The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton. The appendicular skeleton consists of the bones of the limbs and the girdles that connect them to the axial skeleton. It facilitates movement and interaction with the surrounding environment, allowing for a wide range of motion and specialized functions.
Bones of the Pectoral Girdle and Upper Limbs
The pectoral girdle, also known as the shoulder girdle, connects the upper limbs to the axial skeleton. Each side consists of two bones: the clavicle (collarbone) and the scapula (shoulder blade). The clavicle is an S-shaped bone at the front of the shoulder, providing an anterior brace that holds the arm away from the thorax. The scapula is a flat, triangular bone at the back, offering significant flexibility to the upper limb due to its loose attachment to the rib cage.
Extending from the pectoral girdle are the bones of the upper limb, which include the arm, forearm, wrist, and hand. The humerus is the long bone of the upper arm, articulating with the scapula at the shoulder and the radius and ulna at the elbow. In the forearm, the radius and ulna are two parallel bones; the ulna is located medially, while the radius is positioned laterally.
The wrist contains eight small, irregularly shaped carpal bones arranged in two rows, providing flexibility to the wrist joint. Beyond the wrist, the palm of the hand is formed by five metacarpal bones, numbered one through five from the thumb to the little finger. The fingers consist of 14 phalanges. Each finger has three phalanges (proximal, middle, and distal), except for the thumb, which has only two.
Bones of the Pelvic Girdle and Lower Limbs
The pelvic girdle forms the base of the trunk, connecting the lower limbs to the axial skeleton and providing substantial support for the body’s weight. It consists of two large coxal, or hip, bones. Each hip bone is formed by the fusion of three separate bones: the ilium, ischium, and pubis. These fused bones create a stable, weight-bearing structure that also protects internal pelvic organs.
The lower limbs extend from the pelvic girdle, comprising the bones of the thigh, leg, ankle, and foot. The femur, the longest and strongest bone in the body, is the bone of the thigh. It articulates with the hip bone proximally and the tibia and patella distally. The patella, commonly known as the kneecap, is a small, flat, triangular bone located at the front of the knee joint.
In the lower leg, the tibia (shin bone) is the larger, medial bone that bears most of the body’s weight. The fibula, a thinner bone, runs parallel to the tibia on the lateral side and primarily serves as a site for muscle attachment, with minimal weight-bearing function. The ankle and upper foot contain seven irregularly shaped tarsal bones that form the ankle joint and contribute to the foot’s arches.
Distal to the tarsals are the five metatarsal bones, which form the main part of the foot and connect to the toes. These bones are numbered one through five, starting from the big toe. The toes are made up of 14 phalanges. Similar to the fingers, each toe has three phalanges (proximal, middle, and distal), except for the big toe, which has only two.
Role of the Appendicular Skeleton
The appendicular skeleton plays a key role in enabling the body’s diverse movements and interactions with its surroundings. Its structure facilitates locomotion, allowing for activities such as walking, running, and jumping.
Beyond movement, the appendicular skeleton is important for manipulation. The bones of the upper limbs, particularly those in the hands, enable precise grasping, lifting, and the performance of fine motor skills. This manipulative ability is important for daily tasks and complex activities.
The appendicular skeleton also contributes to maintaining balance and posture. The lower limbs and pelvic girdle work together to support the body’s upright position and absorb forces during movement. Additionally, the pelvic girdle offers a degree of protection for certain internal organs within the pelvic cavity.