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Chapter 07: Skeletal System

The Forearm Bones: Radius and Ulna

Radius

  • Location: Situated on the thumb (lateral) side of the forearm, extending from the elbow downward to the wrist.

  • Articulations:

    • The disc-like head of the radius articulates with the capitulum of the humerus.

    • It also articulates with the radial notch of the ulna, which facilitates the rotation of the radius.

  • Key Features:

    • Radial tuberosity: A prominent bump near the neck of the radius.

    • Styloid process: A pointed projection at the distal end of the radius.

Ulna

  • Location: The longer of the two bones in the forearm. It is positioned on the medial side in anatomical position.

  • Articulations:

    • The prominent trochlear notch articulates with the trochlea of the humerus at the elbow joint.

    • The distal end, referred to as the head of the ulna, articulates with the ulnar notch of the radius.

  • Key Features:

    • Olecranon process: The bony projection of the elbow that forms the posterior aspect of the trochlear notch.

    • Coronoid process: A triangular eminence that projects from the anterior aspect of the ulna, forming the anterior part of the trochlear notch.

    • Head: The distal end of the ulna.

    • Styloid process: A pointed projection at the distal end of the ulna.

The Hand

  • Composition: The hand is anatomically divided into the wrist, the palm, and the fingers.

Wrist (Carpus)

  • Bones: Comprised of 8 carpal bones.

  • Arrangement: These carpal bones are organized into 2 rows, with 4 bones in each row.

  • Collective Term: The entire group of carpal bones is collectively known as the carpus.

Palm (Metacarpals)

  • Framework: The structural framework of the palm is formed by 5 metacarpal bones.

Fingers (Phalanges)

  • General Structure: Each finger is typically composed of 3 phalanges: a proximal, a middle, and a distal phalanx.

  • Thumb Exception: The thumb is an exception, having only 2 phalanges (it lacks a middle phalanx).

The Pelvic Girdle

  • Components: Consists of 2 hip bones, also known as coxal bones, pelvic bones, or innominate bones.

  • Articulations:

    • The hip bones articulate with each other anteriorly at the pubic symphysis.

    • They articulate posteriorly with the sacrum.

  • The Pelvis: The term "pelvis" encompasses the sacrum, the coccyx, and the two hip bones (pelvic girdle).

  • Functions:

    • Provides support for the trunk of the body on the lower limbs.

    • Serves as points of attachment for the lower limbs.

    • Protects vital organs situated within it, such as the urinary bladder and the terminal end of the large intestine.

The Coxal (Hip) Bone

  • Composition: Each coxal bone is a fusion of 3 distinct bones: the ilium, ischium, and pubis.

  • Fusion Point: These three bones are fused together in the region of the acetabulum.

    • Acetabulum: A large, cup-like depression that serves as the articulation point for the head of the femur.

Ilium

  • Description: The largest and uppermost portion of the hip bone.

  • Key Features:

    • Iliac crest: The superior border of the ilium.

    • Sacroiliac joint: The articulation point between the ilium and the sacrum.

    • Anterior superior iliac spine: A prominent bony projection at the anterior end of the iliac crest.

Ischium

  • Description: Forms the L-shaped, posterior and inferior portion of the hip bone, crucial for supporting body weight when sitting.

  • Key Features:

    • Ischial tuberosity: A roughened prominence that bears the body's weight during sitting.

    • Ischial spine: A pointed projection superior to the ischial tuberosity.

Pubis

  • Description: Constitutes the anterior portion of each hip bone.

  • Articulation: The two pubic bones join anteriorly at the pubic symphysis.

  • Key Features:

    • Pubic arch: The angle formed by the inferior rami of the two pubic bones.

    • Obturator foramen: A large opening in the hip bone, partially closed by a membrane.

    • Pelvic brim: The boundary line that separates the greater (false) pelvis from the lesser (true) pelvis.

Differences Between Male and Female Pelvic Regions (Table 7.3 Summary)

  • Pelvic Girdle:

    • Female: Hip bones are generally lighter, thinner, and exhibit less prominent evidence of muscular attachments. The obturator foramina are typically triangular. The acetabula are smaller, and the pubic arch is notably wider.

    • Male: Hip bones are heavier, thicker, with more pronounced muscular attachments. The obturator foramina are oval. The acetabula are larger, and the pubic arch is narrower.

  • Pelvic Cavity:

    • Female: The pelvic cavity is wider in all its diameters, shorter, roomier, and less funnel-shaped. The distances between the ischial spines and ischial tuberosities are greater.

    • Male: The pelvic cavity is narrower, longer, and more funnel-shaped, with shorter distances between ischial spines and tuberosities.

  • Sacrum:

    • Female: The sacrum is wider, and its sacral curvature is bent more sharply posteriorly.

    • Male: The sacrum is narrower, and its sacral curvature is less sharply bent.

  • Coccyx:

    • Female: The coccyx is more movable.

    • Male: The coccyx is less movable.

The Lower Limb

  • Function: The bones of the lower limb provide the fundamental framework for the thigh, the lower leg, and the foot.

  • Bones of the Lower Limb:

    • Femur

    • Tibia

    • Fibula

    • Patella

    • Tarsals

    • Metatarsals

    • Phalanges

Femur (Thighbone)

  • Description: The longest bone in the human body, extending from the hip to the knee.

  • Articulations:

    • The spherical head of the femur articulates with the acetabulum of the hip bone.

    • Distally, its medial and lateral condyles articulate with the tibia at the knee joint.

  • Key Features:

    • Fovea capitis: A small pit on the head of the femur for the attachment of a ligament.

    • Neck: The constricted region connecting the head to the shaft.

    • Greater trochanter: A large, irregular prominence located posterolateral to the neck.

    • Lesser trochanter: A smaller, conical prominence located posteromedial to the neck.

    • Gluteal tuberosity: A roughened area on the posterior shaft for muscle attachment.

    • Linea aspera: A prominent ridge running down the posterior shaft of the femur.

    • Medial and Lateral epicondyles: Projections above the condyles for ligament attachment.

    • Patellar surface: A smooth groove on the anterior distal end where the patella articulates.

    • Intercondylar fossa: A deep notch between the medial and lateral condyles posteriorly.

Patella (Kneecap)

  • Description: A triangular bone situated within the tendon that passes anteriorly over the knee.

  • Articulation: Articulates with the patellar surface on the distal anterior surface of the femur.

The Tibia and Fibula

Tibia (Shinbone)

  • Description: The larger and more medically located of the two lower leg bones. It is the primary weight-bearing bone of the lower leg.

  • Articulations:

    • Articulates with the femur at its medial and lateral condyles.

    • Articulates distally with the talus bone of the ankle.

  • Key Features:

    • Anterior tibial tuberosity: A roughened projection on the anterior surface, serving as the point of attachment for the patellar ligament (which is a continuation of the patellar tendon).

    • Medial malleolus: A prominent projection on the medial side of the distal tibia, forming the inner ankle bone.

    • Intercondylar eminence: A projection between the medial and lateral condyles of the tibia.

    • Anterior crest: The sharp ridge forming the anterior border of the tibia.

Fibula

  • Description: A slender bone positioned lateral to the tibia.

  • Weight Bearing: Crucially, the fibula does not bear body weight and is not a part of the knee joint.

  • Key Features:

    • Head: The proximal end of the fibula, articulating with the tibia.

    • Distal lateral malleolus: A prominent projection on the lateral side of the distal fibula, forming the outer ankle bone.

The Foot

  • Composition: The foot is comprised of the ankle (tarsus), the instep (metatarsus), and the toes.

Ankle (Tarsus)

  • Bones: Consists of 7 tarsal bones.

  • Key Tarsal Bones and Functions:

    • Talus: Articulates with both the tibia and fibula, forming the ankle joint.

    • Calcaneus: Commonly known as the heel bone, it plays a vital role in supporting the body's weight.

Instep (Metatarsus)

  • Bones: Composed of 5 long metatarsal bones.

  • Structure: The distal heads of these metatarsal bones form the "ball of the foot."

  • Function: Along with the tarsals, the metatarsals form the arches of the foot, which are essential for weight distribution and shock absorption.

Toes (Phalanges)

  • General Structure: Each toe typically consists of 3 phalanges: a proximal, a middle, and a distal phalanx.

  • Great Toe Exception: The great toe (hallux) is an exception, having only 2 phalanges (it lacks a middle phalanx).

Joints (Articulations)

  • Definition: Joints, also known as articulations, are functional junctions where bones meet.

  • Functions:

    • They enable a wide variety of body movements.

    • Bind various parts of the body together.

    • Make bone growth possible.

    • Permit shape changes, particularly significant during childbirth.

Classification of Joints (By Degree of Movement - Functional)

  • Immovable (Synarthrotic): Joints that allow no movement.

  • Slightly Movable (Amphiarthrotic): Joints that permit limited movement.

  • Freely Movable (Diarthrotic): Joints that allow a wide range of motion.

Classification of Joints (By Type of Tissue - Structural)

  • Fibrous Joints: Bones are bound together by dense connective tissue.

  • Cartilaginous Joints: Bones are connected by cartilage (either hyaline cartilage or fibrocartilage).

  • Synovial Joints: Characterized by a complex structure where bones are joined by ligaments and separated by a fluid-filled joint cavity.

Relationship Between Mobility and Stability in Joints

  • Range of Motion (ROM): Joints exhibit a wide spectrum of motion, from complete immobility to extensive movement.

  • Determinants: The specific structure of each joint dictates its inherent mobility and stability.

  • Inverse Relationship (Tradeoff): There is an inverse relationship between a joint's mobility and its stability. Generally, joints that are highly mobile tend to be less stable, and vice-versa.

    • Example 1: Skull sutures are synarthrotic (immobile) but offer exceptional stability, protecting the brain effectively.

    • Example 2: The shoulder joint is diarthrotic (freely movable) allowing extensive range of motion, but due to its high mobility, it is relatively less stable and more prone to injury or dislocation.

Types of Joints in Detail

Fibrous Joints

  • Description: Found between bones that are in very close contact.

  • Binding Tissue: Held tightly together by a thin layer of dense connective tissue.

  • Movement:

    • Most are immovable (synarthrotic): Classic examples are the sutures found between the bones of the skull.

    • A few are slightly movable (amphiarthrotic): An example includes the distal joint between the tibia and fibula.

Cartilaginous Joints

  • Binding Tissue: Connected by either hyaline cartilage or fibrocartilage.

  • Movement: Allow limited movement.

  • Examples:

    • Intervertebral discs: Located between adjacent vertebrae, these fibrocartilage discs help absorb shock and permit slight movement when the back is bent or twisted.

    • Pubic symphysis: The joint connecting the two pubic bones anteriorly.

    • The joint between the first rib and the sternum.

Synovial Joints

  • Movement: These are diarthrotic joints, meaning they allow free movement and represent the most common type of joint in the skeleton.

  • Complex Structure: Synovial joints possess a more intricate anatomical arrangement compared to fibrous or cartilaginous joints.

  • Components:

    • Articular Cartilage: The articular ends of the bones within a synovial joint are covered with a layer of hyaline cartilage (articular cartilage), which provides a smooth, low-friction surface.

    • Joint Capsule: Surrounds the joint, consisting of two layers:

      • Outer Fibrous Layer: Composed of dense connective tissue, continuous with the periosteum of the bones. It contains ligaments that prevent the bones from being pulled apart.

      • Inner Synovial Membrane: Lines the fibrous layer internally.

    • Synovial Fluid: Secreted by the synovial membrane, this viscous fluid lubricates the joint surfaces, reduces friction, and nourishes the articular cartilage.

    • Menisci (singular: Meniscus): Some synovial joints (e.g., the knee) contain crescent-shaped pads of fibrocartilage. These menisci act as shock absorbers and help to improve the fit between articulating bones.

    • Bursae (singular: Bursa): Fluid-filled sacs located in some synovial joints, often between skin and bone, tendons and bone, or muscles and bone. They reduce friction and aid in the smooth movement of tendons as they slide over bones.

Classification of Synovial Joints (By Shape of Parts and Allowed Movements)

  • 1. Ball-and-Socket (Spheroidal) Joint:

    • Description: Consists of a bone with a globular or nearly egg-shaped head articulating with a cup-shaped cavity of another bone.

    • Movements: Allows the widest range of motion, including movement in all planes (multiaxial), such as flexion, extension, abduction, adduction, circumduction, and rotation.

    • Examples: The shoulder joint and the hip joint.

  • 2. Condylar (Ellipsoidal) Joint:

    • Description: Features an ovoid condyle of one bone fitting into an elliptical cavity of another bone.

    • Movements: Permits back-and-forth and side-to-side movements within two planes (biaxial), but does not allow rotation around a central axis.

    • Examples: The joints between the metacarpals and phalanges (knuckles).

  • 3. Plane (Gliding) Joint:

    • Description: Characterized by articulating surfaces that are nearly flat or slightly curved.

    • Movements: Allows only sliding or twisting movements (nonaxial), where one surface glides over another.

    • Examples: Joints of the wrist (intercarpals), joints of the ankle (intertarsals), sacroiliac joints, and joints between vertebrae (facet joints).

  • 4. Hinge Joint:

    • Description: A convex surface of one bone fits into a concave surface of another bone.

    • Movements: Allows movement in only one plane (uniaxial), similar to the hinge of a door (primarily flexion and extension).

    • Examples: The elbow joint and the joints between the phalanges (interphalangeal joints).

  • 5. Pivot (Trochoid) Joint:

    • Description: A cylindrical surface of one bone rotates within a ring formed by bone and ligament.

    • Movements: Allows only rotation movement around a central axis (uniaxial).

    • Examples: The joint between the dens of the axis (C2 vertebra) and the atlas (C1 vertebra), which allows head rotation.

  • 6. Saddle (Sellar) Joint:

    • Description: Found between bones that possess both concave and convex areas in their articulating surfaces, with complementary shapes fitting together like a saddle.

    • Movements: Permits a variety of movements, mainly in two planes (biaxial), including flexion, extension, abduction, adduction, and circumduction, but restricted rotation.

    • Example: The carpometacarpal joint of the thumb (between the trapezium carpal bone and the first metacarpal bone), which allows the thumb's unique range of motion.

Types of Joint Movements

  • Mechanism: When skeletal muscles contract, their fibers pull the more movable end, known as the insertion, toward the more fixed end, called the origin, resulting in a specific movement at a joint.

Terms Describing Joint Movements:

  • Flexion: Bending parts at a joint, which decreases the angle between the bones involved.

  • Extension: Straightening parts at a joint, which increases the angle between the bones involved.

  • Lateral Flexion: Bending the head, neck, or trunk to the side.

  • Hyperextension: Extending or straightening a joint beyond its normal anatomical position (e.g., bending the head backward past the neutral position).

  • Abduction: Moving a body part away from the midline of the body.

  • Adduction: Moving a body part toward the midline of the body.

  • Dorsiflexion: Ankle movement that brings the foot closer to the shin (lifting the foot).

  • Plantar Flexion: Ankle movement that moves the foot farther from the shin (pointing the toes).

  • Rotation: Movement of a part around its own axis (e.g., twisting the head from side to side).

  • Circumduction: Movement of a part so that its distal end follows a circular path, combining flexion, extension, abduction, and adduction.

  • Pronation: Rotation of the forearm so that the palm is facing downward or posteriorly.

  • Supination: Rotation of the forearm so that the palm is facing upward or anteriorly.

  • Inversion: Turning the sole (plantar surface) of the foot medially.

  • Eversion: Turning the sole (plantar surface) of the foot laterally.

  • Protraction: Moving a part of the body forward (e.g., pushing the jaw forward).

  • Retraction: Moving a part of the body backward (e.g., pulling the jaw backward).

  • Elevation: Raising a part of the body (e.g., shrugging the shoulders).

  • Depression: Lowering a part of the body (e.g., dropping the shoulders).