The torso, or trunk, is the central mass of the human body, acting as the foundation from which the neck, head, and four limbs extend. Defining the torso’s boundaries relies on specific anatomical landmarks that mark the separation between the core body and its appendages, rather than a single surface line. This central structure houses the body’s primary organ systems and provides attachment points for the extremities. Understanding the boundaries requires looking at the skeleton, which provides a fixed, precise anatomical definition.
The Three Major Regions of the Torso
The internal structure of the torso is segmented into three distinct anatomical cavities, stacked vertically. The most superior region is the Thorax, or chest, protected by the rib cage and sternum. This area contains the heart, major blood vessels, and lungs, separated from the region below by the diaphragm.
Immediately inferior to the thorax is the Abdomen, a large cavity primarily housing the organs of the digestive system, such as the stomach, liver, intestines, and kidneys. The abdomen is not protected by a continuous bony structure like the thorax, relying instead on layers of muscle for support.
The lowest region of the trunk is the Pelvis, contained within the protective basin of the pelvic girdle. This region is continuous with the abdominal cavity and contains the bladder, rectum, and internal reproductive organs. These three regions—the thorax, abdomen, and pelvis—collectively form the anatomical core of the torso.
Defining the Superior Boundary
The superior boundary of the torso marks the point where the neck begins. This boundary is defined by the superior thoracic aperture, often called the thoracic inlet, which is a ring of bone and cartilage. This opening establishes the upper limit of the chest cavity and the torso itself.
Anatomically, the superior line of demarcation runs across the highest parts of the collarbones, or clavicles. At the front, this line passes through the sternal notch, the dip at the top center of the breastbone. At the back, the boundary is set by the upper surface of the first thoracic vertebra, T1, which is the first vertebra below the neck vertebrae. This line excludes the entire neck and head from the torso.
Defining the Inferior Boundary
The inferior boundary of the torso defines where the trunk ends and the lower limbs begin. This boundary is fundamentally determined by the pelvic girdle, the bony ring formed by the hip bones and the sacrum. The torso ends exactly where the legs articulate with the pelvis, meaning the hips are considered part of the trunk structure.
The point of articulation is the acetabulum, the socket on the hip bone where the head of the femur inserts to form the hip joint. The torso encompasses the entire bony pelvis, but the lower limbs extend outward from this joint.
A surface landmark defining this lower limit is the inguinal ligament, a fibrous band stretching from the anterior superior iliac spine (the front point of the hip bone) to the pubic bone. This ligament creates the crease separating the lower abdomen from the thigh. Structures passing beneath this ligament are considered to be entering the lower limb, providing a functional separation point.
The lowest point of the trunk is the diamond-shaped perineum, located at the pelvic outlet between the thighs. The perineum marks the lower surface of the torso, acting as the floor of the pelvis. Its boundaries are defined by surrounding bony structures, including the pubic symphysis at the front and the coccyx (tailbone) at the back. The perineum is separated from the pelvic cavity above by the pelvic floor muscles, making it the terminal region of the trunk.