Human Skeletal System: Appendicular Skeleton

Appendicular skeleton includes bones of limbs (forelimbs and hindlimbs) and two girdles; pectoral girdle in the shoulder region and the pelvic girdle present in the pelvis region. Let us discuss the four components of appendicular skeleton in detail:

Pectoral girdle

Pectoral girdle or shoulder girdle connects the bones of upper limbs with the axial skeleton. There are two pectoral girdles, the left and the right one. A pectoral girdle comprises-clavicle or the collar bone and scapula or the shoulder blade.

Scapula 

Scapula is a roughly triangular, flat bone located on the posterior side of thorax, adjacent to vertebral column, between second and seventh thoracic vertebrae. The longitudinal border of scapula closer to vertebral column is called the medial border, whereas the other longitudinal border is called lateral border.

1-Posterior view: The medial and lateral borders taper toward the base of scapula and meet at a point called the inferior angle. Inferior to the superior end of the medial border, a broad ascending bony ridge, called the spine runs through the scapula, broadens and flattens at the superior end of the lateral border forming the acromion. Superior and inferior to the spine are two depressions called the supraspinous fossa and the infraspinous fossa respectively. Glenoid cavity is a shallow depression present inferior to acromion, at the lateral side, which articulates with humerus of forearm.

The upper edge of scapula is called the superior border which articulates with the medial border at a point called the superior angle. The superior angle is itself superior to the superior border. The superior border of scapula also has a scapular notch.

2-Anterior view: The superior border of scapula has an anterior projection at its lateral end called the coracoid process. The anterior surface of scapula has a slight concave area below the superior border called the subscapular fossa.

Picture 2: Scapula
Picture Reference : Source

Clavicle 

Clavicle is the bone that lies perpendicularly to the skull. It articulates with the manubrium of sternum on the medial end (sternal end) and with acromion of scapula on its lateral end (acromial end). It has a curved, flat and somewhat S-shaped shaft. The inferior region at the acromial end of clavicle has a bony protuberance called conoid tubercle, the site of attachment of conoid ligament which articulates clavicle and scapula.

Forelimb

The upper limb has a total of 30 bones (each limb). These include-Humerus (1), Radius (1) and Ulna (1), Carpals (5), Metacarpals (8) and Phalanges (14). Let us study the bones of upper limbs:

Humerus 

Humerus is the longest bone of the upper limb. Proximally, it articulates with the glenoid cavity of scapula (shoulder joint) and distally, with radius and ulna (elbow joint).

1-Anterior view: At the proximal end, humerus has a rounded structure called the head covered by a layer of articular cartilage that articulates with glenoid cavity. Distal to head, is the anatomical neck present obliquely. On the lateral side of anatomical neck is the greater tubercle, medial and inferior to which is the lesser tubercle. Between the tubercles is the intertubercular sulcus. Distal to the tubercles, the proximal end of humerus constricts to form surgical neck. The shaft of humerus has a V-shaped rugged area called the deltoid tuberosity to which deltoid muscle attaches.

The shaft of humerus flattens at the distal end and bears two projections, capitulum and trochlea. Capitulum is a rounded structure on the lateral side that articulates with the head of radius. A depression is present, proximally to capitulum called the radial fossa, it receives the head of radius on flexion. Trochlea, on the other hand, is a pulley-shaped structure which articulates with ulna. Coronoid fossa is depression present proximally to trochlea that receives coronoid process of ulna on flexion. On the lateral and medial side of the distal end of humerus, are the lateral and medial epicondyles respectively which provide site for attachment of muscles of the forearm.

2-Posterior view: The shaft of humerus on the posterior side has a radial groove through which radial nerve passes. The humerus at its distal end, on the posterior side, has a large depression called olecranon fossa which receives olecranon process of ulna when forearm is extended.

Picture 4: Humerus
Picture Reference : Source

Ulna 

Ulna is present on the medial side (little-finger side) and articulates with the trochlea of humerus. The proximal end of ulna has a curved depression by which it fits into trochlea and is called trochlear notch. On the anterior side, trochlear notch has a projection called coronoid process which fits into the coronoid fossa of humerus. On the posterior side of trochlear notch is the olecranon process which articulates with humerus to form the elbow joint. Lateral to the trochlear notch, inferiorly, is the radial notch which articulates with the head of radius to form radio-ulnar joint. Inferior to the coronoid process is the ulnar tuberosity. The head of ulna is located at its distal end. On the posterior side of the distal end of ulna, is a projecting styloid process.

Radius

Radius is present on the lateral side (thumb-side) of the forearm. It is relatively smaller than ulna and has a disc-shaped head that articulates with capitulum of humerus proximally and medially a part of it fits into the radial notch of ulna. The head of radius constricts into a narrow neck. Inferior to the neck, on the anterior side, medially is a rugged radial tuberosity. The shaft of radius flattens and widens distally. On the lateral side, at the distal end, is the styloid process of radius present proximally to the thumb. The shaft of radius and ulna is joined by interosseous membrane.

Carpals 

There are 8 carpals which constitute the wrist region of the forearm. These carpals are interjoined by ligaments (intercarpal joints). The carpals are arranged into two transverse rows, each having four carpals. Carpals are named as per their shape which are mentioned below in lateral to medial order:

1-Proximal row: scaphoid (boat-like), lunate (moon-like), triquetrum (three-cornered), pisiform (pea-shaped). First three bones articulate with distal end of radius.

2-Distal row: trapezium, trapezoid, capitate (head-shaped), hamate (hooked).

Metacarpals

Metacarpals are the bones of palm. Each metacarpal consists of a proximal base, shaft, and a head at the distal end. These bones are numbered 1-5 in the lateral to medial order. The base of metacarpals articulates with the distal row of carpals and the head articulates with the phalanges.

Phalanges 

Phalanges are the bones of the digits that constitute bones of fingers and thumb. A single bone is referred to as phalanx. The thumb has 2 phalanx while rest of the digits have three each. Each phalanx consists of a base present proximally, an intermediate shaft and a distal head.

Read about Pelvic Girdle, hind limbs and metatarsals in our next post.