The thorax consists of the muscles in the chest and abdominals.

The pectoralis major is a superficial muscle of the chest and it flexes the arm.  This muscle can help with pulling and climbing.  The pectoralis minor is under the pectoralis major and it adducts and medially rotates the arm.  The serratus anterior attaches to the rib cage and is found towards the sides of the body.  It pulls the scapula down and helps with flexing the arm.  The external and internal intercostal muscles are found under the rib cage and they assist with inhalation.  The diaphragm lies within the lower part of the rib cage and moves downward to increase the area in the lungs.

The trapezius is mainly located on the back but it does insert onto the clavicle on the chest.  The upper portion elevates the shoulder.  The deltoid also attaches to the clavicle.  It rotates, adducts, and extends the arm.

The most superficial abdominal muscle is the rectus abdominis.  These muscles are also known as your 6-pack, they flex the spine and stablize the pelvis.  The external and internal obliques flex the spine, hold the organs in the abdominal cavity, and aid with rotation.  The deepest layer is the transverse abdominus which compresses and supports the organs.