# Lecture 8: Appendicular Skeleton
## Anatomy and Physiology I
---
## Learning Objectives
By the end of this lecture, students will be able to:
1. Identify the bones of the pectoral (shoulder) girdle and describe their key features
2. Identify the bones of the upper limb and their major landmarks
3. Identify the bones of the pelvic (hip) girdle and describe their key features
4. Compare the male and female pelvis
5. Identify the bones of the lower limb and their major landmarks
6. Describe the arches of the foot and their functional significance
---
## Lecture Content
### I. Overview of the Appendicular Skeleton
The appendicular skeleton consists of **126 bones** comprising the limbs and the girdles that attach them to the axial skeleton. The pectoral (shoulder) girdle together with the upper limbs accounts for 64 bones, while the pelvic (hip) girdle and lower limbs account for 62. The upper limbs are designed primarily for mobility, whereas the lower limbs are built for stability and weight-bearing.
### II. The Pectoral (Shoulder) Girdle
The pectoral girdle is composed of two bones on each side: the **clavicle** and the **scapula**. It attaches the upper limbs to the axial skeleton and is lightweight, allowing a great range of motion at the shoulder. The only bony attachment to the axial skeleton is the **sternoclavicular joint**, where the clavicle meets the manubrium.
#### Clavicle (Collarbone)
The clavicle is an S-shaped bone extending horizontally across the superior thorax. Its rounded **sternal (medial) end** articulates with the manubrium of the sternum, and its flattened **acromial (lateral) end** articulates with the acromion of the scapula at the acromioclavicular joint. The clavicle acts as a brace to hold the scapula and arm laterally and transmits forces from the upper limb to the axial skeleton. It is the most frequently fractured bone in the body, usually resulting from a fall on an outstretched hand or a blow to the shoulder.
#### Scapula (Shoulder Blade)
The scapula is a flat, triangular bone on the posterior thorax, overlying ribs 2 through 7. It has three borders (superior, medial or vertebral, and lateral or axillary) and three angles (superior, inferior, and lateral). Several key features define the scapula. The **spine** is a prominent posterior ridge dividing the dorsal surface into the supraspinous fossa above and the infraspinous fossa below. The **acromion** is the lateral end of the spine, articulating with the clavicle and forming the "point" of the shoulder. The **coracoid process** is an anterior hook-like projection serving as an attachment for the biceps brachii (short head), pectoralis minor, and coracobrachialis. The **glenoid cavity (fossa)** is a shallow depression on the lateral angle that articulates with the head of the humerus to form the glenohumeral (shoulder) joint; its shallow nature allows great range of motion but makes the joint prone to dislocation. The **subscapular fossa** on the anterior (costal) surface provides attachment for the subscapularis muscle. The **suprascapular notch** on the superior border provides passage for the suprascapular nerve.
### III. The Upper Limb (30 bones per limb)
#### Humerus (Arm)
The humerus is the single bone of the arm (brachium) and the largest and longest bone of the upper limb. Its proximal end features a smooth, rounded **head** that articulates with the glenoid cavity of the scapula. The **anatomical neck** is the groove encircling the head, marking the site of the former epiphyseal plate, while the **surgical neck** is the narrow region distal to the tubercles and a common fracture site. The **greater tubercle** is a lateral projection providing attachment for the supraspinatus, infraspinatus, and teres minor (rotator cuff muscles), and the **lesser tubercle** is an anterior projection for the subscapularis (rotator cuff). Between the tubercles lies the **intertubercular (bicipital) groove**, through which the tendon of the long head of the biceps brachii passes.
On the shaft, the **deltoid tuberosity** is a roughened area on the lateral surface serving as the attachment for the deltoid muscle, and the **radial (spiral) groove** on the posterior shaft marks the passage of the radial nerve, which can be damaged in mid-shaft fractures.
The distal end has several important landmarks. The **capitulum** is the rounded lateral condyle articulating with the head of the radius, and the **trochlea** is the spool-shaped medial condyle articulating with the trochlear notch of the ulna. The **medial epicondyle** provides attachment for forearm flexor muscles, and the ulnar nerve passes posterior to it (the "funny bone"). The **lateral epicondyle** serves as the attachment for forearm extensor muscles. The **olecranon fossa** is a posterior depression receiving the olecranon of the ulna during elbow extension, and the **coronoid fossa** is an anterior depression receiving the coronoid process of the ulna during elbow flexion.
#### Radius and Ulna (Forearm)
The forearm contains two parallel bones: the **radius** is lateral (on the thumb side) and the **ulna** is medial (on the pinky side) in anatomical position.
The **ulna** features the **olecranon**, the large proximal process forming the point of the elbow, and the **trochlear (semilunar) notch**, a C-shaped notch that articulates with the trochlea of the humerus. The **coronoid process** is an anterior projection below the trochlear notch, and the **radial notch** on its lateral surface articulates with the head of the radius at the proximal radioulnar joint. The **styloid process** is a small distal projection on the medial side.
The **radius** has a disc-shaped **head** at its proximal end that articulates with the capitulum of the humerus and the radial notch of the ulna. The **radial tuberosity** is a roughened area distal to the neck serving as the attachment for the biceps brachii tendon. The **styloid process** is a large distal projection on the lateral side, and the **ulnar notch** on the medial aspect of the distal end articulates with the head of the ulna at the distal radioulnar joint. The distal end of the radius is broader than the ulna and articulates with the carpal bones to form the main component of the wrist joint. A **Colles' fracture** is a fracture of the distal radius, typically resulting from a fall on an outstretched hand.
#### Hand (27 bones)
The hand contains 27 bones organized into three groups. The **carpals (8)** are arranged in two rows of four. The proximal row, from lateral to medial, consists of the scaphoid, lunate, triquetrum, and pisiform. The distal row consists of the trapezium, trapezoid, capitate, and hamate. A helpful mnemonic is "Some Lovers Try Positions That They Can't Handle." The scaphoid is the most commonly fractured carpal bone. The **metacarpals (5)** are the bones of the palm, numbered I through V from thumb to little finger, each having a base (proximal), shaft, and head (distal, forming the knuckle). The **phalanges (14)** are the bones of the fingers: the thumb (pollex) has two phalanges (proximal and distal), while fingers 2 through 5 each have three (proximal, middle, and distal).
An anterior view of the right upper limb showing all bones. The scapula and clavicle are shown at the top forming the pectoral girdle. The humerus is labeled with the head, greater and lesser tubercles, intertubercular groove, surgical neck, deltoid tuberosity, medial and lateral epicondyles, capitulum, trochlea, and olecranon fossa (shown in a posterior inset). The radius (lateral) and ulna (medial) are shown with the olecranon, trochlear notch, coronoid process, radial head, radial tuberosity, and both styloid processes labeled. The hand shows the 8 carpals arranged in two rows (each bone labeled), 5 metacarpals (I-V), and 14 phalanges. An inset shows the posterior view of the elbow joint with the olecranon fitting into the olecranon fossa.
### IV. The Pelvic (Hip) Girdle
The pelvic girdle is composed of two **coxal (hip) bones** (also called os coxae or innominate bones). Each coxal bone is formed by the fusion of three bones — the **ilium, ischium, and pubis** — which fuse at the **acetabulum** by age 15 to 17. The two coxal bones articulate with each other anteriorly at the **pubic symphysis**, a fibrocartilage joint, and with the sacrum posteriorly at the **sacroiliac joints**. Together with the sacrum and coccyx, the coxal bones form the **bony pelvis**, which supports the trunk, protects pelvic organs, provides attachment for lower limb muscles, and transmits weight to the lower limbs.
#### Ilium
The ilium is the largest portion of the coxal bone, forming the superior flared region. The **iliac crest** is the superior border and the palpable "hip bone" landmark. It terminates anteriorly as the **anterior superior iliac spine (ASIS)**, an important palpable landmark, below which lies the **anterior inferior iliac spine (AIIS)**. Posteriorly, it terminates as the **posterior superior iliac spine (PSIS)**, below which is the **posterior inferior iliac spine**. The **iliac fossa** is the concave internal surface providing attachment for the iliacus muscle. The **auricular surface** articulates with the sacrum at the sacroiliac joint. The **greater sciatic notch** is a large notch posterior to the auricular surface through which the sciatic nerve and piriformis muscle pass. The **gluteal lines** (posterior, anterior, and inferior) on the external surface serve as attachment sites for the gluteal muscles.
#### Ischium
The ischium is the posteroinferior and strongest portion of the coxal bone. The **ischial tuberosity** is a large roughened area that bears body weight when sitting and provides attachment for the hamstring muscles. The **ischial spine** projects medially and serves as a landmark for pudendal nerve block in obstetrics. Below the ischial spine lies the **lesser sciatic notch**. The **ischial ramus** joins with the inferior pubic ramus.
#### Pubis
The pubis forms the anterior portion of the coxal bone and consists of a **superior pubic ramus** and an **inferior pubic ramus**. The **pubic symphysis** is the anterior midline joint between the two pubic bones, united by fibrocartilage. The **pubic crest** and **pubic tubercle** serve as attachment points for abdominal muscles. The **obturator foramen** is a large opening bordered by the pubis and ischium, mostly covered by the obturator membrane, and provides passage for the obturator nerve and vessels.
#### Acetabulum
The acetabulum is a deep socket on the lateral surface of the coxal bone, formed by the fusion of all three component bones (ilium, ischium, and pubis). It articulates with the head of the femur to form the **hip joint**, a deep and inherently stable ball-and-socket joint.
### V. Male vs. Female Pelvis
| Feature | Female Pelvis | Male Pelvis |
|---|---|---|
| General structure | Lighter, thinner, smoother | Heavier, thicker, more prominent markings |
| Pelvic inlet | Wider, oval/round | Narrower, heart-shaped |
| Pelvic outlet | Wider | Narrower |
| Subpubic angle | Greater than 80 degrees (wide) | Less than 70 degrees (narrow) |
| Acetabulum | Smaller, faces more anteriorly | Larger, faces more laterally |
| Iliac crests | Less flared | More flared |
| Sacrum | Shorter, wider, less curved | Longer, narrower, more curved |
| Obturator foramen | Oval | Round |
| Overall shape | Adapted for childbirth | Adapted for heavy muscle attachment |
### VI. The Lower Limb (30 bones per limb)
#### Femur (Thigh)
The femur is the single bone of the thigh and the longest, heaviest, and strongest bone in the body. The proximal end features a ball-shaped **head** that articulates with the acetabulum and has a small pit called the **fovea capitis** for the ligamentum teres, which carries a small artery. The **neck** connects the head to the shaft and is a common fracture site, especially in elderly individuals with osteoporosis (the so-called "broken hip"). The **greater trochanter** is a large lateral projection providing attachment for the gluteal muscles and is palpable at the hip. The **lesser trochanter** is a smaller medial projection serving as the attachment for the iliopsoas muscle. The **intertrochanteric line** is the anterior ridge between the trochanters, and the **intertrochanteric crest** is the posterior ridge.
On the shaft, the **linea aspera** is a roughened ridge on the posterior surface providing attachment for thigh muscles including the adductors and vastus muscles. The shaft angles medially from the hip to the knee at an angle of inclination of approximately 125 degrees.
The distal end features the **medial and lateral condyles**, large smooth rounded surfaces that articulate with the tibia. Above the condyles are the **medial and lateral epicondyles**, which serve as attachment points for knee ligaments. The **patellar surface (trochlear groove)** is a smooth anterior groove between the condyles that articulates with the patella, and the **intercondylar fossa** is a deep notch between the condyles posteriorly through which the cruciate ligaments pass.
#### Patella (Kneecap)
The patella is the largest sesamoid bone, formed within the tendon of the quadriceps femoris. It protects the anterior knee joint and improves the leverage of the quadriceps during knee extension.
#### Tibia (Shin Bone)
The tibia is the larger, medial bone of the leg and the main weight-bearing bone. Its proximal end features the **medial and lateral condyles**, flat surfaces articulating with the femoral condyles, and the **intercondylar eminence**, a raised area between the condyles serving as the attachment for the cruciate ligaments. The **tibial tuberosity** is an anterior roughened prominence providing attachment for the patellar ligament. The shaft is triangular in cross-section, and its anterior border (the shin) is subcutaneous and palpable. The distal end features the **medial malleolus**, a medial projection forming the inner ankle bump, and an inferior surface that articulates with the talus at the ankle joint.
#### Fibula
The fibula is the thin, lateral bone of the leg and does NOT bear body weight. Its primary functions are muscle attachment and stabilization of the ankle. The proximal **head** articulates with the lateral condyle of the tibia at the proximal tibiofibular joint. The distal end forms the **lateral malleolus**, the outer ankle bump, which articulates with the talus.
#### Foot (26 bones)
The foot contains 26 bones in three groups. The **tarsals (7)** are the ankle bones. The **talus** articulates with the tibia and fibula at the ankle joint and transfers weight from the leg to the foot. The **calcaneus** is the heel bone, the largest tarsal, and provides attachment for the Achilles (calcaneal) tendon. The remaining tarsals are the navicular, cuboid, medial cuneiform, intermediate cuneiform, and lateral cuneiform. The **metatarsals (5)** are the bones of the sole, numbered I through V from the great toe to the little toe. The **phalanges (14)** are the bones of the toes: the great toe (hallux) has two phalanges (proximal and distal), while toes 2 through 5 each have three (proximal, middle, and distal).
#### Arches of the Foot
The arches of the foot are maintained by the interlocking shapes of the tarsal and metatarsal bones, along with supporting ligaments and tendons. They distribute body weight and absorb shock, acting as spring-like levers during walking and running. The **medial longitudinal arch** is the higher arch, with the talus as its keystone, and is the most important arch for shock absorption. The **lateral longitudinal arch** is lower and flatter, with the cuboid as its keystone, and provides stability. The **transverse arch** runs from side to side across the foot at the level of the cuneiforms and cuboid. **Fallen arches (flat feet or pes planus)** result from weakening of the tendons and ligaments that support the arches.
An anterior view of the right lower limb showing all bones from the pelvis to the foot. The femur is labeled with the head, fovea capitis, neck, greater and lesser trochanters, linea aspera (shown in a posterior inset), medial and lateral condyles, and patellar surface. The patella is shown anterior to the knee. The tibia (medial) shows the condyles, tibial tuberosity, medial malleolus. The fibula (lateral) shows the head and lateral malleolus. The foot is shown from a medial view with all 7 tarsals labeled (talus, calcaneus, navicular, three cuneiforms, cuboid), 5 metatarsals (I-V), and 14 phalanges. An inset shows the medial longitudinal arch from the calcaneus through the talus to the metatarsal heads, with the transverse arch shown in a cross-sectional view of the mid-foot.