The muscles of the torso are often thought of as chest and back muscles, when in fact most, other than the obliques and the abdominals, are really muscles of the arm.
I've posted these diagrams that I've begun to prepare for my anatomy master classes this summer in order to help others who find this area confusing. All these muscles coming off the ribcage and the scapula can seem confusing, and though not without complexity, are not that hard to understand.
Once again I emphasize the need to know the skeleton since once you understand the origins and insertions of the muscles as cables, then no matter where you position the arm, it becomes self evident how the form should follow.
More about future anatomy workshops this summer to follow shortly.
Feel free to copy these images for your use.