Thoracic Vertebra
Irregular bone; part of the axial (core) skeleton
Also known as: Mid-back spinal segment, vertebrae of the chest
Classification
Irregular bone; part of the axial (core) skeleton
Dimensions
Medium-sized compared to other segments of the spine, significantly taller and deeper than the small neck bones but smaller than the chunky lower back bones. They feature a heart-shaped main body when viewed from above.
Key Features
The most striking feature is the long, slender spike (the spinous process) that points sharply downward, resembling a giraffe's head in profile. It also possesses flat, smooth circular patches on the sides of the thick body where the ribs attach. The central hole for the spinal cord is small and circular compared to other vertebrae.
Similar Bones
Often confused with Lumbar (lower back) vertebrae; however, Lumbar bones have a much larger, kidney-shaped main body and lack the rib attachment spots. It can also be confused with Cervical (neck) vertebrae, but neck bones usually have extra holes on the sides and are more delicate in structure.
Across Species
In many animals, these bones are the anchors for the ribs. In primates, the long bony 'cactus arm' pointing toward the back is angled downward, whereas in many four-legged mammals like dogs or horses, these points are often upright to support heavy neck muscles. In birds, many of these mid-back bones are fused together to provide a rigid platform necessary for the mechanics of flight.
Evolutionary History
These bones evolved from the simple segmented rods found in early fish. As vertebrates moved onto land, the mid-back segments became specialized to support the rib cage, protecting internal organs and facilitating a more efficient way of breathing. Over millions of years, they shifted from serving as a flexible swimming aid to a sturdy structural bridge supporting the weight of the body against gravity.
Photography Tips
To identify this bone, photograph it from the side to show the downward angle of the back spike, and from the top looking down through the central canal. Placing a coin or ruler next to it helps show the medium-scale size relative to other spine segments.
Notes
What is the blue highlighted one?