Your fingers are able to flex and extend. This type of motion is bidirectional, but along only one axis. This type of joint is called a hinge joint, at which you are able to flex and extend along an axis, with no side to side motion.

The nucleus pulposus is a gelatinous, spongy material located in the core of the vertebral disk. This material responds directly to the pressures of the spinal joints, allowing for twisting, compressing, and stretching forces acting upon the spinal joints and vertebrae. This material can become less resilient with age, but generally does not disintegrate or die as other structures do.

When the rhomboids contract, they pull on the scapula, causing the inferior angle of the scapula to swing up toward the spine. This causes the glenoid fossa to orient downward. Thus, the scapula is downwardly rotated by the rhomboids at the scapulocostal joint. 

When the psoas minor contracts to flex the lower trunk relative to the upper trunk, this posterior tilting of the pelvis occurs at the lumbosacral joint. 

While ligaments are used to connect a bone to another bone, tendons are used to attach muscles to bones. Tendons are made of collagen and are able to withstand great amounts of tension.

Joints refer to the general articulation between two bones can house several different structures. Tendons and ligaments often play key roles in joint mobility and stability. Fascia is the region of connective tissue found between and around muscles and can help anchor them to other nearby muscles or organs.

Tendons are formed at the ends of muscles, and are a compact version of muscle. The tendon connects the muscle to the bone to allow flexing and extension motions, transferring the force of muscle contraction to the bone to cause movement. Ligaments connect bone to bone and provide further support to the bones and skeleton by stabilizing joints.

The skeletal system is comprised of bones, bone marrow, ligaments, and cartilage. The skeletal system is responsible for a number of functions. Most prominently, the bones bear the weight of the body, provide attachment points for muscles, and are responsible for locomotion. Bones also serve as stores for calcium and phosphorus. Red bone marrow is responsible for producing erythrocytes and the cells of the immune system, while yellow bone marrow plays a key role in fat storage.

The bones can be broken into two main categories: the axial skeleton and the appendicular skeleton. The bones that make up the axial skeleton are the vertebral column, thoracic cage, hyoid bone, and the skull. The appendicular skeleton is made of the upper and lower limbs, the pelvic girdle, and the pectoral girdle.

Cartilage serves to provide cushioning between bones, and ligaments are essential for maintaining stability.

Functions of the skeletal system include support, protection, mineral storage, and hematopoiesis. The structure of bones and their coordinated function with muscles, ligaments, and tendons allow for movement and support of the body. They also provide protection of vital organs, such as the rib cage surrounding the heart and pelvis protecting reproductive organs. Calcium and phosphate are stored in the hydroxyapatite matrix of the bone and hematopoiesis, the generation of red blood cells, takes place in red bone marrow, which houses hematopoietic stem cells.

Several systems coordinate thermoregulation, but ultimately this function is performed by the muscles. Muscles are responsible for most ATP usage in the body. Break down of ATP releases heat, which is why we shiver when we are cold. Regulation of blood flow and actions of the integumentary system also contribute to thermoregulation in other manners.

The correct answer is osteons. An osteon (or haversian system) is the fundamental unit of most compact bone. Osteons are formed during the process of bone remodeling, but are not one of the three types of cells involved in bone homeostasis.

Bone homeostasis involves the development, growth and remodeling of bones. The three cell types responsible for these processes are osteoblasts, osteocytes and osteoclasts. Each cell type refers to a different process:

• osteoblasts = bone forming cells
• osteocytes = mature bone cells
• osteoclasts = break down and reabsorb bone

All of these layers make up hyaline cartilage. The superficial layer is distinguished by the parallel (relative to the articular surface) arrangement of it's collagen fibers. The middle zone is characterized by the oblique orientation of its fibers. The deep zone has collagen fibers that are oriented perpendicularly to the articular surface. The calcified layer connects directly to the underlying (subchondral) bone.