Skeletal Sys

The Skeletal System

Our skeleton is an internal support framework made of bone, cartilage and ligaments.

1. Body Shape 2. Support 3. Protection 4. Movement

The arrangement of bones in our skeleton gives our body a particular shape that has evolved and is adapted to carry out the many varied activities of survival and reproduction.

The soft parts of our body, especially the muscles, are held in position by direct or indirect attachment to the skeleton. If the skeleton was removed our body would collapse like a pile of clothes when a clothes line breaks. Bone is made of hard material and can form a protective cover around soft organs

a) Skull: protects the brain, eyes and ears.

b) Backbone: protects the spinal cord.

c) Rib Cage: protects the heart and lungs. Many of the 206 bones act as levers and will turn at a joint when a pulling force is exerted on them by muscle contraction.

The Muscular System

A muscle is a mass of contractile tissue responsible for movement when it contracts. Muscles in the walls of hollow organs, other than the heart, cause the movement of material along those organs by alternate contraction and relaxation e.g. the movement of food along the gut. Special muscle, cardiac muscle, in the wall of the heart, pushes blood through the blood vessels to and from all the organs of the body. The skeletal muscles are attached to bones and when they contract they pull on the bones causing body movements.

Antagonistic Muscles There are about 700 muscles in the body. A muscle cannot lengthen. A muscle is capable of very strong shortening when stimulated to contract. Therefore a muscle is only able to exert a pulling force.

When a bone is pulled in one direction by a contracting muscle that bone can only be moved back to its original position by a different muscle pulling it in the opposite direction. All skeletal muscles operate in pairs called antagonistic muscles Antagonistic muscles are pairs of skeletal muscle where the contraction of each moves a bone in the opposite direction to the other. The biceps and triceps muscles of the upper arm are an example. Contraction of the biceps (triceps relaxed) causes the arm to bend i.e. the lower arm moves towards the shoulder.

Contraction of the triceps (biceps relaxed) causes the arm to straighten as the lower arm mover away from the shoulder.

The Function of Bone

Bone is a hard specialised living tissue. It is composed of living cells surrounded by a material which is 65% salts and 35% organic matter. The special combination gives bone it hardness without being brittle to resist compression but also gives it enough flexibility to resist stretching and twisting. Bone is half as strong as steel in resisting compression and equal to steel in resisting stretching.

The Major Bones of the Body

a) Skull Composed of the cranium (brain case) and the flexible lower jaw.

d) Collar Bone The collar bone is also called the clavicle. It is at the top front of the rib cage and supports the shoulder preventing it collapsing inward. There are two collar bones, one to each shoulder.

e) Shoulder Blade (Scapulla) There are two shoulder blades, one behind each shoulder at the back. They are broad bones form the attachment of very strong muscles.

The arms are attached to the shoulder blades by a ball and socket joint. The two clavicles and two shoulder blades make up a ring of bones called the pectoral girdle.

f) Humerus The humerus is a long bone in the upper arm from shoulder to elbow. The biceps and triceps muscles are found here.

c) Ribs Twelve pairs of ribs make up the ribcage forming the thorax or chest of the body. The ribcage not only protects the heart and lungs but also has a very important role in breathing.

b) Vertebrae These are the 33 small bones arranged in an S-shaped line from skull to ‘tail’ tip just below the pelvis.Each small bone is called a vertebra.

g) Radius The radius is the bone of the lower arm from the elbow to the wrist at the thumb side.

h) Ulna The other bone of the lower arm forming a hinge joint with the humerus.

i) Pelvis Also known as the hips. The lower limbs (legs) are attached to the pelvis by a ball and socket joint.The pelvis is attached very strongly to the backbone at vertebrae 25 to 29.

j) Femur This is the upper leg bone forming a ball and socket joint with the hip and a hinge joint at the knee with the tibia. The femur is the largest, longest strongest bone of the body.

k) Tibia This is the larger of the two bones of the lower leg and is known as the shinbone – it is the second largest and second strongest bone of the body

l) Fibula The fibula is the smaller of the two bones of the lower leg.

The skeleton also provides the suspended solids of the blood – red blood cells, white blood cells and platelets – are made by the bone marrow.

A joint is a point of attachment between two bones. There are several types of joint depending on the type of movement required at that point.. The three major types of joint are fused, ball and socket plus hinged.

Fused/Fixed/Immoveable Joint

Movement is not possible at this type of joint. The bones are held very tightly together by special connective tissue. Example: the bones of the skull’s cranium (braincase) that protects the brain;

Ball and Socket Joint Example; shoulders and hips. A wide range of movement in all three dimensions is possible with this type of joint. The opposing ends have complementary shapes, spherical (ball) and cupped (socket), that allows the great freedom of movement. At the shoulder the humerus has the ball and the shoulder blade the socket.

At the hip the femur has the ball and the pelvis the socket. The ball and socket surfaces have a protective layer of cartilage reducing both impact compression forces and friction. Synovial fluid, reduces friction between the bones.

The bones are held in position by strong flexible ligaments. Ligaments are special tough bands of connective tissue that connect bone to bone and their function is to limit the range of movement at the joint and prevent the dislocation of bones at the joint.

Tendons are tough inelastic cords connecting skeletal muscle to bone. The entire pull of the contracting muscle is transferred to the bone. Tendons take up much less space than muscle and so allow freer movement at the joint and/or allow more muscles to attach producing more complex movement at the joint.

Skeletal muscles, bones, ligaments and tendons have specific roles to play in the body movements that take place at joints.

Hinge Joint Sites: elbows and knees.

The opposing ends have complementary shapes a cylindrical end fits into a depression-shaped surface of the other. Movement is along one plane and is similar to that of a mechanical hinge on a door.

Both opposing surfaces are covered in a layer of cartilage reducing both impact compression and friction.

Synovial fluid again reduces friction between the bones. The bones are held in position by strong flexible ligaments.