|Derivation||The widest (latissimus) muscle of the back (dorsi).|
|Origin||All vertebrae from the sixth thoracic to the sacrum, and the posterior, upper, medial border of pelvis.|
|Insertion||Ridge on anterior surface of the humerus.|
|Action||Extends the shoulder - pulls upper arm downward towards the torso and behind the back.|
Latissimus dorsi is basically a triangular sheet covering the lower half of the back. Its simple shape is influenced by a number of deeper forms, which are considered below from the bottom up.
The first is a column of muscles running up either side of the spine called erector spinae (
ee-RECK-tor SPINE-ee), literally, the muscles that raise (erect) the spine. These columns plunge the lower backbone into a protective furrow.
There is a second, rounded form of erector spinae lying next to the column, whose lower border lies between the ribcage and pelvis.
External oblique pushes out on the lower, lateral border as it emerges.
Serratus anterior can be easily seen beneath, dropping away from the lower angle of the scapula in a downward, sweeping arc toward the front of the body. The lower angle is often a distinct form itself.
The medial side of the upper border is covered by trapezius.
The upper lateral corner joins with teres major, and together they form the dorsal wall of the armpit before inserting onto the humerus in front.
It's possible to see the lower ribcage pushing up through latissimus dorsi on a slender subject, or one with arms raised.