Chapter 8: The Skeletal System - The Appendicular Skeleton

Overview of the Appendicular Skeleton

• The appendicular skeleton consists of a total of $126$ bones.

• Primary Function: These bones are primarily involved in body movements.

• Components: As "appendages" to the central (axial) skeleton, they include:

  • Bones of the upper limbs.

  • Bones of the lower limbs.

  • The girdles (pectoral and pelvic) that attach these limbs to the axial skeleton.

The Pectoral (Shoulder) Girdle

• Structure: There are two pectoral girdles, each consisting of two bones:

  • Clavicle (collarbone).

  • Scapula (shoulder blade).

• The Clavicle:

  • Shape: S-shaped.

  • Medial (Sternal) End: Articulates with the manubrium of the sternum.

  • Lateral (Acromial) End: Articulates with the acromion of the scapula.

• The Scapula:

  • Type: A flat bone.

  • Location: Located in the superior part of the posterior thorax between the second ($2^{nd}$) and seventh ($7^{th}$) ribs.

  • Glenoid Cavity: This cavity serves as the attachment point for the humerus.

Upper Limb: The Humerus

• Definition: The humerus is the bone of the arm.

• Proximal Articulation: The rounded head of the humerus fits into the glenoid cavity of the scapula.

• Distal Articulation: Articulates with the radius and ulna.

  • The trochlea articulates with the ulna.

  • The capitulum articulates with the radius.

• Key Surface Markings:

  • Head and Anatomical neck.

  • Greater tubercle and Lesser tubercle.

  • Intertubercular sulcus (groove).

  • Surgical neck (a common site of fracture).

  • Body (shaft) and Deltoid tuberosity.

  • Radial groove.

  • Radial fossa (anterior) and Olecranon fossa (posterior).

  • Coronoid fossa.

  • Lateral epicondyle and Medial epicondyle.

Upper Limb: The Forearm (Ulna and Radius)

• The Ulna:

  • Location: Medial side of the forearm.

  • Proximal End: Features the olecranon and coronoid process which form the trochlear notch.

  • The trochlear notch wraps around the trochlea of the humerus to form the elbow joint.

  • Ulnar tuberosity: Located at the proximal end.

  • Distal End: Features the head of the ulna and the styloid process of the ulna.

• The Radius:

  • Location: Lateral (thumb) side of the forearm.

  • Proximal End: Features the head and neck of the radius. The head articulates with the capitulum of the humerus and the radial notch of the ulna, allowing for forearm rotation.

  • Radial tuberosity: A point of attachment for muscles.

  • Distal End: Features the styloid process of the radius and the ulnar notch of the radius.

• Interosseous Membrane: A fibrous tissue that connects the shafts of the ulna and radius.

Upper Limb: Carpals, Metacarpals, and Phalanges

• Carpals (Wrist Bones):

  • Total: $8$ small bones connected by ligaments.

  • Arrangement: Two rows of four bones each.

  • Proximal Row (Lateral to Medial): Scaphoid, Lunate, Triquetrum, Pisiform.

  • Distal Row (Lateral to Medial): Trapezium, Trapezoid, Capitate, Hamate.

  • Mnemonic: "Stop Letting Those People Touch The Cadaver's Hand" (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).

• Metacarpals (Palm Bones):

  • Total: $5$ bones making up the palm and back of the hand.

  • Numbering: I through V (or $1-5$) starting from the thumb (lateral) to the little finger (medial).

  • Bases: Articulate with the distal carpal bones.

  • Heads: Articulate with the proximal phalanges (commonly known as knuckles).

• Phalanges (Digit Bones):

  • Total: $14$ bones in each hand.

  • Thumb (Pollex): Contains $2$ phalanges (proximal and distal).

  • Fingers: The other four fingers each contain $3$ phalanges (proximal, middle, and distal).

The Pelvic (Hip) Girdle

• Composition: Made up of two hip bones (os coxa or coxal bones).

• Posterior Articulation: The hip bones articulate with the sacrum.

• Anterior Articulation: The hip bones articulate with each other at the pubic symphysis, which contains a disc of fibrocartilage.

• Individual Bone Components: Each hip bone is composed of three fused bones:

  • Ilium (superior portion).

  • Ischium (inferior, posterior portion).

  • Pubis (inferior, anterior portion).

• Acetabulum: The deep socket where the head of the femur articulates to form a ball-and-socket joint. It is composed of parts of all three bones (ilium, ischium, and pubis).

• Key Markings:

  • Ilium: Iliac crest, Anterior Superior Iliac Spine (ASIS), Iliac fossa, Greater sciatic notch.

  • Ischium: Ischial spine, Ischial tuberosity (the bone you sit on), Lesser sciatic notch.

  • Pubis: Pubic tubercle, Pubic crest, Obturator foramen (the large hole formed by the ischium and pubis).

True and False Pelves

• Pelvic Brim: The boundary line that divides the pelvis into superior and inferior portions; indicates where the abdomen meets the pelvic cavity.

• False (Greater) Pelvis: The area of the bony pelvis superior to the pelvic brim.

• True (Lesser) Pelvis: The area of the bony pelvis inferior to the pelvic brim.

Comparison of Female and Male Pelves

• General Principles: Male bones are usually larger and heavier; female bones are structured for pregnancy and childbirth (wider and shallower).

• Specific Comparisons (Table 8.1):

  • General Structure: Female is light and thin; Male is heavy and thick.

  • False Pelvis: Female is shallow; Male is deep.

  • Pelvic Brim (Inlet): Female is wide and more oval; Male is narrow and heart-shaped.

  • Acetabulum: Female is small and faces anteriorly; Male is large and faces laterally.

  • Obturator Foramen: Female is oval; Male is round.

  • Pubic Arch Angle: Female is greater than a $90^{\circ}$ angle; Male is less than a $90^{\circ}$ angle.

  • Iliac Crest: Female is less curved; Male is more curved.

  • Ilium: Female is less vertical; Male is more vertical.

  • Greater Sciatic Notch: Female is wide (almost $90^{\circ}$); Male is narrow (about $70^{\circ}$, inverted V).

  • Sacrum: Female is shorter, wider, and less curved anteriorly; Male is longer, narrower, and more curved anteriorly.

  • Pelvic Outlet: Female is wider; Male is narrower.

  • Ischial Tuberosity: Female bones are shorter, farther apart, and more medially projecting; Male bones are longer, closer together, and more laterally projecting.

Lower Limb: The Femur and Patella

• The Femur:

  • Characteristics: The longest, heaviest, and strongest bone in the human body.

  • Proximal End: The head inserts into the acetabulum of the hip bone. Features the neck, greater trochanter, lesser trochanter, and intertrochanteric line/crest.

  • Body (Shaft): Features the gluteal tuberosity and the linea aspera.

  • Distal End: Articulates with the tibia and patella. Features the medial and lateral condyles, medial and lateral epicondyles, adductor tubercle, and intercondylar fossa.

• The Patella (Kneecap):

  • Type: A triangular sesamoid bone.

  • Development: It develops within the quadriceps tendon.

  • Articulation: Its posterior surface articulates with the femur.

Lower Limb: The Leg (Tibia and Fibula)

• The Tibia (Shin Bone):

  • Location: Medial side of the leg.

  • Proximal end articulates with the femur (medial and lateral condyles).

  • Distal end articulates with the talus bone of the ankle.

  • Tibial Tuberosity: Located on the anterior surface; serves as the attachment point for the patellar ligament.

  • Medial Malleolus: The medial bulge at the ankle.

  • Anterior Border: Known as the crest.

• The Fibula:

  • Location: Lateral side of the leg.

  • Size: Parallel to the tibia and significantly thinner.

  • Head: Located at the proximal end.

  • Lateral Malleolus: The lateral bulge at the ankle.

  • Mnemonic for Location: "The fibuLA is LAteral."

• Interosseous Membrane: Connects the shafts of the tibia and fibula.

Lower Limb: Tarsals, Metatarsals, and Phalanges

• Tarsus (Ankle Bones):

  • Total: $7$ tarsal bones.

  • Bones: Talus (articulates with tibia/fibula), Calcaneus (heel bone), Navicular, Cuboid, and three cuneiforms (Third/Lateral, Second/Intermediate, and First/Medial).

  • Mnemonic: "Tall Centers Never Take Shots From Corners" (Talus, Calcaneus, Navicular, Third cuneiform, Second cuneiform, First cuneiform, Cuboid).

• Metatarsus (Foot Bones):

  • Total: $5$ metatarsal bones.

  • Numbering: I through V (or $1-5$) starting from the big toe (medial).

  • Anatomy: Make up the sole and dorsal surface of the foot. Consist of a base, shaft, and head.

  • Proximal Articulation: Articulate with the three cuneiform bones and the cuboid.

  • Distal Articulation: Articulate with the proximal phalanges.

• Phalanges (Toe Bones):

  • Arrangement: Exactly like the hand.

  • Great Toe (Hallux): Contains $2$ phalanges (proximal and distal).

  • Other Toes: Contain $3$ phalanges (proximal, middle, and distal).

Arches of the Foot

• Structure: Supported by ligaments and tendons.

• Purposes:

  • Allow the foot to support body weight.

  • Provide leverage during walking.

  • Distribute body weight over the foot.

• Types of Arches:

  1. Longitudinal Arch: Consists of a medial part and a lateral part.

  2. Transverse Arch.

Development of the Skeletal System

• Origin: Most skeletal tissue arises from the mesoderm (middle primary germ layer). Most of the skull arises from the ectoderm (outer layer).

• Skull Development:

  • Cartilaginous Neurocranium: Hyaline cartilage undergoing endochondral ossification.

  • Membranous Neurocranium: Undergoes intramembranous ossification.

• Facial Development: Bones of the face form from the viscerocranium.

  • Cartilaginous Viscerocranium: Comes from the pharyngeal arches; forms the ear bones (stapes, incus, malleus) and hyoid bone.

  • Membranous Viscerocranium: Comes from the mesenchyme of the first pharyngeal arch; undergoes intramembranous ossification to form facial bones.

• Limb Development: Derived from mesoderm.

  • Timeline: Week $4$ to Week $8$ after fertilization involves extensive growth.

  • Week 4: Development of free limb buds.

  • Week 6: Development of hand and foot plates.

  • Week 7: Development of arm, forearm, hand, thigh, leg, and foot segments.

  • Week 8: Free limb buds have developed into distinct upper and lower limbs.

The Skeletal System and Homeostasis

• General Role: The skeletal system maintains homeostasis for every body system directly and indirectly.

• Specific System Contributions:

  • Integumentary: Provides support for overlying skin and muscles.

  • Lymphatic/Immune: Red bone marrow produces lymphocytes (white blood cells).

  • Muscular: Provides attachment points and leverage for movement. Bone store is a source of calcium ions ($Ca^{2+}$) required for muscle contraction.

  • Respiratory: Thoracic axial skeleton protects lungs. Rib movements assist breathing. Muscles for breathing attach to bones.

  • Nervous: Skull protects the brain; vertebrae protect the spinal cord. Calcium is required for neuron and neuroglia function.

  • Endocrine: Bones store and release calcium needed for hormone exocytosis and hormone actions.

  • Digestive: Teeth chew food; rib cage protects esophagus, stomach, and liver; pelvis protects intestines.

  • Urinary: Ribs protect kidneys; pelvis protects the bladder and urethra.

  • Cardiovascular: Red bone marrow performs hemopoiesis (blood cell formation). Heart rhythm requires calcium.

  • Reproductive: Pelvis protects ovaries, uterus, and fallopian tubes in females, and the ductus deferens and accessory glands in males. Bones provide calcium for milk synthesis during lactation.

  • General: Bones provide support and protection for internal organs and act as a reservoir for calcium for most body tissues.