Mammals_Concept_2

Subject Facts

The discovery of magnets occurred in ancient times.They were known about and used by the early Greek and Chinese civilisations.The word magnet comes from Magnesia, a place in Turkey where the natural magnet lodestone can still be found.

Magnets and magnetic materials

A magnet is an object that is able to exert a strong force of attraction on materials that contain the metals iron, cobalt, steel or nickel.These are called ferromagnetic materials. Alternatively, we can describe ferromagnetic materials as materials that are susceptible to the attractive forces of magnets and have the potential to become permanent magnets themselves.

Magnets lose their magnetism permanently when they are heated beyond a specific temperature, the Curie point, because the movement of the individual atoms becomes too great to maintain a static magnetic field. All other materials are either paramagnetic (very slightly attracted by a magnetic field) or diamagnetic (very slightly repelled by a magnetic field).'Very slight' here means not detectable by anything but highly sensitive, specialised detectors.

Permanent magnets

Before 1 820, the only way to make a permanent magnet was

to stroke a steel or iron rod with a natural magnet (a piece of lodestone).The accepted model of how this works is that a lump of any magnetic material is made up of particles that are themselves 'mini-magnets'. Normally, these magnets will be pointing randomly in different directions, so their effects 'cancel out'. Stroking them with a magnet aligns them so that they all take up the same orientation, turning the magnetic material into a magnet.

Magnets produced in this way need very careful handling, as they are likely to lose their magnetism and revert to simply being magnetic material. Dropping and banging will cause the particles to become random once more as their alignment is disturbed. Materials which can be magnetised (and demagnetised) easily are known as 'soft magnetic materials'.They include nails, paper clips and an iron/silicon alloy called Stalloy, which is used in the core of electromagnets.

Since the discovery of the electromagnetic effect by Hans Christian Oersted in 1 820, it has been possible to create a permanent magnet by placing a steel rod in a coil of wire which is then attached to a strong battery.The effect of the electric coil is similar to that of the lodestone (see Chapter l, page 28).The strength of the electromagnet created depends on the number of turns in the coil and the electrical current passed through the coil.

The strongest type of permanent metallic magnet in widespread current use is an alloy of aluminium, nickel, cobalt, and copper, called alnico. An alternative is an artificial ceramic material called magnadur, which contains powdered ferromagnetic oxides. These and others are collectively known as 'hard magnetic materials'. Although they are permanent as magnets, alnico and magnadur are very brittle and prone to breakage as materials, If you keep dropping a steel magnet, you will end up with a lump of steel; if you drop a magnadur magnet, you will end up lots of tiny magnets. Alnico and magnadur magnets can easily be made into a variety of shapes, including bars and rings, for different purposes.

Most magnets found in toys and used in schools tend not to be of the steel type, because steel magnets are more likely than alloy or ceramic ones to lose their magnetism through rough handling. Often bar and horseshoe magnets, which would have been made from steel in the past, are made from plastic with correctly orientated ceramic magnets inserted in the pole areas.

This tends to make them more powerful and reliable; but it can also mean, particularly in the case of horseshoe magnets, that the magnetic field produced is not what you would get from a continuous steel magnet.

Temporary electromagnets

If a rod of a soft magnetic material (such as iron) is used instead of a steel rod in an electromagnet, the magnet will only last as long as the current is flowing: the rearrangement of the particles in the rod is only temporary.This effect has a number of applications: separating magnetic and non-magnetic materials; lifting and moving magnetic materials; operating bells and switching devices.

Subject *acts

Protection

Obviously, not all animals have their internal organs encased in a hard shell such as the exoskeleton of an arthropod. External blows can damage internal organs, and bone provides the first stage of an excellent impact protection system in mammals. The brain, which is rather exposed as an appendage at one end of the body, is almost completely encased in bone. The bones that fuse together to form the skull only join fully towards the end of infancy — a degree of suppleness allows the head of a baby to squeeze through the birth canal. Further 'impact protection' is offered by a liquid-filled sac that surrounds the brain and acts as a shock absorber.

The rib cage offers a semi-flexible protected area for many of the vital organs, particularly those concerned with respiration. The ribs are connected to the sternum at the front and the spine at the back with one pair of ribs attached to each vertebra or spinal bone. Having ribs rather than a shell offers advantages in terms of both weight and movement This flexibility can be felt while breathing. You can breathe either by pushing your tummy in and out (your rib cage remains relatively still) or by raising and lowering your rib cage.

If the organs of the thorax (chest) are protected in this way, then why not those of the abdomen (belly)? There is a payoff between flexibility and protection — in general, the better the protection, the less flexibility there is. To allow the body to bend and rotate above the hips, the body does not have bones surrounding the abdomen. To compensate, thick muscular walls (the proverbial 'six-pack') surround it. These muscles allow movement, but also give protection. When they are flexed, they can become quite rigid: a blow to the abdomen can be absorbed with much less damage than a blow to the head (the brain can rattle around quite a bit inside the skull). As well as needing flexibility for movement, the mammalian abdomen needs to be flexible to allow expansion as a result of eating or foetal growth.

Bone structure

The rigidity of bone is derived from its inorganic content (65—70% by mass), which is mostly calcium. This provides bone with a structure that is very strong in compression but weaker in bending (in other words, bones can withstand great pressure, but are liable to snap when bent). Without a rigid internal structure, mammals and other vertebrates would lose their shape and end up looking like rather badly filled bean bags. Animals that don't have a rigid body structure (either internally or externally) tend to be either very small, or restricted to an aquatic life form where the surrounding water can act as a support.

The long structural bones within the body, particularly those in the limbs, contain a central core of organic or living material (see below) surrounded by a rigid inorganic component. The structure of bone has similarities to that of wood, in that it consists of parallel fibres or strands running the length of the bone. Bundling drinking straws or paper tubes together can model this: the structure is strong in compression, but not so strong when you try to bend it. Healthy bones are not brittle: they tend to be quite tough, due to the gelatine and fatty tissues between the fibres providing some elasticity. The bones of young mammals, due to their rapid early growth, tend to be less rigid than adult bones and thus more flexible and less likely to break,

Cell production

The organic or living part of the bone (apart from its blood supply system) is in the central core or marrow. There are two types of bone marrow cell: the yellow fat cells and the red blood producing cells. Most of the production of blood cells (both red and white) occurs in the ribs and the pelvis. More details on mammalian blood are given on page 67.

Movement

Bones allow movement: muscles do the actual pulling, but bones are what they pull against. The bones act as levers within the body allowing movement about a joint (where two or more bones are connected).There are various different forms of joint Some do not allow any movement at all, such as those where the bones of the skull meet; hinge and 'ball and socket' joints allow the greatest degree of controlled movement (see Figure 2).

Bones that meet at a joint are held together by a ligament: a collar of tough, fibrous material that prevents the bones from moving out of alignment, and also prevents the liquid that lubricates the joint (synovial fluid) from leaking away.

To increase ease of movement and reduce friction, the ends of the bones are also coated with a rubbery cartilage. When mammals and other animals are borne most of their skeleton is made up of cartilage rather than bone (for greater flexibility and faster growth), in old age, the cartilage between bones can wear away, allowing the bones at joints to come into direct contact and rub against each other. This process, known as arthritis, can result in pain and loss of mobility

To achieve movement about a joint, muscles have to act Muscles are bundles of long cells that can contract (shorten) and relax (lengthen). They are attached to the bones across a joint by very strong cords or tendons. Muscles are usually attached in pairs: one to pull the bone one way, and another to pull it back. When one of a pair of antagonistic muscles contracts and the other goes limp, the bone will move about the joint (see Figure 3).

Why you need to know these facts

Children often associate bones and the skeleton with death, because the bones only become visible when the rest of the body has decayed. Knowing about the skeleton as a part of the body will demystify it and help children to understand what functions it performs for living mammals.

Vocabulary

Cartilage — a flexible, hard-wearing material that protects bones from damage within joints.

Ligament — tissue that connects bones together and surrounds joints.

Tendon — very strong, inelastic tissue that connects muscles to bones.

Amazing facts

There are 206 bones in the body of an adult human.

A giraffe has the same number of neck bones (cervical vertebrae) as a human — but the giraffe's neck bones are longer.

One third of the mass of a bone is water.

3—5% of your body weight is bone marrow.

The gelatine in animal bones can be used to make jellies, non-dairy cream and glue.

Common Misconceptions

All hard bits in the body are bone.

By feeling your body, you can identify the positions of bones. Some of the hard bits may also be either tendon or cartilage. The ear and the end of the nose have a hard but semi-flexible structure. This is cartilage: a tough, rubbery substance that is shaped (feel your ears) and can be bent, but will spring back into its original shape, The hard parts of your throat and larynx are also made of cartilage.

If you flex your arm or leg into a right angle, you can feel some hard strands in the pit of your elbow or knee. These are the tendons, which attach the muscles to the bones. The most noticeable tendons are the Achilles tendons at the back of the ankles. Some fish, such as sharks, don't have any bones: their skeletons are entirely composed of cartilage.

Questions

What are bones like inside?

Bones are not the same wherever you look. They have a skin, called the periosteum, which is where new bone grows. At the ends of a long bone (such as those in the limbs) it looks like a sponge: full of holes, but fight and strong. Running down the centre of the bone is a jelly-like material called bone marrow, which is where blood cells are made. Between the marrowbone core and the outer skin is a dense, compact form of bone which is very strong. It does have some small holes in it to allow tiny blood vessels access to the bone cells.

Are all bones hard?

Newborn mammals start life with many of their bones made only from cartilage.This allows them to be more flexible and grow faster. If you leave a chicken bone in a pot of vinegar for a few days, the acid in the vinegar will eat away the calcium in the bone, allowing you to bend it without it snapping.

Do your teeth count as bones?

Although teeth are made mostly from the same material as bones (calcium), they are considerably harder, They have to resist acid in the saliva, and have a very hard outer covering (enamel) so that they do not wear away too fast. Teeth are much more solid than the spongy interior of most large bones,

How long do bones go on growing?

Once bones reach adult size, they stop getting any bigger — but they never stop growing. Bones are living, not lumps of rock, so the cells have to be continually renewed. This means that if a bone is broken it can knit back together again (rather like the skin when it is cut) and be as strong as before.

Teaching ideas

Skeletal drawings (observing, presenting)

Ask the children to draw what they think their skeleton would look like, using chalk on black paper. Encourage them to feel along their limbs to identify the joints between bones, Repeat with an animal for which you can obtain a skeleton in a case. This activity is particularly interesting to use as a 'before and after' assessment activity where the children can compare their two drawings.

Bone structure (observing, modelling)

The children can make a model bone by taping together a bundle of five or six drinking straws or tightly rolled up sheets of paper. They can try to compress their bundle from each end (lengthways) and note how strong it is. Now they can try to bend the bundle — in this plane, it is relatively weak. Explain that long bones are fibrous, like the straws: they are strong in compression, but quite weak when you try to bend them.

Joint effort (observing, testing)

Ask the children to work their way around their bodies* operating one joint at a time. Can they identify how each joint moves? Is it in one plane, like the joints in the fingers — a hinge joint? Is there greater movement, as in the shoulder — a ball and socket joint?