Human skeletal system

Hydrostatic skeleton:

1. Found in soft-bodied animals.
2. The body tube is filled with fluid, and muscles contract around the fluid, creating pressure for movement.

Example: Earthworm.

Exoskeleton:

1. Found outside the body.
2. Common in arthropods such as insects.
3. Provides protection and support.

Endoskeleton:

1. A rigid framework of bones and cartilage located inside the body.
2. Muscles are attached to the bones, and their contraction and relaxation produce movement.
3. Bone is a hard tissue found only in vertebrates.
4. Cartilage is softer and more flexible than bone, found in the nose, ears, and at the ends of bones.

1. Support: Provides a rigid framework that supports softer body parts and attaches muscles and organs.
2. Locomotion: Enables movement of the organism from one place to another.
3. Protection: Protects internal organs. For example, the skull protects the brain, the ribcage protects the heart and lungs, and the sternum protects the spinal cord.
4. Blood cell formation: The bone marrow manufactures red and white blood cells.
5. Shape: Gives the body a definite shape.
6. Mineral storage: Stores minerals such as calcium and phosphorus.

The human skeleton is divided into two major parts:

1. The axial skeleton: Includes the skull, vertebral column, ribcage, and sternum.
2. The appendicular skeleton: Includes the limbs and limb girdles.

The axial skeleton consists of four main parts:

The skull:

1. Composed of small bones joined together by sutures (immovable joints).
2. It protects the brain, inner ear, nose, and eyes.
3. The skull has hollow spaces (orbits) for the eyes and cavities for the ears.
4. The skull holds the upper and lower jaws and supports teeth.

1. The ribcage is composed of ribs and the sternum.
2. It forms a thoracic cage, which encloses the thoracic cavity, protecting the heart, lungs, and major blood vessels.
3. The ribcage consists of 12 pairs of ribs joined to the thoracic vertebrae at the back and the sternum at the front.
4. The last two ribs, which are not joined to the sternum, are called floating ribs.
5. Between the ribs are intercostal muscles, which assist in breathing.
6. The sternum is composed of small bones called sternebrae and serves as a surface for attachment of ribs.

1. The vertebral column is the main axis of the body and consists of 33 vertebrae.
2. Intervertebral disks (cartilage) between adjacent vertebrae act as shock absorbers and reduce friction.
3. The vertebral column supports the body and the spinal cord.

It has five types of vertebrae:

1. Cervical vertebrae (7)
2. Thoracic vertebrae (12)
3. Lumbar vertebrae (5)
4. Sacral vertebrae (fused)
5. Caudal vertebrae (tail region)

Cervical vertebrae

1. There are 7 cervical vertebrae, located in the neck region.
2. The first cervical vertebra, atlas, allows nodding movement of the head.
3. The second cervical vertebra, axis, allows rotational movement of the head (e.g., shaking head to say "no").
4. These vertebrae also support the head and protect blood vessels passing through their canals.

Thoracic vertebrae

1. There are 12 thoracic vertebrae in the chest region.
2. These vertebrae, together with the ribs and sternum, form the thoracic cage that protects the heart, lungs, and major blood vessels.
3. They also play a key role in the movement of breathing.

Lumbar vertebrae

1. There are 5 lumbar vertebrae in humans, providing support for the lower back.
2. The lumbar vertebrae have large thick centra that support the upper body and provide attachment surfaces for abdominal and lower back muscles.
3. They allow bending, sideways movement, and rotation of the trunk.

Sacral vertebrae

1. The sacral vertebrae are fused to form the sacrum in the sacral region.
2. The sacrum provides a large surface area for muscle attachment, especially for back muscles.

Caudal vertebrae

1. These vertebrae are found in the tail region, but humans have no external tail.
2. Humans have 4 caudal vertebrae that form the coccyx (tailbone), with no significant function.

The appendicular skeleton consists of the limbs and limb girdles, which are attached to the axial skeleton.

There are two types of limbs:

1. Forelimbs (anterior limbs)
2. Hindlimbs (posterior limbs)

Forelimbs

The forelimbs are attached to the axial skeleton at the anterior part of the body.

Key components of the forelimbs:

1. Pectoral girdle (shoulder girdle)
2. Humerus: The long bone of the upper arm, providing attachment for muscles.
3. Ulna and radius: The two bones of the forearm.

Carpals, metacarpals, and phalanges:

1. Carpals: Nine small bones forming the wrist.
2. Metacarpals: Five slightly elongated bones in the palm.
3. Phalanges: The bones of the fingers.
4. These bones allow free movement of the hand and provide surfaces for muscle attachment.

Pectoral girdle

Humerus: Is long bone of the upper arm and provide surface for attachment of muscle

Ulna and radius

Phalanges

Phalanges form the skeleton of the fingers

Hindlimbs

The hindlimbs are attached to the posterior part of the body and consist of several important components:

Pelvic girdle: Consists of bones around the hip region, supporting the hind limbs.

1. Each half of the pelvic girdle contains the pubic bones, which are made up of three bones: ischium, ilium, and pubis.
2. The pelvic girdle supports the hindlimbs and forms a protective cage for vital organs such as the reproductive organs in females.
3. It articulates with the head of the femur to form the hip joint.
4. It also articulates with the sacrum and supports the tail where present.

It articulates with sacrum and provides for a tail where it is present.

Femur

The femur is the long bone located in the upper part of the hind limb (thigh region).

Key features:

1. The head of the femur fits into the pelvic girdle to form the hip joint.
2. It articulates with the tibia at its lower end to form the knee joint.
3. The femur provides a surface for the attachment of leg muscles and supports the weight of the body during activities like standing, walking, and running.

Tibia and fibula

Tibia: The tibia is a long bone found on the medial (inner) side of the lower leg, next to the big toe.

Key features:

1. The tibia is larger than the fibula and supports most of the body's weight.
2. It articulates with the femur at the knee joint and with the tarsals (foot) at the ankle joint.

Red blood cells are produced in the tibia's bone marrow.

Fibula: The fibula is a smaller bone located on the lateral (outer) side of the lower leg, alongside the tibia.

Key features:

1. It provides support and a surface for muscle attachment but does not bear much weight.
2. It articulates with the tibia and helps support the lower leg.

Patella (knee cap): The patella is a small, round bone located in front of the knee joint.

Key features:

1. It helps prevent the leg from bending upwards at the knee.
2. It provides mechanical advantage to the quadriceps muscles, aiding in efficiency during leg movement.

Tarsals, metatarsals, and phalanges

Tarsals: The tarsals are six small bones located in the ankle region.

Key features:

1. Two of the tarsals are elongated, and one projects backward to form the heel bone.
2. The heel bone prevents the foot from bending backward.

Function: The tarsals provide surfaces for the attachment of ankle muscles and allow for movement at the ankle joint.

Metatarsals: The metatarsals are elongated bones in the foot, located between the tarsals and phalanges (toes).

Key features:

1. There are five metatarsals in humans.
2. They provide a surface for muscle attachment and support the shape of the foot.

Function: The metatarsals support the foot, help maintain its shape, and assist in weight-bearing during walking or running.

Phalanges: The phalanges are the bones of the toes.

Key features:

• Each toe consists of three phalanges, except for the big toe, which has two.

Function: The phalanges enable movement of the toes and help with balance during activities such as standing and walking.

Functions of the tarsals, metatarsals, and phalanges

1. Tarsals: Articulate with the fibula to form the ankle joint, and with the metatarsals to form the foot.
2. Metatarsals: Articulate with the phalanges to form the toes, supporting the foot's shape and function.
3. Phalanges: Form the toes, aiding in balance and movement during walking and standing.

Skeleton of human fore limb

Skeleton of human hind limb

Bone

A hard, tough connective tissue composed primarily of mineral salts such as calcium and phosphate. Bones provide structure, protection, and support to the body.

Cartilage

A soft, flexible tissue found in areas such as the trachea, ears, nose, and at the ends of bones, particularly in joints. It serves to reduce friction and provide cushioning in these areas.

Ligaments

Fibrous connective tissues that join one bone to another. Ligaments are elastic to allow movement at joints while maintaining stability.

Tendon

Tough connective tissue that attaches muscles to bones. Tendons are inelastic, ensuring that muscles are firmly attached to bones for efficient movement.

Joints

The areas or regions where two or more bones meet. Joints provide articulation between bones, making movement possible.

Fixed/immovable joints

Joints that do not allow movement of the bones. These joints provide stability but restrict motion. Examples: The pelvic girdle and sutures (found in the skull).

Movable joints

Joints that allow movement of bones. These joints are designed for flexibility and motion. Examples: The hip joint and shoulder joint.

Movable joints are classified according to the type of movement they allow and their structure. There are four main types:

Ball and socket joints

A type of joint that allows movement of bones in many directions. These joints provide the greatest flexibility of all joint types.

• Examples: Hip joint and shoulder joint.

Structure: The joint involves a rounded head of one bone (like a ball) that fits into a socket of another bone.

Function: The ball and socket joint allows rotation, flexion, extension, and abduction/adduction, offering a wide range of motion.

Example: At the shoulder, the humerus (upper arm bone) fits into the socket of the scapula (shoulder blade).

Synovial fluid: Some ball and socket joints, like the hip joint and shoulder joint, contain synovial fluid, which reduces friction and lubricates the joint for smoother movement.

Hinge joints

Joints that allow movement in one direction, like the hinge of a door. These joints restrict movement to flexion and extension. Examples: Elbow joint, knee joint, and joints between the phalanges (fingers and toes).

Gliding joints

Joints where two bones slide past each other with limited movement, allowing only small gliding motions.

• Examples: Wrist joint and ankle joint.

Pivot joints (peg and socket joints)

Joints that allow one bone to rotate around another bone. Examples: The joint between the first and second cervical vertebrae (atlas and axis), which allows the head to rotate from side to side.

Gliding (sliding) joints

1. Location: These joints are found between the vertebrae and in other areas where two or more bones slide over each other.
2. Movement: Gliding joints allow movement in two directions, such as up and down or slightly rotating. For example, at the wrist and ankle, the bones can move slightly in multiple directions, allowing flexibility and limited rotation.
3. Structure: Unlike other joints, gliding joints lack synovial fluid between the bones. Instead, they have a layer of cartilage that reduces friction between the surfaces of the bones, allowing smooth movement.

Example:

• Between the wrist bones and ankle bones, where slight gliding movements occur during activities like walking and hand movements.

Hinge joints

1. Location: Hinge joints are found in areas like the elbow, knee, and fingers (including the knuckles and phalanges of the toes).
2. Movement: These joints allow movement in one direction only, similar to how a door hinge works. The bone moves in one plane, either flexing (bending) or extending (straightening), like how the elbow bends and straightens or how the knee allows walking and running.

Example:

• Elbow joint: Allows the arm to move in one direction, bending and straightening, just like a door opening and closing.
• Knee joint: Allows the leg to bend and straighten when walking or running.

Pivot joint (on the neck)

Joints are specialized connections between bones that allow for movement. However, the design of joints is adapted to ensure efficient and smooth movement while minimizing the risk of injury such as dislocation and friction. Here's how joints adapt to movement:

Freely movable joints

Examples: Joints like those in the limbs (shoulders, elbows, knees, and wrists) are freely movable. These joints allow a wide range of motion and enable the body to perform various actions.

Risk of dislocation: Because these joints allow for such extensive movement, they are at risk of dislocation (when bones are forced out of their normal position in the joint). This is especially common when the joint is overstretched or subjected to an unnatural force.

Role of ligaments

1. Function: Ligaments are strong connective tissues that hold bones together at a joint. They stabilize the joint by limiting excessive movement that could lead to injury or dislocation.
2. Prevention of dislocation: Ligaments help to prevent dislocation by restricting the range of motion and ensuring the bones stay in their proper position during movement.
3. Reducing friction: Ligaments also help in reducing friction between bones, which can occur during movement if the bones rub directly against each other. This friction is minimized by the smooth cartilage that lines the joints, which also helps with shock absorption.

Strain and compression

1. Bone strain: Joints allow bones to move in ways that generate forces or stress. If the bones are not well protected, these forces can lead to strain in the bones, especially if the joint is overstretched or misaligned during movement.
2. Compression: In some joints, especially weight-bearing ones like the knee and hip, the bones may experience compression when subjected to pressure (e.g., while standing, walking, or running). This compression can cause stress on the bones, but the cartilage within the joint acts as a cushion to reduce damage from this pressure.
3. Protection: The cartilage in the joints acts as a protective layer, reducing friction, cushioning the impact of movement, and preventing the bones from rubbing directly against each other.