Let's start at the back side of the shoulders.
The trapezius is a wide kite on your back. It starts from the bottom of the skull and goes down to mid back. It's corners touch the top of the shoulder. The trapezius is working when you shrug up, rotate the scapula, or do a shoulder press.
To the front:
Pectoralis major -
Structure:
Origin: Clavicle, Lower part of the sternum (not higher), lower true ribs
Insertion: Ends in a broad tendon inserted into the biceptical groove, near the "greater tubercle of the humerus" (The upper outer bump of the shoulder) This is tucked under the deltoid.
Function - Moves the humerus (arm)
Deltoid -
Origin big matches the trapezius
Action - Raises the arm from the side to bring it to right angle with the trunk.
Draw the arm forward (helped by pecs)
Pull the arm back (helped by teres major & latissimus dorsi)
Seratus Anterior (I like to call them fingers)
8 or 9 along the ribs. "Saw" like see 3 or 4 of them
Carries the scapula forward - like when throwing a punch "boxers muscle"
Terres Major
An important visual indicator for artists. Located near the back under armpit. This muscle runs diagonally and, when drawn, indicates the bulk of the muscle and an important horizontal / diagonal visual line. (I've omitted the anatomical function in this post)
The upper arm!
The first action is flexion of the humerus, as in throwing a ball underhand, and in lifting a child. Secondly, it adducts the humerus, as when flapping the arms. Thirdly, it rotates the humerus medially, as occurs when arm-wrestling.
Triceps. A threebodied muscle on the back of the arm. (We can revisit this in greater detail later)
Not pictured:
Trapezius does not engage.
Rhomboid Major & Minor (adjuction of the scapula, slight upper rotation of the scapula - ROWS seated rows, etc..., chin up and dips)