The Appendicular Skeleton

Overview

The appendicular skeleton consists of the bones that attach the limbs to the trunk and those of the limbs themselves. It is divided into the pectoral girdle (shoulder), pelvic girdle (hip), and the bones of the upper and lower limbs.

• Pectoral girdle: Attaches the upper limbs to the trunk.

• Pelvic girdle: Attaches the lower limbs to the trunk.

• Upper and lower limbs differ in function but share a similar structural plan.

The Pectoral Girdle

Structure and Function

The pectoral girdle is composed of the clavicle and scapula. It provides attachment for muscles that move the upper limb and allows for a high degree of mobility.

• Clavicle: Articulates medially with the manubrium and first rib, laterally with the scapula.

• Scapula: Does not join each other or the axial skeleton, allowing free movement.

• Only the clavicle articulates with the axial skeleton.

• The socket of the shoulder joint (glenoid cavity) is shallow, favoring flexibility over stability.

Clavicle

The clavicle extends horizontally across the superior thorax and serves as a strut to keep the scapula and arms laterally positioned.

• Sternal end: Articulates with the manubrium.

• Acromial end: Articulates with the scapula.

• Transmits compression forces from the upper limbs to the axial skeleton.

• Provides attachment for muscles.

Scapula

The scapula lies on the dorsal surface of the rib cage, between ribs 2–7. It has three borders (superior, medial, lateral) and three angles (lateral, superior, inferior).

• Provides attachment for muscles.

• Allows for a wide range of motion of the upper limb.

The Upper Limb

Bone Groups

Each upper limb contains thirty bones, grouped into three sets:

• Arm: Humerus

• Forearm: Radius and ulna

• Hand: Carpals, metacarpals, phalanges

Arm (Humerus)

The humerus is the only bone of the arm, articulating with the scapula at the shoulder and with the radius and ulna at the elbow. It is the longest and strongest bone of the upper limb.

Forearm

The forearm is formed from the radius and ulna.

• Proximal ends articulate with the humerus; distal ends with the carpals.

• Radius and ulna articulate with each other at the proximal and distal radioulnar joints.

• The interosseous membrane interconnects the radius and ulna.

• In anatomical position, the radius is lateral and the ulna is medial.

Ulna

• Main bone responsible for forming the elbow joint with the humerus.

• Hinge joint allows forearm to bend on arm.

• Distal end separated from carpals by fibrocartilage; plays little role in hand movement.

• Major landmarks: Olecranon, radial notch, trochlear notch, coronoid process, ulnar styloid process.

Radius

• Superior surface of the head articulates with the capitulum.

• Medially, head articulates with the radial notch of the ulna.

• Contributes heavily to the wrist joint; distal radius articulates with carpal bones.

• Major landmarks: Head, neck, radial tuberosity, radial styloid process.

Hand

The hand includes the carpals (wrist), metacarpals (palm), and phalanges (fingers).

Carpus

• Forms the true wrist; composed of eight marble-sized bones arranged in two rows.

• Proximal row (lateral to medial): Scaphoid, lunate, triquetrum, pisiform.

• Distal row (lateral to medial): Trapezium, trapezoid, capitate, hamate.

• Mnemonic: "Sally left the party to take Carmen home".

Metacarpus

• Five metacarpals radiate distally from the wrist, forming the palm.

• Numbered beginning with the pollex (thumb).

• Bases articulate proximally with carpals; heads articulate distally with proximal phalanges.

Phalanges

• Numbered beginning with the pollex (thumb).

• Each finger (except thumb) has three phalanges: proximal, middle, distal.

The Pelvic Girdle

Structure and Function

The pelvic girdle attaches the lower limbs to the spine and supports visceral organs. It is more stable than the pectoral girdle, with less freedom of movement.

• Consists of paired hip bones (coxal bones) and the sacrum.

• Coxal bones unite anteriorly and articulate posteriorly with the sacrum.

• In childhood, consists of three separate bones: ilium, ischium, pubis.

• Acetabulum: Deep socket on lateral pelvic surface, receives the head of the femur.

Ilium

• Large, flaring bone forming the superior region of the coxal bone.

• Site of attachment for many muscles.

• Articulates with the sacrum at the sacroiliac joint.

Ischium

• Forms posteroinferior region of the coxal bone.

• Joins the pubis anteriorly.

• Ischial tuberosities are the strongest part of the hip bone.

Pubis

• Forms the anterior region of the coxal bone.

• Pubic symphysis: Two pubic bones joined by fibrocartilage at the midline.

• Pubic arch: Angle helps distinguish male from female pelvis.

Pelvic Structure and Childbearing

The bony pelvis is divided into two regions:

• False (greater) pelvis: Bounded by alae of the iliac bones.

• True (lesser) pelvis: Inferior to pelvic brim, forms a bowl containing pelvic organs.

Comparison of Male and Female Pelves

The Lower Limb

Overview

The lower limb carries the entire weight of the erect body. Its bones are thicker and stronger than those of the upper limb. It is divided into three segments: thigh, leg, and foot.

Thigh

• Region between hip and knee.

• Femur: Single bone of the thigh; longest and strongest bone in the body.

• Ball-shaped head articulates with the acetabulum.

Patella

• Triangular sesamoid bone embedded in the tendon securing the quadriceps muscles.

• Protects the knee anteriorly and improves leverage of thigh muscles across the knee.

Leg

• Region between knee and ankle.

• Tibia: More massive medial bone, receives weight from femur.

• Fibula: Sticklike lateral bone, stabilizes the ankle joint.

• Interosseous membrane connects tibia and fibula.

Foot

The foot is composed of the tarsus, metatarsus, and phalanges. It supports body weight and acts as a lever for walking. Segmentation makes the foot pliable and adapted to uneven ground.

Tarsus

• Posterior half of the foot; contains seven tarsal bones.

• Body weight is borne primarily by the talus and calcaneus.

• Other tarsals: Cuboid, navicular, medial/intermediate/lateral cuneiforms.

Metatarsus

• Five small long bones called metatarsals, numbered beginning with the hallux (great toe).

• First metatarsal supports body weight.

Phalanges of the Toes

• Fourteen phalanges; smaller and less nimble than fingers.

• Each toe has proximal, middle, and distal phalanges, except the great toe (hallux), which has only two.

Summary Table: Bones of the Appendicular Skeleton

Key Terms and Definitions

• Girdle: Structure that attaches limbs to the trunk.

• Acetabulum: Socket in the pelvic bone for the femur.

• Interosseous membrane: Fibrous sheet connecting two bones (radius/ulna, tibia/fibula).

• Sesamoid bone: Bone embedded within a tendon (e.g., patella).

Additional info:

• The appendicular skeleton is essential for movement and manipulation of the environment, as well as for supporting the body during locomotion.

• Differences in male and female pelvises are important for childbirth and can be used in forensic identification.