The human skeletal system provides the body with its framework, offering support, protection, and enabling movement. It divides into two main components: the axial and appendicular skeletons. While the axial skeleton forms the central axis (skull, vertebral column, rib cage), the appendicular skeleton comprises bones facilitating movement and interaction with the environment, connecting to the axial framework through specialized girdles.
The Pectoral Girdle and Upper Limbs
The pectoral, or shoulder, girdle connects the upper limbs to the axial skeleton. Each pectoral girdle consists of two bones: the clavicle (collarbone) and the scapula (shoulder blade). The clavicle is an S-shaped bone that extends horizontally across the anterior shoulder, articulating with the sternum and the scapula. The scapula is a flat, triangular bone located at the back of the pectoral girdle, providing attachment sites for numerous muscles and connecting with the humerus.
Extending from the pectoral girdle are the upper limbs, each containing 30 bones. The humerus is the single bone of the upper arm, articulating with the scapula at the shoulder and with the forearm bones at the elbow. The forearm contains two bones: the ulna, positioned medially, and the radius, located laterally on the thumb side. The radius can pivot around the ulna, allowing for movements of the forearm.
The hand and wrist together contain 27 bones. The wrist, or carpus, consists of eight small carpal bones arranged in two rows. Distal to the carpals are the five metacarpal bones, which form the palm of the hand. Finally, the fingers and thumb are made up of 14 phalanges. Each finger has three phalanges (proximal, middle, and distal), while the thumb has two (proximal and distal).
The Pelvic Girdle and Lower Limbs
The pelvic, or hip, girdle attaches the lower limbs to the axial skeleton and bears the body’s weight. It is formed by two large coxal bones, commonly known as hip bones. Each coxal bone results from the fusion of three separate bones during development: the ilium, ischium, and pubis. These fused bones create a deep, cup-shaped cavity called the acetabulum, which articulates with the thigh bone to form the hip joint.
Each lower limb contains 30 bones, which are thicker and stronger than those of the upper limbs to support body weight. The femur, or thigh bone, is the single bone of the thigh and is the longest, heaviest, and strongest bone in the body. At the knee, the patella, or kneecap, is a triangular bone that protects the knee joint and improves leverage for thigh muscles.
The lower leg contains two bones: the tibia and the fibula. The tibia, often called the shin bone, is the larger and stronger of the two, bearing most of the body’s weight. The fibula is a slender bone located laterally, providing stability to the ankle joint and serving as an attachment site for muscles.
The foot consists of 26 bones, divided into tarsals, metatarsals, and phalanges. There are seven tarsal bones that make up the ankle and upper part of the foot, followed by five metatarsal bones forming the main body of the foot. Similar to the hand, the toes contain 14 phalanges, with the great toe having two and the other toes having three each.
Total Count and Overall Role
The appendicular skeleton totals 126 bones, including the four bones of the pectoral girdles, the two coxal bones of the pelvic girdle, and 60 bones for each pair of upper and lower limbs. Unlike the axial skeleton, which primarily offers protection and central support, the appendicular skeleton’s structure, characterized by numerous synovial joints, allows for a broad range of motion and interaction with the environment.