Your back has 18 muscles, almost half of them are deeply laid but most are superficial.

Lats run from your pelvis to the inside of your upper arm.  When the lats are well-developed, they give your upper body a wing-shaped form.  They establish compound movements such as a dead lift.

3 FUNCTIONS OF LATS THAT WORK THROUGH THE SHOULDERS

1.  Extension.  When you have your arm extended in front of you, the lats pull [or contract] the arm down.

2.  Adduction.  The lats pull the upper arm to the side of the body.

3.  Internal Rotation.

4.  Assist in Hyperextension of the spine and lateral flexion of the pelvis when the insertion, the end points, are fixed.  

11 EXERCISES [from easiest to most difficult] FOR THE LATISSIMUS DORSI, aka, the LATS

1.