Final answer:
The latissimus dorsi muscle originates from the lower spine and the iliac crest, and inserts onto the humerus. Its main action is to adduct the upper arm towards the midline of the body.
Step-by-step explanation:
The latissimus dorsi muscle originates from the lower spine and the iliac crest, which is a bony prominence of the pelvis. It then inserts onto the humerus, which is the bone of the upper arm.
The main action of the latissimus dorsi is to adduct the upper arm towards the midline of the body. It also plays a role in extension, medial rotation, and depression of the arm at the shoulder joint.