The major muscle in the chest is the pectoralis major. This large fan-shaped muscle stretches from the armpit up to the collarbone and down across the lower chest region on both sides of the chest. The two sides connect at the sternum, or breastbone.

This muscle moves each shoulder joint in four distinct ways as well as keeps the arms attached to the body. Injuries to this muscle are rare, but symptoms include pain in the chest, bruising, and decreased strength of the muscle.

The pectoralis minor resides under the pectoralis major. This thin triangle-shaped muscle runs up and down along the upper ribs.  

The major muscles in the upper torso of the body include:

  • Trapezius: This muscle extends across the neck, shoulder, and back. It allows for movement of the shoulders and shoulder blades.
  • Rhomboid major: Attached to the shoulder blade, this muscle is one of many that aids shoulder movement.
  • Infraspinatus: This rotator cuff muscle helps raise and lower the arm.
  • Teres major: This muscle helps rotate the upper arm.
  • Serratus anterior: Located in the rib cage, this muscle keeps the shoulder blade against the chest wall and helps rotate the shoulder blade upward.
  • Deltoid: This muscle gives the shoulder its rounded shape as well as raises and rotates the arm.
  • Latissimus dorsi: This flat rectangular muscle of the back helps the arms rotate as well as move away from and toward the body.

Women’s chests typically have higher fat deposits than men’s to protect their accessory sex glands, or breasts. The fat deposits help protect a woman’s milk-producing glands and ducts.

The mammary glands are responsible for producing milk for a newborn following childbirth. Each gland is made up of a series of lobules, which are glands that produce milk. These connect to ductal lobes, which connect to the lactiferous ducts.

The lactiferous ducts form a tree-branch-like network of ducts that converges at the nipple. These ducts dilate behind the areola to hold accumulating milk.

The milk leaves the body through tiny pores in the nipple. These are known by many names, including milk ducts, mammary ducts, and galactophores.

The mammary glands and tissue surrounding them begin developing as accessory sex organs at the onset of puberty. However, the breasts remain inactive until hormonal changes during pregnancy signal milk production to begin.

A mother can continually produce milk for years as long as there is regular suckling or pumping of the milk.