The vertebral column, commonly called the spine, is a flexible structure composed of a series of bones called vertebrae. This column provides central structural support for the upper body and protects the delicate spinal cord housed within its central canal. To accurately locate specific points, medical professionals rely on a standardized system of identification. This system divides the spine into distinct regions and assigns a letter and number to each individual bone.
Defining the Five Regions of the Spine
The human spine is divided into five major anatomical sections, each designated by a specific letter. Starting at the base of the skull, the Cervical spine (‘C’) makes up the neck region. This segment supports the weight of the head and allows for a wide range of motion.
Moving downward, the Thoracic spine (‘T’) corresponds to the mid-back or chest area. These vertebrae are distinguished by their connection to the rib cage. Below this is the Lumbar spine (‘L’), which forms the lower back. This region bears the greatest amount of the body’s weight.
The final two regions are located in the pelvis area. The Sacrum (‘S’) is a large, triangular bone situated between the hip bones. At the bottom is the Coccyx, or tailbone, which is often grouped with the sacrum for clinical designation. These regional designations provide the necessary context for locating any specific vertebra.
The Standardized Numbering System
The system for numbering vertebrae is a consistent, sequential process that moves from the skull downward. Once the region is identified by its letter, each vertebra is assigned a number based on its position, starting with number one at the top. For example, the first bone in the cervical region is C1, and the last is C7, providing a precise anatomical address.
The three movable sections—cervical, thoracic, and lumbar—have a fixed number of segments. The cervical spine is composed of seven vertebrae (C1 through C7). The thoracic spine contains twelve bones, labeled T1 through T12.
The lumbar spine consists of five segments, numbered L1 through L5. Clinicians use this system to reference a specific bone, such as L4, which is understood to be the fourth vertebra in the lower back. This nomenclature ensures medical teams can accurately communicate the precise location of an injury or condition.
Special Designations and Fused Segments
Unique Cervical Vertebrae
While most vertebrae follow the simple letter-number rule, the first two cervical bones have unique anatomical names due to their specialized function. The C1 vertebra is called the Atlas, a ring-shaped bone that supports the skull and allows for the head’s nodding motion. The C2 vertebra, known as the Axis, features a bony projection that acts as a pivot, enabling the head to rotate.
Fused Segments
The lower segments of the spine are treated differently because they are fused together in adults. The Sacrum is counted as five separate sacral vertebrae (S1 through S5) that have merged into a single, solid bone. Similarly, the Coccyx is formed from the fusion of three to five smaller coccygeal vertebrae.
These fusions begin in late adolescence and are completed by middle age. The adult spine contains 24 movable vertebrae, plus the two fused complexes of the sacrum and coccyx, totaling approximately 33 segments.