Chapter 32.1

Lesson OverviewLesson Overview The Skeletal SystemThe Skeletal System Lesson OverviewLesson Overview The Skeletal SystemThe Skeletal System

Lesson OverviewLesson Overview32.1 The Skeletal System32.1 The Skeletal System

Lesson OverviewLesson Overview The Skeletal SystemThe Skeletal System

The Skeleton

What are the functions of the skeletal system?

Structure of the Skeleton

There are 206 bones in the adult human skeleton. As seen in the figure, some of these bones are in the axial skeleton and others are in the appendicular skeleton.

The axial skeleton supports the central axis of the body and consists of the skull, the vertebral column, and the rib cage.

The bones of the arms and legs, along with the bones of the pelvis and shoulder area, form the appendicular skeleton.

Functions of the Skeletal System

The skeleton supports the body, protects internal organs, assists movement, stores minerals, and is a site of blood cell formation.

What is the structure of a typical human bone?

Most of the mass of bone is mineral salts—mainly calcium and phosphorus. However, bones are living tissue. Bones are a solid network of living cells and protein fibers that are surrounded by deposits of calcium salts.

Structure of Bones

The structure of a typical long bone is shown.

Structure of Bones

The bone is surrounded by a tough layer of connective tissue called periosteum.

Structure of Bones

Beneath the periosteum is a thick layer of compact bone.

Structure of Bones

Nerves and blood vessels run through compact bone in channels called Haversian canals.

Structure of Bones

A less dense tissue known as spongy bone may be found under the outer layer of compact bone.

Structure of Bones

Spongy bone is found in the ends of long bones such as the femur and in the middle of short, flat bones such as those found in the skull. The latticework structure in spongy bone adds strength without adding excess mass.

Structure of Bones

Within many bones are cavities that contain a soft tissue called bone marrow.

Yellow marrow consists primarily of cells that store fat.

Red marrow contains stem cells that produce most types of blood cells.

Development of Bones

The skeleton of a human embryo is composed almost entirely of a type of connective tissue called cartilage. Cartilage is gradually replaced by bone during the process of bone formation called ossification, which begins up to seven months before birth.

Bone tissue forms as cells called osteoblasts secrete mineral deposits that replace the cartilage in developing bones. As bone tissue completes its development, most osteoblasts mature into osteocytes, which help to maintain the minerals in bone tissue and continue to strengthen the growing bone.

Many long bones have growth plates at either end.

The growth of cartilage at these plates causes the bones to lengthen. The cartilage is gradually replaced by bone. During late adolescence or early adulthood, growth plates become completely ossified, and the person “stops growing.”

Cartilage remains in parts of the body that are flexible, such as the tip of the nose and the external part of ears. It also cushions the areas where bones meet, such as in the knee.

Bone Remodeling and Repair

Some older adults, especially women, develop a disorder called osteoporosis. Osteoporosis leads to weak bones due to excessive decrease in bone density.

Joints

What is the role of joints?

Joints

A place where one or more bones meet another bone is called a joint. The knee joint is shown. Joints contain connective tissues that hold bones together. Joints permit bones to move without damaging each other.

Types of Joints

Depending on its type of movement, a joint is classified as immovable, slightly movable, or freely movable.

Immovable Joints

Immovable joints, often called fixed joints, allow no movement. The bones at an immovable joint are interlocked and grow together until they are fused. The places where the bones in the skull meet are examples of immovable joints.

Slightly Movable Joints

Slightly movable joints permit a small amount of movement. The bones of slightly movable joints are separated from each other. The joints between the two bones of the lower leg and the joints between vertebrae are examples of slightly movable joints.

Freely Movable Joints

Freely movable joints permit movement in two or more directions and are grouped according to the shapes of the surfaces of the adjacent bones.

-Hinge-Ball and Socket-Pivot-Saddle

Types of Joints

Many freely movable joints are involved in the movements of a gymnast.

Structure of Joints

In freely movable joints, cartilage covers the surfaces where two bones come together and protects the bones from damage as they move against each other. The joints are also surrounded by a fibrous joint capsule that helps hold the bones together while still allowing for movement.

Structure of Joints

The outer layer of the joint capsule forms strips of tough connective tissue called ligaments, which hold bones together in a joint and are attached to the membranes that surround bones.

Structure of Joints

The inner layer of the joint capsule, called the synovial cavity, produces a substance called synovial fluid. Synovial fluid enables the surfaces of the bones connected at the joint to slide over each other smoothly.

Structure of Joints

In some freely movable joints, such as the knee, there are small sacs of synovial fluid called bursae (singular: bursa). Bursae reduce the friction between the bones of a joint and any tissues they come in contact with and also act as tiny shock absorbers.

Joint Injuries

A common injury among athletes is damage to the anterior cruciate ligament (ACL) of the knee. This ligament is found in the center of the knee between the femur and the tibia. It prevents the tibia from shifting too far forward during movements. ACL damage can be caused by the rapid pivoting, leaping, and forceful contacts that occur when playing sports like basketball and soccer.

Joint Injuries

Excessive strain on a joint may produce inflammation, a response in which excess fluid causes swelling, pain, heat, and redness. Inflammation of a bursa is called bursitis.

Joint Injuries

Wear and tear over the years often leads to osteoarthritis, which develops as the cartilage of often-used joints in the fingers, knees, hips, and spine begins to break down. The affected joints become painful and stiff as unprotected bones start to rub together.

Chapter 32.1

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