This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The human skeleton consists of two parts, called the axial skeleton and the appendicular skeleton.
The axial skeleton embraces the bones of the head, the spine, the ribs, the hyoid bone, and the breast bone. In the spine are included not only the vertebrae of the cervical, thoracic, and lumbar regions, but likewise the sacrum and coccyx.
The appendicular skeleton embraces the bones of the limbs, or extremities, including the shoulder-girdle, formed by the scapula and clavicle, and the pelvic girdle, formed by the innominate bone (Fig. 226).
The upper extremity in man is an organ of prehension. As such, mobility is its chief characteristic. To permit of this mobility the bones and joints are many, and the latter are comparatively loose; the muscles, also, are both numerous and complex. Hence it is that slight injuries are frequently followed by considerable disturbance of function. They are readily produced and with difficulty repaired, either by nature or by art. Orthopaedic surgery has done much for the disabilities and deformities of the lower extremities, but comparatively little for those of the upper. An artificial leg in many cases satisfactorily substitutes the natural one, but an artificial arm is comparatively useless.
The hand is the essential part of the upper extremity, and the rest of the limb is subsidiary. If the forearm were lacking and the hand were attached to the end of the humerus it would still be a very useful appendage, far more so than the stump which is left after the hand has been amputated.
The extremities proper are joined to the trunk by what are called girdles. The upper extremity is attached through the medium of the shoulder-girdle and the lower extremity by the pelvic girdle. The interposition of these girdles adds to the mobility of the extremities, and as the upper extremity is more mobile than the lower we find the shoulder-girdle composed of two bones instead of one as in the pelvic girdle; also, as the lower extremity bears the weight of the body it requires strength in addition to mobility, hence we find that it is joined to the trunk by a single big strong bone, the innominate, instead of by two comparatively slight, narrow bones like the clavicle and scapula which form the shoulder-girdle.
The extremities are termed appendicular because they are simply appendages to the essential part, which is the head and trunk; a person can live without extremities.
Fig. 226. - The bones of the head and trunk forming the axial skeleton and those of the upper and lower extremities constituting the appendicular skeleton.