   the route by which lymph circulates in the body and is returned to
    the blood system
-   the location and function of the spleen, thymus and lacteals

## Lymphatic System

When **tissue fluid** enters the small blind-ended **lymphatic
capillaries** that form a network between the cells it becomes
**lymph**. Lymph is a clear watery fluid that is very similar to blood
plasma except that it contains large numbers of white blood cells,
mostly **lymphocytes**. It also contains protein, cellular debris,
foreign particles and bacteria. Lymph that comes from the intestines
also contains many fat globules following the absorption of fat from the
digested food into the lymphatics (**lacteals**) of the villi (see
chapter 11 for more on these). From the lymph capillaries the lymph
flows into larger tubes called **lymphatic vessels.** These carry the
lymph back to join the blood circulation (see diagrams 10.1 and 10.2).

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

Diagram 10.1 - A capillary bed with lymphatic capillaries

### Lymphatic vessels

Lymphatic vessels have several similarities to veins. Both are thin
walled and return fluid to the right hand side of the heart. The
movement of the fluid in both is brought about by the contraction of the
muscles that surround them and both have valves to prevent backflow. One
important difference is that lymph passes through at least one **lymph
node** or gland before it reaches the blood system (see diagram 10.2).
These filter out used cell parts, cancer cells and bacteria and help
defend the body from infection.

Lymph nodes are of various sizes and shapes and found throughout the
body and the more important ones are shown in diagram 10.3. They consist
of lymph tissue surrounded by a fibrous sheath. Lymph flows into them
through a number of incoming vessels. It then trickles through small
channels where white cells called **macrophages** (derived from
**monocytes**) remove the bacteria and debris by engulfing and digesting
them (see diagram 10.4). The lymph then leaves the lymph nodes through
outgoing vessels to continue its journey towards the heart where it
rejoins the blood circulation (see diagrams 10.2 and 10.3).

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

Diagram 10.2 - The lymphatic system

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

Diagram 10.3 - The circulation of lymph with major lymph nodes

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

Diagram 10.4 - A lymph node

As well as filtering the lymph, lymph nodes produce the white cells
known as **lymphocytes**. Lymphocytes are also produced by the
**thymus**, **spleen** and **bone marrow**. There are two kinds of
lymphocyte. The first attack invading micro organisms directly while
others produce **antibodies** that circulate in the blood and attack
them.

The function of the lymphatic system can therefore be summarized as
transport and defense. It is important for returning the fluid and
proteins that have escaped from the blood capillaries to the blood
system and is also responsible for picking up the products of fat
digestion in the small intestine. Its other essential function is as
part of the immune system, defending the body against infection.

### Problems with lymph nodes and the lymphatic system

During infection of the body the lymph nodes often become swollen and
tender because of their increased activity. This is what causes the
swollen 'glands' in your neck during throat infections, mumps and
tonsillitis. Sometimes the bacteria multiply in the lymph node and cause
inflammation. Cancer cells may also be carried to the lymph nodes and
then transported to other parts of the body where they may multiply to
form a secondary growth or **metastasis**. The lymphatic system may
therefore contribute to the spread of cancer. Inactivity of the muscles
surrounding the lymphatic vessels or blockage of these vessels causes
tissue fluid to 'back up' in the tissues resulting in swelling or
**oedema**.

## Other Organs Of The Lymphatic System

The **spleen** is an important part of the lymphatic system. It is a
deep red organ situated in the abdomen caudal to the stomach (see
diagram 10.3). It is composed of two different types of tissue. The
first type makes and stores lymphocytes, the cells of the immune system.
The second type of tissue destroys worn out red blood cells, breaking
down the haemoglobin into iron, which is recycled, and waste products
that are excreted. The spleen also stores red blood cells. When severe
blood loss occurs, it contracts and releases them into the circulation.

The **thymus** is a large pink organ lying just under the sternum
(breastbone) just cranial to the heart (see diagram 10.1). It has an
important function processing lymphocytes so they are capable of
recognising and attacking foreign invaders like bacteria.

Other lymph organs are the **bone marrow** of the long bones where
lymphocytes are produced and **lymph nodules**, which are like tiny
lymph nodes. Large clusters of these are found in the wall of the small
intestine (called Peyer's Patches) and in the tonsils.

## Summary

-   Fluid leaks out of the thin walled capillaries as they pass through
    the tissues. This is called **tissue fluid**.
-   Much of tissue fluid passes back into the capillaries. Some enters
    the blind-ended lymphatic capillaries that form a network between
    the cells of the tissues. This fluid is called **lymph**.
-   Lymph flows from the **lymphatic capillaries** to **lymph vessels**,
    passing through **lymph nodes** and along the thoracic duct to join
    the blood system.
-   Lymph nodes filter the lymph and produce **lymphocytes**.
-   Other organs of the lymphatic system are the **spleen, thymus, bone
    marrow**, and **lymph nodules**.

## Worksheets

Lymphatic System
Worksheet

## Test Yourself

1\. What is the difference between tissue fluid and lymph?

2\. By what route does lymph make its way back to join the blood of the
circulatory system?

3\. As the lymphatic system has no heart to push the lymph along what
makes it flow?

4\. What happens to the lymph as it passes through a lymph node?

5\. Where is the spleen located in the body?

6\. Where is the thymus located in the body?

7\. What is the function of lymphocytes?

/Test Yourself Answers/

## Websites

-   <http://www.cancerhelp.org.uk/help/default.asp?page=117> Cancerhelp

A nice clear explanation here with great diagrams of the (human)
lymphatic system.

-   <http://www.jdaross.cwc.net/lymphatics2.htm> Lymphatic system

Introduction to the Lymphatic System. A good description of lymph
circulation with an animation.

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

Good information here on the (human) lymphatic system, lymph circulation
and lymphoid organs.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/The Gut and Digestion

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

## Objectives

After completing this section, you should know:

-   what is meant by the terms: ingestion, digestion, absorption,
    assimilation, egestion, peristalsis and chyme
-   the characteristics, advantages and disadvantages of a herbivorous,
    carnivorous and omnivorous diet
-   the 4 main functions of the gut
-   the parts of the gut in the order in which the food passes down

## The Gut And Digestion

Plant cells are made of organic molecules using energy from the sun.
This process is called **photosynthesis**. Animals rely on these
ready-made organic molecules to supply them with their food. Some
animals (herbivores) eat plants; some (carnivores) eat the herbivores.

## Herbivores

Herbivores\'\' eat plant material. While no animal produces the
digestive enzymes to break down the large **cellulose** molecules in the
plant cell walls, micro-organisms\' like bacteria, on the other hand,
can break them down. Therefore herbivores employ micro-organisms to do
the job for them.

There are two types of herbivore:

:   The first, **ruminants** like cattle, sheep and goats, house these
    bacteria in a special compartment in the enlarged stomach called the
    **rumen**.

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```

:   The second group has an enlarged large intestine and caecum, called
    a **functional caecum**, occupied by cellulose digesting
    micro-organisms. These non-ruminant herbivores include the horse,
    rabbit and rat.

Plants are a primary pure and good source of nutrients, however they
aren\'t digested very easily and therefore herbivores have to eat large
quantities of food to obtain all they require. Herbivores like cows,
horses and rabbits typically spend much of their day feeding. To give
the micro-organisms access to the cellulose molecules, the plant cell
walls need to be broken down. This is why herbivores have teeth that are
adapted to crush and grind. Their guts also tend to be lengthy and the
food takes a long time to pass through it.

Eating plants have other advantages. Plants are immobile so herbivores
normally have to spend little energy collecting them. This contrasts
with another main group of animals - the carnivores that often have to
chase their prey.

## Carnivores

**Carnivorous animals** like those in the cat and dog families, polar
bears, seals, crocodiles and birds of prey catch and eat other animals.
They often have to use large amounts of energy finding, stalking,
catching and killing their prey. However, they are rewarded by the fact
that meat provides a very concentrated source of nutrients. Carnivores
in the wild therefore tend to eat distinct meals often with long and
irregular intervals between them. Time after feeding is spent digesting
and absorbing the food.

The guts of carnivores are usually shorter and less complex than those
of herbivores because meat is easier to digest than plant material.
Carnivores usually have teeth that are specialised for dealing with
flesh, gristle and bone. They have sleek bodies, strong, sharp claws and
keen senses of smell, hearing and sight. They are also often cunning,
alert and have an aggressive nature.

## Omnivores

Many animals feed on both animal and vegetable material -- they are
**omnivorous.** There are currently two similar definitions of
omnivorism:

1\. Having the ability to derive energy from plant and animal material.

2\. Having characteristics which are optimized for acquiring and eating
both plants and animals.

Some animals fit both definitions of omnivorism, including bears,
raccoons, dogs, and hedgehogs. Their food is diverse, ranging from plant
material to animals they have either killed themselves or scavenged from
other carnivores. They are well equipped to hunt and tear flesh (claws,
sharp teeth, and a strong, non-rotational jaw hinge), but they also have
slightly longer intestines than carnivores, which has been found to
facilitate plant digestion. The examples also retain an ability to taste
amino acids, making unseasoned flesh palatable to most members of the
species.

Classically, humans and chimpanzees are classified as omnivores.
However, further research has shown chimpanzees typically consume 95%
plant matter (the remaining mass is largely termites), and their teeth,
jaw hinge, stomach pH, and intestinal length closely matches herbivores,
which many suggest classified them as herbivores. Humans, conversely,
have chosen to eat meat for much of the archaeological record, although
their teeth, jaw hinge, and stomach pH, and intestinal lengths also
closely match other herbivores.

The dispute of human/ chimps classifications is caused by two things.
First, there is research that both plant-only and some-animal diets
promote health (longevity and freedom from disease) in humans. Second,
well-off humans have often chosen to eat meat and dairy products
throughout written history, which some argue shows that we prefer meat
and dairy by latent instinct.

Per the classical definition, omnivores lack the specialized teeth and
guts of carnivores and herbivores but are often highly intelligent and
adaptable reflecting their varied diet.

## Treatment Of Food

Whether an animal eats plants or flesh, the **carbohydrates**, **fats**
and **proteins** in the food it eats are generally giant molecules (see
chapter 1). These need to be split up into smaller ones before they can
pass into the blood and enter the cells to be used for energy or to make
new cell constituents.

For example:

:   **Carbohydrates** like cellulose, starch, and glycogen need to be
    split into **glucose** and other **monosaccharides**;
:   **Proteins** need to be split into **amino acids**;
:   **Fats** or **lipids** need to be split into **fatty acids** and
    **glycerol**.

## The Gut

The **digestive tract, alimentary canal** or **gut** is a hollow tube
stretching from the mouth to the anus. It is the organ system concerned
with the treatment of foods.

At the mouth the large food molecules are taken into the gut - this is
called **ingestion**. They must then be broken down into smaller ones by
digestive enzymes - **digestion**, before they can be taken from the gut
into the blood stream - **absorption**. The cells of the body can then
use these small molecules - **assimilation**. The indigestible waste
products are eliminated from the body by the act of **egestion** (see
diagram 11.1).

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

Diagram 11.1 - From ingestion to egestion

The 4 major functions of the gut are:

:   1\. Transporting the food;
:   2\. Processing the food physically by breaking it up (chewing),
    mixing, adding fluid etc.
:   3\. Processing the food chemically by adding digestive enzymes to
    split large food molecules into smaller ones.
:   4\. Absorbing these small molecules into the blood stream so the
    body can use them.

The regions of a typical mammals gut (for example a cat or dog) are
shown in diagram 11.2.

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

Diagram 11.2 - A typical mammalian gut

The food that enters the **mouth** passes to the **oesophagus**, then to
the **stomach**, **small intestine**, **cecum**, **large intestine**,
**rectum** and finally undigested material exits at the **anus**. The
**liver** and **pancreas** produce secretions that aid digestion and the
**gall bladder** stores **bile**. Herbivores have an appendix which they
use for the digestion of cellulose. Carnivores have an appendix but is
not of any function anymore due to the fact that their diet is not based
on cellulose anymore.

## Mouth

The mouth takes food into the body. The lips hold the food inside the
mouth during chewing and allow the baby animal to suck on its mother's
teat. In elephants the lips (and nose) have developed into the trunk
which is the main food collecting tool. Some mammals, e.g. hamsters,
have stretchy cheek pouches that they use to carry food or material to
make their nests.

The sight or smell of food and its presence in the mouth stimulates the
**salivary glands** to secrete **saliva**. There are four pairs of these
glands in cats and dogs (see diagram 11.3). The fluid they produce
moistens and softens the food making it easier to swallow. It also
contains the enzyme, **salivary amylase**, which starts the digestion of
starch.

The **tongue** moves food around the mouth and rolls it into a ball
called a bolus for swallowing. **Taste buds** are located on the tongue
and in dogs and cats it is covered with spiny projections used for
grooming and lapping. The cow's tongue is prehensile and wraps around
grass to graze it.

Swallowing is a complex reflex involving 25 different muscles. It pushes
food into the oesophagus and at the same time a small flap of tissue
called the **epiglottis** closes off the windpipe so food doesn't enter
the trachea and choke the animal (see diagram 11.4).

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

Diagram 11.3 - Salivary glands

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

Diagram 11.4 - Section through the head of a dog

## Teeth

Teeth seize, tear and grind food. They are inserted into sockets in the
bone and consist of a crown above the gum and root below. The crown is
covered with a layer of **enamel**, the hardest substance in the body.
Below this is the **dentine**, a softer but tough and shock resistant
material. At the centre of the tooth is a space filled with **pulp**
which contains blood vessels and nerves. The tooth is cemented into the
**socket** and in most teeth the tip of the root is quite narrow with a
small opening for the blood vessels and nerves (see diagram 11.5).

In teeth that grow continuously, like the incisors of rodents, the
opening remains large and these teeth are called **open rooted teeth**.
Mammals have 2 distinct sets of teeth. The first set, the **milk
teeth**, are replaced by the **permanent teeth**.

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

Diagram 11.5 - Structure of a tooth

### Types Of Teeth

All the teeth of fish and reptiles are similar but mammals usually have
four different types of teeth.

The **incisors** are the chisel-shaped 'biting off' teeth at the front
of the mouth. In rodents and rabbits the incisors never stop growing
(open-rooted teeth). They must be worn or ground down continuously by
gnawing. They have hard enamel on one surface only so they wear unevenly
and maintain their sharp cutting edge.

The largest incisors in the animal kingdom are found in elephants, for
tusks are actually giant incisors. Sloths have no incisors at all, and
sheep have no incisors in the upper jaw (see diagram 11.6). Instead
there is a horny pad against which the bottom incisors cut.

The **canines** or 'wolf-teeth' are long, cone-shaped teeth situated
just behind the incisors. They are particularly well developed in the
dog and cat families where they are used to hold, stab and kill the prey
(see diagram 11.7).

The tusks of boars and walruses are large canines while rodents and
herbivores like sheep have no (or reduced) canines. In these animals the
space where the canines would normally be is called the **diastema**. In
rodents like the rat and beaver it allows the debris from gnawing to be
expelled easily.

The cheek teeth or **premolars** and **molars** crush and grind the
food. They are particularly well developed in herbivores where they have
complex ridges that form broad grinding surfaces (see diagram 11.6).
These are created from alternating bands of hard enamel and softer
dentine that wear at different rates.

In carnivores the premolars and molars slice against each other like
scissors and are called **carnassial** teeth see diagram 11.7). They are
used for shearing flesh and bone.

### Dental Formula

The numbers of the different kinds of teeth can be expressed in a
**dental formula**. This gives the numbers of incisors, canines,
premolars and molars in **one half** of the mouth. The numbers of these
four types of teeth in the left **or** right **half of the upper jaw**
are written above a horizontal line and the four types of teeth in the
right **or** left **half of the lower jaw** are written below it.

Thus the dental formula for the sheep is:

:

    :   0.0.3.3
    :   3.1.3.3

It indicates that in the upper right (or left) **half** of the jaw there
are no incisors or canines (i.e. there is a **diastema**), three
premolars and three molars. In the lower right (or left) **half** of the
jaw are three incisors, one canine, three premolars and three molars
(see diagram 11.6).

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

Diagram 11.6 - A sheep's skull

The dental formula for a dog is:

:

    :   3.1.4.2
    :   3.1.4.3

The formula indicates that in the right (or left) **half** of the upper
jaw there are three incisors, one canine, four premolars and two molars.
In the right (or left) **half** of the lower jaw there are three
incisors, one canine, four premolars and three molars (see diagram
11.7).

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

Diagram 11.7 - A dog's skull

## Esophagus

The **Esophagus** transports food to the stomach. Food is moved along
the esophagus, as it is along the small and large intestines, by
contraction of the smooth muscles in the walls that push the food along
rather like toothpaste along a tube. This movement is called
**peristalsis** (see diagram 11.8).

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

Diagram 11.8 - Peristalsis

## Stomach

The **stomach** stores and mixes the food. Glands in the wall secrete
**gastric juice** that contains enzymes to digest protein and fats as
well as **hydrochloric acid** to make the contents very acidic. The
walls of the stomach are very muscular and churn and mix the food with
the gastric juice to form a watery mixture called **chyme** (pronounced
kime). Rings of muscle called **sphincters** at the entrance and exit to
the stomach control the movement of food into and out of it (see diagram
11.9).

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

Diagram 11.9 - The stomach

## Small Intestine

Most of the breakdown of the large food molecules and absorption of the
smaller molecules take place in the long and narrow small intestine. The
total length varies but it is about 6.5 metres in humans, 21 metres in
the horse, 40 metres in the ox and over 150 metres in the blue whale.

It is divided into 3 sections: the duodenum (after the stomach), jejunum
and ileum. The duodenum receives 3 different secretions:

:   1\) **Bile** from the liver;
:   2\) **Pancreatic juice** from the pancreas and
:   3\) **Intestinal juice** from glands in the intestinal wall.

These complete the digestion of starch, fats and protein. The products
of digestion are absorbed into the blood and lymphatic system through
the wall of the intestine, which is lined with tiny finger-like
projections called **villi** that increase the surface area for more
efficient absorption (see diagram 11.10).

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

Diagram 11.10 - The wall of the small intestine showing villi

## The Rumen

In ruminant herbivores like cows, sheep and antelopes the stomach is
highly modified to act as a "fermentation vat". It is divided into four
parts. The largest part is called the **rumen**. In the cow it occupies
the entire left half of the abdominal cavity and can hold up to 270
litres. The **reticulum** is much smaller and has a honeycomb of raised
folds on its inner surface. In the camel the reticulum is further
modified to store water. The next part is called the **omasum** with a
folded inner surface. Camels have no omasum. The final compartment is
called the **abomasum**. This is the 'true' stomach where muscular walls
churn the food and gastric juice is secreted (see diagram 11.11).

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

Diagram 11.11 - The rumen

Ruminants swallow the grass they graze almost without chewing and it
passes down the oesophagus to the rumen and reticulum. Here liquid is
added and the muscular walls churn the food. These chambers provide the
main fermentation vat of the ruminant stomach. Here bacteria and
single-celled animals start to act on the cellulose plant cell walls.
These organisms break down the cellulose to smaller molecules that are
absorbed to provide the cow or sheep with energy. In the process, the
gases methane and carbon dioxide are produced. These cause the "burps"
you may hear cows and sheep making.

Not only do the micro-organisms break down the cellulose but they also
produce the **vitamins E, B and K** for use by the animal. Their
digested bodies provide the ruminant with the majority of its protein
requirements.

In the wild grazing is a dangerous activity as it exposes the herbivore
to predators. They crop the grass as quickly as possible and then when
the animal is in a safer place the food in the rumen can be regurgitated
to be chewed at the animal's leisure. This is 'chewing the cud' or
**rumination**. The finely ground food may be returned to the rumen for
further work by the microorganisms or, if the particles are small
enough, it will pass down a special groove in the wall of the oesophagus
straight into the omasum. Here the contents are kneaded and water is
absorbed before they pass to the abomasum. The abomasum acts as a
"proper" stomach and gastric juice is secreted to digest the protein.

## Large Intestine

The **large intestine** consists of the **caecum**, **colon** and
**rectum**. The chyme from the small intestine that enters the colon
consists mainly of water and undigested material such as cellulose
(fibre or roughage). In omnivores like the pig and humans the main
function of the colon is absorption of water to give solid faeces.
Bacteria in this part of the gut produce vitamins B and K.

The caecum, which forms a dead-end pouch where the small intestine joins
the large intestine, is small in pigs and humans and helps water
absorption. However, in rabbits, rodents and horses, the caecum is very
large and called the **functional caecum**. It is here that cellulose is
digested by micro-organisms. The **appendix**, a narrow dead end tube at
the end of the caecum, is particularly large in primates but seems to
have no digestive function.

## Functional Caecum

The caecum in the rabbit, rat and guinea pig is greatly enlarged to
provide a "fermentation vat" for micro-organisms to break down the
cellulose plant cell walls. This is called a **functional caecum** (see
diagram 11.12). In the horse both the caecum and the colon are enlarged.
As in the rumen, the large cellulose molecules are broken down to
smaller molecules that can be absorbed. However, the position of the
functional caecum after the main areas of digestion and absorption,
means it is potentially less effective than the rumen. This means that
the small molecules that are produced there can not be absorbed by the
gut but pass out in the faeces. The rabbit and rodents (and foals) solve
this problem by eating their own faeces so that they pass through the
gut a second time and the products of cellulose digestion can be
absorbed in the small intestine. Rabbits produce two kinds of faeces.
Softer night-time faeces are eaten directly from the anus and the harder
pellets you are probably familiar with, that have passed through the gut
twice.

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

Diagram 11.12 - The gut of a rabbit

## The Gut Of Birds

Birds' guts have important differences from mammals' guts. Most
obviously, birds have a **beak** instead of teeth. Beaks are much
lighter than teeth and are an adaptation for flight. Imagine a bird
trying to take off and fly with a whole set of teeth in its head! At the
base of the oesophagus birds have a bag-like structure called a
**crop**. In many birds the crop stores food before it enters the
stomach, while in pigeons and doves glands in the crop secretes a
special fluid called **crop-milk** which parent birds regurgitate to
feed their young. The stomach is also modified and consists of two
compartments. The first is the true stomach with muscular walls and
enzyme secreting glands. The second compartment is the **gizzard**. In
seed eating birds this has very muscular walls and contains pebbles
swallowed by the bird to help grind the food. This is the reason why you
must always supply a caged bird with grit. In birds of prey like the
falcon the walls of the gizzard are much thinner and expand to
accommodate large meals (see diagram 11.13).

![](Anatomy_and_physiology_of_animals_Stomach_&_small_intestine_of_hen.jpg "Anatomy_and_physiology_of_animals_Stomach_&_small_intestine_of_hen.jpg")

Diagram 11.13 - The stomach and small intestine of a hen

## Digestion

During digestion the large food molecules are broken down into smaller
molecules by **enzymes**. The three most important groups of enzymes
secreted into the gut are:

:# **Amylases** that split carbohydrates like starch and glycogen into
monosaccharides like glucose.

:# **Proteases** that split proteins into amino acids.

:# **Lipases** that split lipids or fats into fatty acids and glycerol.

Glands produce various secretions which mix with the food as it passes
along the gut.

These secretions include:

:# **Saliva** secreted into the mouth from several pairs of **salivary
glands** (see diagram 11.3). Saliva consists mainly of water but
contains salts, mucous and salivary amylase. The function of saliva is
to lubricate food as it is chewed and swallowed and salivary amylase
begins the digestion of starch.

:# **Gastric juice** secreted into the stomach from glands in its walls.
Gastric juice contains **pepsin** that breaks down protein and
hydrochloric acid to produce the acidic conditions under which this
enzyme works best. In baby animals rennin to digest milk is also
produced in the stomach.

:# **Bile** produced by the liver. It is stored in the **gall bladder**
and secreted into the duodenum via the **bile duct** (see diagram
11.14). (Note that the horse, deer, parrot and rat have no gall
bladder). Bile is not a digestive enzyme. Its function is to break up
large globules of fat into smaller ones so the fat splitting enzymes can
gain access the fat molecules.

![](Anatomy_and_physiology_of_animals_Liver,_gall_bladder_&_pancreas.jpg "Anatomy_and_physiology_of_animals_Liver,_gall_bladder_&_pancreas.jpg")

Diagram 11.14 - The liver, gall bladder and pancreas

## Pancreatic juice

The **pancreas** is a gland located near the beginning of the duodenum
(see diagram 11.14). In most animals it is large and easily seen but in
rodents and rabbits it lies within the membrane linking the loops of the
intestine (the **mesentery**) and is quite difficult to find.
**Pancreatic juice** is produced in the pancreas. It flows into the
duodenum and contains **amylase** for digesting starch, **lipase** for
digesting fats and **protease** for digesting proteins.

## Intestinal juice

**Intestinal juice** is produced by glands in the lining of the small
intestine. It contains enzymes for digesting disaccharides and proteins
as well as mucus and salts to make the contents of the small intestine
more alkaline so the enzymes can work.

## Absorption

The small molecules produced by digestion are absorbed into the
**villi** of the wall of the **small intestine**. The tiny finger-like
projections of the villi increase the surface area for absorption.
Glucose and amino acids pass directly through the wall into the blood
stream by diffusion or active transport. Fatty acids and glycerol enter
vessels of the lymphatic system (**lacteals**) that run up the centre of
each villus.

## The Liver

The liver is situated in the abdominal cavity adjacent to the diaphragm
(see diagrams 2 and 14). It is the largest single organ of the body and
has over 100 known functions. Its most important digestive functions
are:

:# the production of **bile** to help the digestion of fats (described
above) and

:# the control of **blood sugar** levels

Glucose is absorbed into the capillaries of the villi of the intestine.
The blood stream takes it directly to the liver via a blood vessel known
as the **hepatic portal vessel** or **vein** (see diagram 11.15).

The liver converts this glucose into glycogen which it stores. When
glucose levels are low the liver can convert the glycogen back into
glucose. It releases this back into the blood to keep the level of
glucose constant. The hormone **insulin**, produced by special cells in
the **pancreas**, controls this process.

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

Diagram 11.15 - The control of blood glucose by the liver

Other functions of the liver include:

:   3\. making **vitamin A**,
:   4\. making the **proteins** that are found in the **blood plasma**
    (**albumin, globulin** and **fibrinogen**),
:   5\. storing **iron**,
:   6\. removing **toxic substances** like alcohol and poisons from the
    blood and converting them to safer substances,
:   7\. producing **heat** to help maintain the temperature of the body.

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

Diagram 11.16 - Summary of the main functions of the different regions
of the gut

## Summary

-   The **gut** breaks down plant and animal materials into nutrients
    that can be used by animals' bodies.
-   Plant material is more difficult to break down than animal tissue.
    The gut of **herbivores** is therefore longer and more complex than
    that of **carnivores**. Herbivores usually have a compartment (the
    **rumen** or **functional caecum**) housing micro-organisms to break
    down the **cellulose** wall of plants.
-   Chewing by the teeth begins the food processing. There are 4 main
    types of teeth: **incisors, canines, premolars** and **molars**. In
    dogs and cats the premolars and molars are adapted to slice against
    each other and are called **carnassial** teeth.
-   **Saliva** is secreted in the mouth. It lubricates the food for
    swallowing and contains an enzyme to break down starch.
-   Chewed food is swallowed and passes down the **oesophagus** by waves
    of contraction of the wall called **peristalsis.** The food passes
    to the stomach where it is churned and mixed with acidic **gastric
    juice** that begins the digestion of protein.
-   The resulting **chyme** passes down the small intestine where
    enzymes that digest fats, proteins and carbohydrates are secreted.
    **Bile** produced by the liver is also secreted here. It helps in
    the breakdown of fats. **Villi** provide the large surface area
    necessary for the absorption of the products of digestion.
-   In the **colon** and **caecum** water is absorbed and micro
    organisms produce some **vitamin B and K**. In rabbits, horses and
    rodents the caecum is enlarged as a **functional caecum** and
    micro-organisms break down cellulose cell walls to simpler
    carbohydrates. Waste products exit the body via the **rectum** and
    **anus**.
-   The **pancreas** produces **pancreatic juice** that contains many of
    the enzymes secreted into the small intestine.
-   In addition to producing bile the liver regulates blood sugar levels
    by converting glucose absorbed by the villi into glycogen and
    storing it. The liver also removes toxic substances from the blood,
    stores iron, makes vitamin A and produces heat.

## Worksheet

Use the Digestive System
Worksheet to
help you learn the different parts of the digestive system and their
functions.

## Test Yourself

Then work through the Test Yourself below to see if you have understood
and remembered what you learned.

1\. Name the four different kinds of teeth

2\. Give 2 facts about how the teeth of cats and dogs are adapted for a
carnivorous diet:

:   1\.
:   2\.

3\. What does saliva do to the food?

4\. What is peristalsis?

5\. What happens to the food in the stomach?

6\. What is chyme?

7\. Where does the chyme go after leaving the stomach?

8\. What are villi and what do they do?

9\. What happens in the small intestine?

10\. Where is the pancreas and what does it do?

11\. How does the caecum of rabbits differ from that of cats?

12\. How does the liver help control the glucose levels in the blood?

13\. Give 2 other functions of the liver:

:   1\.
:   2\.

/Test Yourself Answers/

## Websites

-   <http://www.second-opinions.co.uk/carn_herb_comparison.html> Second
    opinion. A good comparison of the guts of carnivores and herbivores
-   <http://www.chu.cam.ac.uk/~ALRF/giintro.htm> The gastrointestinal
    system. A good comparison of the guts of carnivores and herbivores
    with more advanced information than in the previous site.
-   <http://www.westga.edu/~lkral/peristalsis/index.html> Peristalsis
    animation.
-   <http://en.wikipedia.org/wiki/Digestion> Wikipedia on digestion with
    links to further information on most aspects. Like most websites
    this is mainly about human digestion but much is applicable to
    animals.

## Glossary

-   Link to
    Glossary