Skeleton and Muscles

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The Human skeleton is located internally and is called an endoskeleton. Arthropods (e.g. Cockroaches and Crabs) have an external skeleton called an exoskeleton. Annelids (e.g. Earthworms) have water inside a cavity as a skeleton called a hydrostatic skeleton.

Endoskeleton - The Human skeleton

Click here for a diagram of the human skeleton

Functions:
1. Movement - the limbs bend at their joints, so they can move by the action of the skeletal muscles.
2. Support - it holds the body upright and gives it shape.
3. Protection - the skull protects the brain, the rib cage protects the lungs and heart and the vertebrae protect the spinal cord.

Structure of the Human Skeleton
1. The Axial skeleton: skull, vertebral column, sternum and ribs.
2. The Appendicular skeleton: arms and the pectoral girdle, legs and the pelvic girdle.

The Skull: The skull is made up of 22 bones, most of which are connected by joints which do not move called sutures, but the lower jaw can move to open and close the mouth.

Vertebral Column: The vertebral column consists of 33 bones, the vertebrae are stacked end to end forming a column. Discs of cartilage separate them.
Click here for a diagram of the vertebral column

The typical vertebra has:

A solid body, the Centrum.
Neural spine
Neural canal to enclose the spinal cord.
Transverse processes (projections) for attachment of muscles.

Click here for a diagram of a typical vertebra

The Sternum and Ribs:
The Sternum or breastbone is a long flat bone found in the centre of the chest. There are 12 pairs of ribs. All ribs are attached to the vertebrae at the back.
a) True ribs: The first 7 pairs are attached directly to the breastbone in the front.
b) False ribs: The next 3 pairs (pairs 8,9 and 10) are attached to the seventh pair.
c) Floating ribs: The bottom 2 pairs (pairs 11 and 12) are attached at the front.

The Pectoral girdle (Shoulder): Consists of the collar bone (Clavicle) and the shoulder blade (Scapula).

The Pelvic girdle (Hip): The Pelvic girdle is made up of two halves. Each half consists of three bones fused together. The Pelvic girdle helps support the weight of the body and to allow articulation with the leg bones.

Limbs: The arms and legs have a similar design pattern.
Click here for a diagram of a limb

Joints:

Joints are formed where bones meet with each other. There are different types of joint depending on the amount of movement they allow.
1. Moveable Joints (Synovial Joints):
Click here for a diagram of a joint

Ball and Socket: Found in the shoulder and the hip. Allows movement in all directions.
Hinge: Found in the elbow and the knee. Allows movement in one direction only.
Gliding: Found at the ankles and wrists. Bones glide over each other, allowing movement.
Pivot: Found between the atlas and axis bones in the neck. Allow the turning of the head from side to side.

2. Slightly moveable joints:
The joints between the vertebrae. The cartilage between the vertebrae allows a limited amount of movement. The bones are held together by ligaments.

3. Immovable joints:
An example of this joint is the bones of the skull, which are fused together.

Muscles

There are three kinds of muscle:
a) Skeletal muscles
b) Smooth muscles
c) Cardiac muscles

a) Skeletal muscle:

Attached to the skeleton.
Striped muscle.
Voluntary.

b) Smooth muscle:

In arteries, veins and intestines.
Not striped.
Involuntary.

c) Cardiac muscle:

In the heart.
Striped.
Involuntary
Contracts rhythmically and does not fatigue.

Skeletal muscles and movement:
Voluntary muscles are attached to the skeleton and used for movement. They work in pairs called antagonistic pairs when one muscle contracts, the other muscle relaxes. One end of the muscle is attached to a bone, which will not move. This is called the origin. The other end of the muscle is attached to a bone, which will move. This is called the insertion.
Click here for a diagram of the muscles of the arm.

To raise the arm:
The biceps contracts (triceps relaxes) and pulls on the radius. The arm is raised.

To lower the arm:
The triceps contracts (biceps relaxes) and pulls on the ulna. The arm is pulled downwards.

Bone

Bone is made up of living (organic) and non living (inorganic) parts. The living part consists of bone cells and protein fibres which give the bone its flexibility.
The non living part consists of calcium, phosphorus and other minerals which give the bone strength.

Structure of long bones:
Click here for a diagram of a long bone.

The surface of bone is covered with periosteum, a thin membrane that contains a network of blood vessels and nerves.
Beneath the periosteum is a hard dense shell called compact bone.
Inside this is a mesh like structure called spongy bone.
The central cavity found in some bones and the spaces in spongy bone contain bone marrow. This is where red blood corpuscles , platelets and some white blood cells are produced.
The two ends of a long bone are called the epiphyses and the shaft of the bone is called the diaphysis.

Compact bone:
Click here for a diagram of a TS through a compact bone.
When compact bone is cut in T.S and examined under the microscope it is seen to be composed of closely packed units called Haversian systems. Each Haversian system has a central hollow called a Haversian canal which contains an artery, a vein and a lymph vessel.
The Haversian canal is surrounded by concentric layers of calcium salts called lamellae.
Between the lamellae are small cavities or lacunae. The lacunae contain bone cells (osteoblasts) The osteoblasts produce the calcium salts (the lamellae).
The lacunae are connected to each other by tiny channels called canaliculi.

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