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.
On examining the back of a person standing upright a median furrow is seen (Fig. 477). In the bottom of this the tips of the spinous processes can be felt. If the back is bent these processes can be distinctly seen; they should form a straight line. The second cervical spine can be felt by deep pressure in a relaxed neck. The sixth is usually the first one visible and the seventh cervical and first dorsal are very prominent, often the latter the more so. The furrow ends abruptly at the top of the sacrum. From this point down to near the top of the gluteal fold is a triangular space with its base above and apex downward. Its apex marks the third sacral spine, and just above this latter, opposite the second sacral spine, on each side can be felt the posterior superior iliac spines. The erector spina (sacrolumbalis) muscles form elevations on each side of the furrow, most marked in the lumbar region. In muscular people the erector spinae is seen to consist of two parts: an inner longissimus dorsi muscle, and an outer iliocostalis. Above, the projections of the scapulae are visible, If the arms are by the sides the posterior border of the scapula is parallel to the median line. The root of the spine of the scapula in a muscular person makes a depression. It is opposite the third dorsal spine or the body of the fourth thoracic vertebra, and marks the upper end of the fissure of the lungs. The spine of the scapula is subcutaneous and can be traced out to the acromion process. The lower angle of the scapula is opposite the upper border of the eighth rib; the upper angle covers the second rib but its tip is level with the first.
Fig. 477. - Surface anatomy of the back.
In the lumbar region the erector spinae muscle forms a clearly marked prominence. The twelfth rib usually projects beyond its outer edge, which is marked by a depression separating it from the abdominal muscles in front. It is through this depression that operations on the kidney are performed (see page 428). The distance between the twelfth rib and crest of the ilium is usually 6.25 cm. (2 1/2 in.) but it may be more and is often less. Just above the middle of the crest of the ilium is Petit's triangle (page 394); and to the inner side of the lower third of the posterior edge of the scapula is another small triangle. Its upper side is formed by the trapezius, its lower by the latissimus dorsi, and its outer by the posterior edge of the scapula. As the lung is nearest the surface at this point it is often chosen for physical examination, puncture, etc.