Bones Of The Adult Skeleton Part Two: The Upper Appendicular Skeleton

Posted on 14 Oct 2012 19:06




This is part two of our description of the 206 bones of the human body. Part one covered the axial skeleton, which comprises the vertebral column, the skull, and the thoracic cage. Part two will cover the upper appendicular skeleton, which is the 64 bones of the upper limbs and the bones which connect them to the axial skeleton, the pectoral girdle. Part three will cover the lower appendicular skeleton.



The bones that connect the arms and legs to the axial skeleton are known as girdles. The bones connecting the arms are the pectoral girdle, which is commonly called the shoulder girdle. We will consider the upper arm, forearm, and hand bones, followed by the pectoral girdle bones. When talking about the arm, anatomists are usually only referring to the upper arm, although most of us consider the entire limb from the shoulder to the wrist to be the arm. In anatomy, the forearm us not always included when the term arm is used.

The Upper Limbs (60 total)

The upper limb bones consist of the bones of the arm, forearm, wrist, and hands. The arm, or upper arm, contains the humerus bone while the forearm, or lower arm, contains two bones, the radius and ulna. The wrists bones are called the carpals, the palm bones the metacarpals, and the finger bones the phalanges.


labelled diagram of the human upper limb bones showing humerus, radius, ulna, carpus (carpal bones), metacarpals, and phalanges

Bones of the Upper Limb


The Arms (Upper Arm and Forearm, 6 total)

The upper arms and forearms contain 6 bones, the humerus of the upper arm and the radius and ulna of the forearm.

Humerus (Upper Arm Bone, 2)

The humerus is named from a Latin word for shoulder or upper arm: humerus, humeri or umerus, umeri. It is a long, heavy bone extending from the scapula to the elbow, where it articulates with the radius and ulna. At its upper end it fits into the glenoid cavity of the scapula via a smooth rounded head. There are two projections, called tubercles, just below the head, that provide attachment points for muscles that move the arm at the shoulder. These are the greater tubercle and the lesser tubercle. Between the tubercles is a narrow groove, called the intertubercular groove. A tendon passes through this furrow from the biceps brachii to the shoulder.

Underneath the head of the humerus runs a narrow groove called the anatomical neck. Below this neck is a tapering region called the surgical neck, which is named for it's tendency
to sustain fractures.


labelled diagram of the humerus bone showing humeral head, anatomical neck, surgical neck, lesser and greater tuberosity, deltoid tuberosity, epicondyles, trochlea, and capitulum

The Humerus



About midway along the shaft of the humerus, on the lateral side, is the deltoid tuberosity, which provides attachment for the deltoid muscle. At the lower end are two smooth condyles, the capitulum, which articulates with the radius of the forearm, and the trochea, which articulates with the ulna. These two articulations form the elbow joint. Since more than two bones form the elbow joint, it is a compound joint.

Above the condyles on either side are the epicondyles, medial and lateral, which provide attachments for muscles and ligaments. Between these epicondyles is the coronoid fossa, which allows the coronoid process of the ulna to move over it when the elbow is bent. On the other side is the olecranon fossa which receives the ulnar process (or olecranon process) when the elbow is extended (arm straightened).


closeup view of upper and lower end of human humerus bone, labelled

Upper and Lower End of Humerus Bone
image by Anatomist90 via wikimedia


Radius (2)

The Latin word radius, radi, refers to a ray or rod. It may also refer to the spoke of a wheel. Unlike the upper arm, the forearm has two bones. The radius is located on the thumb side which, to anatomists, is the lateral (side) part of the arm as viewed from the anatomical position. At the upper end of the radius, the head is shaped more like a disc that a round knob. This disc articulates with the capitulum of the humerus and with the ulna via a notch on the side of the ulna called the radial notch. This lets the radius rotate freely without getting hung up by the ulna. The radius crosses over the ulna when the hand is turned palm up (supinated).


labelled image of human radius bone showing head, radial tuberosity and styloid process

Radius Bone
image via wikimedia



Just below the head is radial tuberosity, to which the biceps brachii attaches. At the lower end, the styloid process provides attachments of the wrist ligaments. Just above on the medial side is a notch for articulation with the ulna called the ulnar notch. The radius articulates with the carpal bones of the wrist via the Baze-Carpal articular surface.


closeup of proximal and distal end of radius bone, labelled

Proximal and Distal End of Radius
image by Anatomist90 via wikimedia


Ulna (2)

The Latin word ulna means elbow or forearm. The ulna is longer than the radius bone and overlaps the end of the humerus in the back (posteriorly). At its top end it has a notch that articulates with the trochlea of the humerus, called the trochlear notch or semilunar notch. There are two process, above and below the notch. The one above is the olecranon process and it provides attachment for the triceps brachii muscle. When the triceps straightens (extends) the elbow, the olecranon process of the ulna fits into the olecranon fossa of the humerus. The process below the trochlear notch is called the coronoid process and it fits into the coronoid fossa of the humerus when the elbow is bent.


anterior and medial view of ulna bone, labelled

Two Ulna Views
image via wikimedia



At the distal or lower end of the ulna is a knob shaped head that articulates with the ulnar notch of the radius, laterally and with a fibrocartilage disc at its inferior end, as well as to the triquetrum radial bone of the wrist. On its medial (inside) surface is a styloid process that provides attachment for wrist ligaments.


closeup view of proximal and distal ends of ulna bone, labelled

Proximal and Distal Ulna
image by Anatomist90 via wikimedia


Wrist and Hands (54)

The wrists and hands consist of 54 total bones. The wrist is made up of 16 carpal bones and the hands are made up of 10 metacarpals and 28 phalanges. Although the wrist itself is a joint, for this description, wrist refers to the carpal bones. The articulation of the radius and carpal bones, and specifically the saphoid, lunate, and triquetrum, called the radiocarpal joint is sometimes considered to be the "wrist proper." However, the articulation between the proximal and distal rows of carpals, the midcarpal joint, is also part of the wrist. As well, there is the carpometacarpals, which are the articulations between the distal carpals and metacarpal bones of the hand. Even the spaces between the rows of carpals from proximal to distal are considered to be joints, the intercarpals. Obviously, the wrist can be considered in many complex ways and the complexities of the joint itself are beyond the scope of this description.

The Carpal Bones (Carpus, 16 total)

The wrist is composed of eight small short bones called carpal bones. These bones are set together in a stable complex of two rows, each having four bones. The rows are considered as one being proximal, or closer to the radius and ulna of the forearm, and the other being distal, or closer to the hands (the metacarpals). Together, this complex of bones is called the carpus, which is the Latin word for wrist.


labelled view of hand skeleton showing carpal bones, metacarpals o hand, and phalanges of fingers

Carpus and Hand
image by Anatomist90 via wikimedia



On its proximal side, the carpus is rounded. This proximal row of bones articulates with the radius and with a fibrocartilage disc on the ulna side. The distal surface row of carpals articulates with the metacarpal bones of the hand. The carpus is concave on its anterior side so that a canal or arch is formed for the passage of tendons and nerves that extend through to the palm. The "pillars" of this arch is formed by the trapezium and the scaphoid on the lateral side and the hamate and pisiform on the medial side. These pillars are connected by a deep fascial band called a flexor retinaculum, which ties the pillars together and forms a tunnel, called the carpal tunnel. The long flexor tendons of the fingers and the median nerve pass through this tunnel.


closeup view of wrist, carpus and metacarpals, with labelled carpal bones and articulation of each carpal with the metacarpal clearly shown

Labelled Carpal Bones in Carpus with the Metacarpals
image by Anatomist90 via wikimedia



Keep in mind the anatomical position for this description: The proximal row of carpals, from lateral to medial, are the scaphoid (navicular), lunate, triquetrum, and pisiform.

The distal row of carpal bones, from lateral to medial, are the trapezium (greater multangular or multangulus major), trapezoid (lesser mutangular or multangulus minor), capitate, and hamate. The capitate is the largest carpal bone and the pisiform is the smallest. The pisiform is considered a sesamoid bone because it lies in the tendon of the flexor carpi ulnaris muscle.

The following image shows closeup views of each of the carpal bones, set in the order they appear in the wrist. The image is labelled to indicate the distal, proximal, medial, and lateral orientations.


closeup view of carpal bones set in order they appear in wrist, labelled to indicate orientation from distal to proximal, medial to lateral showing hamate, capitate, trapezoid, trapezium, pisiform, triquetral, lunate, and scaphoid bones

Carpal Bone Closeups
image by Anatomist90 via wikipedia


Metacarpals and Phalanges of Hand (38)

The hands are made up of the metacarpals of the palm and the phalanges of the fingers. There are five metacarpals in each hand, one for each finger. Each finger has three phalanges, and the thumb has two. Despite how small they may be, the bones of the hand are long bones, meaning they are longer than they are wide.

The metacarpal bones are cylindrical. Their distal ends are rounded to form the knuckles of your fist. They articulate with the carpals of the wrist at their proximal ends and with the first (proximal) phalanges of the fingers at their distal ends. The metacarpals are numbered 1 to 5, with the thumb being metacarpal one. The thumb's articulation with the wrist is the most freely moveable, allowing it to move in opposition to the fingers. See the image of the carpus and metacarpals above to see them numbered and to see the way they articulate with the carpal bones of the wrist. The x-ray image of the hand, below, also shows the metacarpals clearly numbered.


high quality hand x-ray with metacarpals, phalanges, carpals and distal ends of radius and ulna labelled

X-ray of Bones of Hand
image by Hellerhoff via wikimdiea



The 14 phalanges are the finger bones. As above, each finger has three phalanges, a proximal, middle, and distal phalanx (singular form of phalange). The thumb does not have a middle phalanx, and so has only two, a proximal and distal phalanx. Each phalanx has a base, a shaft, and a head.

The bases of each of the proximal phalanges articulates with the head of the metacarpal in line with it. This joint is called the metacarpophalangeal joint (MP joint) and is the joint that forms your knuckles when you clench your fist. The heads of the proximal phalanges articulate with the base of middle phalanges, except for the thumb, whose proximal phalanx articulate directly with the distal phalanx. The head of each middle phalanx articulates with the base of the distal phalanx. These joints formed by the middle and distal phalageal articulations are called the interphalangeal joints (IP joints). The bases of the middle and distal phalanges have concave facets to allow articulation with the head of the phalanx proximal to it. The head of the distal phalanx has a horse-shoe shaped tuberosity on the palmar side and a smooth dorsal side, which is covered by the finger nails. As well, the phalanges provide attachment for several muscles of the hand, extrinsic and intrinsic.


labelled phalanges of fingers, finger bones

Labelled Phalanges
image by Anatomist90via wikimdiea


Pectoral Girdle (Shoulder Girdle, 4 total)

The bones that connect the limbs to the axial skeleton are called girdles. There are two girdles in the human body, the pelvic or hip girdle, described below, and the pectoral girdle. The pectoral girdle is made up of four bones, two clavicles and two scapulae. Although the word girdle usually describes a circular structure, the pectoral girdle is not a complete circle, being open in the back between the scapulae (shoulder blades) and in front at the manubrium of the sternum. The pectoral girdle is more often, today, called the shoulder girdle. This can be confusing because the shoulder girdle is considered as a separate feature to the shoulder joint. The arms, basically, are suspended from the shoulder girdle. The image below shows the pectoral girdle from its superior aspect (viewed from above).


Labelled superior aspect(viewed from above) of pectoral or shoulder girdle

Superior Aspect of Pectoral Girdle
BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan



This entire girdle system is only connected to the axial skeleton by one small joint, the sternoclavicular joint, which is the joint that connects each clavicle to the sternum.
Most of the joining of the pectoral (or shoulder girdle) with the body is through muscular attachments, which is how the shoulder girdle has so much mobility compared to the lower limbs.

Clavicles (Collar bones, 2)

The clavicles, or collarbones, are long bones located at the base of the neck and they run horizontally from the middle of the chest at the manubrium of the sternum to the shoulder at the scapular acromium process. The end of the scapula that joins with the sternum is called the sternal end and the end that joins with the scapula is called the acromial end. Since the scapula is a free-moving bone, the clavicles, firmly attached to the front of the chest, serve as braces to keep the scapula from moving forward on the torso. Essentially, they help keep the shoulder in place. They also provide attachments for muscles of the arms, chest, and back.

Each clavicle is joined to the scapulae at the acromium process through the acromioclavicular joint. They are joined to the manubrium of the sternum at the sternoclavicular joint, as stated above. You can read more about these joints, and the movements of the shoulder girdle and shoulder joint in the shoulder complex.

Although a long bone is normally very strong under compression, the clavicle has double curve, each curve going a different direction much like an S shape. This makes the clavicle a structurally weak bone and if it gets compressed from one end because of unusual pressure on the shoulder, it may fracture, which happens often in sport or in accidents.


closeup view of human clavicle bone (collar bone), labelled

Clavicle Bone
image by Anatomist90 via wikimedia


Scapulae (Shoulder Blades, 2)

The scapulae are broad, roughly triangular, flat bones that sit on either side of the upper back. Their anterior surfaces, the part facing and connecting with the thorax, is concave compared to their posteriors. The scapulae have no direct attachment to axial skeleton; they are held in position on the rib cage by muscles. This gives them a considerable range of movement, necessary for the correct functioning of the shoulder girdle.

Each scapula consists of a flat blade divided into unequal portions by the scapular spine, a ridge of bone that running laterally and upwards across the top one-third of the broad superior part of the scapula. This spine forms a crest that you can feel under your skin. Its front and back surface forms a V-shape trough called the supraspinous or supraspinatous fossa. It leads to a head which has two processes, the acromion process, which articulates with the clavicle, provides attachment for arm and chest muscles, and forms the tip of the shoulder at the top. Below the acromion is the coracoid process which curves downward and forward below the clavicle, its tip in front of the shoulder joint. This process also providing attachment for arm and chest muscles. Their arrangement is indicated in the superior view of the pectoral girdle, above.


Labelled diagram of shoulder girdle showing colored scapula and clavicles, with shoulder joint, humerus, and ribs

Shoulder Girdle



Between the acromion and coracoid process is the glenoid cavity or glenoid fossa, a large depression into which the head of the humerus fits, forming the shoulder joint proper.

The angle of the blade that points directly downward is called the superior angle. The uppermost angle opposite the head is called the superior angle. The large front surface of the blade facing the body is called the subscapular fossa. The large posterior surface of the blade is the infraspinatous or infraspinous fossa. Since so many muscles attach to the scapula, their roles can be confusing, as they are mixed and serve to move and stabilize the scapula and the shoulder joint as well. The muscles attached to the coracoid process, the acromion process, the crest of the spine, and the medial border of the scapula are generally concerned with movement of the scapula and the clavicle on the sternoclavicular and acromioclavicular joints, as well as the humerus upon the shoulder joint. The subscapular, supraspinous, and infraspinous processes, on the other hand, provide attachments for muscle mainly concerned with stabilizing the shoulder joint so that the humeral head and glenoid fossa stay in close contact with one another.


anterior and posterior (front and back) view of scapula bone, labelled

Anterior and Posterior Scapula
images via wikimedia



This concludes part two of the bones of the adult skeleton, the upper appendicular bones. The image below shows the full upper appendicular skeleton with all its parts well labelled. Part three will cover the lower appendicular skeleton: the pelvic girdle, and lower limbs.


diagram of upper appendicular skeleton with all parts labelled


References
1. Hole, John W. Human Anatomy and Physiology. Dubuque, IA: W.C. Brown, 1987
2. Watkins, James. Structure and Function of the Musculoskeletal System. Champaign, IL: Human Kinetics, 2010.
3. Behnke, Robert S. Kinetic Anatomy. Champaign, IL: Human Kinetics, 2001.
4. Clark, Robert K. Anatomy and Physiology: Understanding the Human Body. Sudbury, MA: Jones and Bartlett, 2005.
5. Hamill, Joseph. Biomechanical Basis Of Human Movement. Lippincott Williams & Wilkins, 2006.
6. Last, R. J., and R. M. H. McMinn. Last's Anatomy, Regional and Applied. Edinburgh: Churchill Livingstone, 1994.


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