Which of the following systems is not classified as a body system?

The thoracic system is not classified as a distinct body system in the same way that the muscular, skeletal, or endocrine systems are. The muscular system pertains to the muscles and their functions, enabling movement and maintaining posture. The skeletal system consists of bones and cartilage that provide structure and protect vital organs. The endocrine system includes glands that produce hormones, regulating various bodily functions such as metabolism, growth, and mood.

The term "thoracic" typically refers to the area of the body known as the thorax, which houses vital organs like the heart and lungs, as well as various structures, such as ribcage and muscles involved in breathing. However, it does not denote a standalone system; instead, it is more accurately described as part of the anatomy relating to the chest area, which falls under broader systems. Thus, it does not fit the classification of a body system like the others listed.