Appendicular skeleton
The appendicular skeleton contains the bones of the pectoral girdle, upper limbs (arms), pelvic girdle, and lower limbs (legs).
Pectoral Girdle
The pectoral girdle (shoulder girdle) contains four bones: two clavicles and two scapulae. It supports the arms and serves as a place of attachment for muscles that move the arms. The bones of this girdle are not held tightly together and are weakly attached and held in place by ligaments and muscles. This arrangement allows great flexibility but means that the pectoral girdle is easily dislocated.
Clavicles
The clavicles (collarbones) are slender and S-shaped. Each clavicle articulates medially with the manubrium of the sternum. This is the only place of attachment of the pectoral girdle to the axial skeleton.
Each clavicle also articulates with a scapula. The clavicle serves as a brace for the scapula and helps stabilize the shoulder. It is structurally weak, however, and if undue force is applied to the shoulder, the clavicle will fracture.
Scapulae
The scapulae (shoulder blades) are broad bones that somewhat resemble triangles. One reason for the pectoral girdle's flexibility is that the scapulae are not joined to each other.Each scapula has a spine and also the following
features:
acromion process, which articulates with a clavicle and provides a place of attachment for arm and chest muscles;
coracoid process, which serves as a place of attachment for arm and chest muscles; glenoid cavity, which articulates with the head of the upper arm bone (humerus). The pectoral girdle's flexibility is also a result of the glenoid cavity being smaller than the head of the humerus.
The pectoral (shoulder) girdle contains two clavicles and two scapulae.
Upper Limb (Arm)
The upper limb includes the bones of the upper arm (humerus), the forearm (radius and ulna), and the hand (carpals, metacarpals, and phalanges).
The humerus is the bone of the upper arm. It is a long bone with the following features at the proximal end:
head, which fits into the glenoid cavity of the scapula; greater and lesser tubercles, which provide attachments for muscles that move arm and shoulder;
intertubercular groove, which holds the tendon from the biceps brachii, a muscle of the upper arm;
deltoid tuberosity, which provides an attachment for the deltoid, a muscle that covers the shoulder joint.
The humerus has the following features at the distal end: capitulum, a lateral condyle that articulates with the radius;
trochlea, a spool-shaped condyle that articulates with the ulna;
coronoid fossa, a depression for a process of the ulna when the elbow is bent;
olecranon fossa, a depression for a process of the ulna when the elbow is extended.
Radius
The radius and ulna are the bones of the forearm (lower arm). The radius is on the thumb side when the palm faces forward, but crosses over the ulna when the hand is turned so that the palm faces backward. Proximally, the radius has the following features:
head, which articulates with the capitulum of the humerus and fits into the radial notch of the ulna;
radial tuberosity, which serves as a place of attachment for a tendon from the biceps brachii.
Ulna
The ulna is the longer bone of the forearm. Proximally, the ulna has the following features:
coronoid process, which articulates with the coronoic. fossa of the humerus when the elbow is bent;
olecranon process, which articulates with the olecranon fossa of the humerus when the elbow is extended;
trochlear notch, which articulates with the trochlea of: the humerus.
Hand
Each hand has a wrist, a palm, and five fingers. The wrist contains eight small carpal bones (tightly bound by ligaments in two rows offour each). The palm has five metacarpal bones that form the knuckles when a fist is made. The fingers contain the phalanges. The thumb has only two phalanges, but the other fingers have three each.
The upper limb contains the humerus. the radius. the ulna. and the bones of the hand (the carpals. metacarpals. and phalanges).
Pelvic Girdle
The pelvic girdle, or pelvis, contains two coxal bones (hipbones), as well as the sacrum and coccyx. The strong bones of the pelvic girdle are firmly attached to one another and bear the weight of the body. The pelvis also serves as the place of attachment for the legs and protects the urinary bladder, the internal reproductive organs, and a portion of the large intestine.
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