Lesson 1, Topic 1
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Anatomy of the Skeletal System

April 11, 2024

Anatomy of the Skeletal System

Learning Objective: Examine the anatomy of the axial skeleton and the appendicular skeleton.

Human bones appear in a variety of shapes and sizes that suit their function in the body. Bones generally are categorized by shape:

• Flat bones protect internal organs. Examples include the cranium, ribs, and sternum.
• Short bones provide stability with their cube shape. Examples include carpals and tarsals.
• Long bones support weight and help with movement. Examples include the humerus, radius, femur, and tibia.
• Irregular bones have an irregular shape and help protect internal organs. The vertebrae are an example of irregular bones.
• Sesamoid bones are embedded in the tendons and help protect the tendons. The patella is the largest sesamoid bone in the body.

The human skeleton is composed of more than 200 bones. Bones are divided into two categories: the axial skeleton and the appendicular skeleton (FIGURE 20.1).

Axial Skeleton

Learning Objective: Identify the bones of the axial skeleton.

The axial skeleton is composed of 80 bones:

• The skull is made up of the following structures:

• Cranium, which encloses and protects the brain. The eight bones that make up the cranium include the frontal, parietal (2), temporal (2), occipital, sphenoid, and ethmoid bones.
• Facial bones, which include the mandible and vomer bone, and two each of the following bones: nasal, zygomatic, lacrimal, palatine, inferior nasal conchae, and maxillary (also called the maxilla or upper jawbone)

• Ossicles, or the three small bones of the middle ear that transmit sound vibrations from the eardrum to the inner ear. The small bones include the malleus (hammer), incus (anvil), and stapes (stirrup).

FIGURE 20.1  Axial skeletal bones (outside columns) and appendicular skeletal bones (middle column).

• The hyoid bone, in the neck above the larynx, is the attachment point for the extrinsic tongue muscle and other mouth muscles.
• The spinal or vertebral column is divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. The spinal column is composed of 26 vertebrae. Soft disks filled with a jelly-like substance are between the vertebrae to cushion and keep them in place.
• The rib cage is made up of these structures:

• Sternum (breastbone), which is made up of the manubrium, body, and xiphoid process.
• Seven pairs of true ribs, which attach to the sternum by costal cartilage.
• Five pairs of false ribs. The first three pairs of false ribs attach to the seventh rib and indirectly to the sternum by costal cartilage. The last two pairs of false ribs are called floating ribs because they are not attached in the front of the body.

The ribs, thoracic vertebral column, and sternum form the thorax.

• Tarsals, which form the heel and the posterior side of the foot. The tarsals include the calcaneus (heel bone), talus, navicular, cuboid, medial cuneiform, intermediate cuneiform, and the lateral cuneiform.
• Metatarsals (feet bones) and the phalanges (toe bones).

20.1

Critical Thinking Application

Suzanne is thinking about Walter as she prepares the exam room for his appointment. Walter’s hands and knees are bothering him. The hands and knees are both a part of the appendicular skeleton. She thought for a moment and quickly named the major structures in the appendicular and axial skeleton. Can you name them?

Anatomy of Long Bones

The outer covering of a long bone is called the periosteum, and the inner lining is called the endosteum. The long bone is composed of the following parts:

• Diaphysis: The long shaft, which is made of hard, compact bone (FIGURE 20.2).
• Medullary cavity: The hollow space inside the diaphysis. Yellow bone marrow is found in the medullary cavity. Yellow bone marrow is a soft, gelatinous tissue that consists mostly of fat cells and a small amount of primitive blood cells.
• Epiphysis: The end of the long bone, which is made of spongy bone. The epiphysis is covered with articular cartilage and is attached by ligaments to the epiphysis of another bone, forming a joint. Articular cartilage reduces the stress of weight bearing and the friction of movement. The thickness of the cartilage depends largely on the amount of stress placed on a particular joint.
• Metaphysis: The narrow strip between the diaphysis and epiphysis. The metaphysis contains the epiphyseal plates (also called growth plates). This is where bone growth normally occurs.

Compact bone is made up of structural units called osteons. Osteons are composed of osteocytes (bone cells) and calcified matrix. The matrix stores phosphorus and calcium for the body to use. The nutrient foramina are small passageways that contain blood vessels that supply osteocytes with nutrients.

Joints

Learning Objective: Describe the different types of joints.

Bones are connected to each other at junctions known as joints (articulations). The range through which a joint can extend and flex is called its range of motion (ROM). Joints can be classified by their ROM:

FIGURE 20.2  Long bone features. From Applegate E: The anatomy and physiology learning system, ed 4, St. Louis, 2011, Saunders.

• Synarthroses: Immovable (no ROM) joints held together by fibrous cartilaginous tissue. An example of this joint is the suture lines of the skull.
• Amphiarthroses: Limited ROM joints, which are joined together by cartilage that is slightly movable. Examples of these joints include the vertebrae and the pubic bones of the pelvic girdle.
• Diarthroses (or synovial joints): Full ROM joints. Examples include the hinge joints in the knee and the ball-and-socket joint in the hip. The following section will provide more details on diarthrotic joints.

20.2

Critical Thinking Application

As Dr. Kahn examines Walter’s hands, he has Walter open and close his hands, wiggle his fingers, grip tightly around two of Dr. Kahn’s fingers, and move his wrist. Throughout the exam, Dr. Kahn watches Walter’s hands and wrists. Dr. Kahn is looking at the ROM in Walter’s hands. Describe in your own words what the acronym ROM means. List the three joint types (based on ROM), and give one example of each.

Diarthrotic Joints

Many of the diarthroses, or synovial joints, have bursae. Bursae are sacs of synovial fluid located between the bones of the joint and the tendons that hold the muscles in place. Bursae help cushion and support the joints when they move. Synovial joints also have joint capsules that enclose the ends of the bones (FIGURE 20.3). A synovial membrane lines the joint capsules. This membrane secretes synovial fluid that lubricates the joint. Joints also have cartilage that covers and protects the bone. The meniscus consists of crescent-shaped cartilage in the knee joint that also cushions the joint. Ligaments are strong bands of white, fibrous connective tissue that connect one bone to another at the joints.

FIGURE 20.2  Long bone features. From Applegate E: The anatomy and physiology learning system, ed 4, St. Louis, 2011, Saunders.

There are six classifications of diarthroses (or synovial joints). Each type has its own unique movement.

Hinge joint: Permits flexion and extension (TABLE 20.1). Examples include the elbow, knee, and finger joints.
Pivot joint: Permits rotation. Examples of pivot joints include the joint found between the atlas and the axis (first and second cervical vertebrae) and the joint between the ulna and radius.
Saddle joint: Allows for flexion, extension, and other movements. An example is the thumb crossing over the palm of the hand.
Condyloid joint: Permits flexion, extension, and circular motion. An example would be the movement of the atlas.
Ball-and-socket joint: Allows free movement (rotation). Examples include the shoulder and hip joints.
Gliding joint: Allows a bone to slide over another bone. This occurs in the wrist and between the vertebrae.