Bones have various surface features that help form joints and attach to other soft tissues. Depending on the function, bone markings are categorized into articulating projections, processes for attachment, depressions, and openings.
Articulating Projections
Articulating projections are found where two bones meet to form a joint. These structures are usually found at the ends of bones. The largest articulation is a rounded projection called the head, supported by a narrow neck at the ends of large, long bones. These articulations form a ball-and-socket joint, as seen at the hip and shoulder joints. A similarly rounded knuckle-like projection is the condyle, also found at the ends of long bones. Both the head and condyle are usually covered in articular cartilage for ease of movement at the joints. Another articulating feature is the flat, broad surface called the facet. This is seen in bones such as the vertebrae, which form the facet joints with other vertebrae. Facets may also be slightly convex or concave.
Processes for Attachment
Bones can only provide structure and movement to the body if they are attached to surrounding tissues, such as ligaments, tendons and aponeuroses, and muscles. These points of attachment are usually raised projections from the bone surface. Any projection is called a process, and it can vary in shape and size. For example, large muscles can attach at long, narrow, and usually curved projections called crests, or along sharp, slender, and usually pointed processes called spines. In contrast, tubercles are small, knobby processes that help attach to smaller muscles, tendons, and ligaments. An epicondyle is a tubercle-like process that arises atop a condyle. Other rounded projections with a roughened surface are called tuberosities.
Depressions and Openings
In addition to projections, bones also have depressions and openings that allow vasculature and nerves to pass through or along the bone without being compressed against the surface. A large, shallow, basin-like depression, such as those found in cranial bones, is a fossa, which helps protect and house vital organs such as the brain. Smaller fossae are found at certain joints of the ball-and-socket type and help articulate with the rounded head of the other bone. In contrast, the sulcus is a narrow, furrow-like groove on the surface that provides a pathway for blood vessels, lymphatic vessels, and nerves. These vessels and nerves can also pass through bones at openings such as the foramen (round- or oval-shaped holes), and the fissure (narrow, slit-like opening). Openings also include the meatus (a tube-like passageway), and the sinus (an air-filled cavity within a bone).
