Comprehensive Anatomy Notes: The Skeletal System and Axial Skeleton

The course schedule for the following weeks involves covering Muscle and the Nervous System (parts 1, 2, and 3) over a two-week period.
The current material marks the beginning of the content for Practical 2. While earlier introductory material (such as directional terms) applies, students are not directly tested on the previous basic definitions but must apply them to the skeletal and muscular systems.
Important directional terms that remain in use include: * Superior and Inferior. * Posterior and Anterior.
Students are encouraged to review the diagram of body regions, as many bones and muscles are named according to the specific region of the body where they are located.

The adult human skeleton consists of exactly $206$ bones.
The primary functions of the skeletal system include:
* Providing internal structure and support (without which the body would behave like a "jellyfish blotting").
* Protecting vital organs (e.g., the skull protects the brain; the rib cage protects the heart and lungs).
* Facilitating movement by acting as levers for muscles (primarily the bones of the arms and legs).
* Production of red blood cells (erythrocytes). Because erythrocytes have no nucleus, they cannot divide; they must be produced in the red marrow of certain bones.
The skeleton is divided into two main groups: * Axial Skeleton: The core of the body, including the skull, facial bones, vertebrae, ribs, and sternum. * Appendicular Skeleton: The limbs and their attachments, including the shoulders, arms, pelvis, legs, and feet.

There are five primary classifications of bones: 1. Long Bones: These support weight and facilitate movement. Examples include the bones of the arms, legs, and fingers. 2. Flat Bones: Primarily serve to protect internal organs. These include the cranial bones (skull), scapula (protects the back of the lungs), sternum, and ribs (protecting the heart and lungs). 3. Short Bones: Small, cuboidal bones located in the inner wrists (carpals) and ankles (tarsals). These allow for movement in multiple directions and axes, unlike the long bones which typically move on a single axis. 4. Irregular Bones: Bones with complex shapes that do not fit other categories. Primary examples include the vertebrae (spine) and the pelvic bones. 5. Sesamoid Bones: These reinforce tendons and protect them from stress and wear. The primary example is the patella (kneecap).

Macroscopic Structure:
* Compact Bone: The dense outer layer seen under a microscope.
* Spongy Bone: A layer found inside the compact bone.
* Medullary Cavity: A large central cavity within the bone filled with marrow.
Bone Marrow Types: * Red Bone Marrow: Site of red blood cell production. It is found in the axial skeleton (skull, ribs, sternum, vertebrae), as well as the pelvis and scapula. * Yellow Bone Marrow: Composed mostly of fat. Found primarily in the appendicular skeleton.
Cellular Components: * Osteocytes: Mature bone cells found within the compact bone. * Osteoblasts: Dividing cells responsible for making more bone tissue. The term "blast" refers to cells that are dividing and creating new osteocytes. * Osteoclasts: Cells responsible for breaking down bone. They remove old or damaged bone tissue to make room for new growth.

A fracture is the clinical term for a broken bone. There are five main types discussed: 1. Closed (Simple) Fracture: A clean break where the bone is broken straight through, but the skin remains intact. 2. Open (Compound) Fracture: A break where the bone penetrates through the skin. 3. Greenstick Fracture: A partial break where one side of the bone breaks and the other side stays intact, often resulting from bending. This is most common in children because their bones are softer and more pliable. 4. Comminuted Fracture: A break where the bone fragments into several places (more than two pieces). 5. Impacted Fracture: Occurs when pressure is applied to the ends of the bone, causing one part of the bone to be driven into the rest of the bone. This often occurs from a high-impact fall where the limbs are locked.

The repair process occurs over several stages: 1. Hematoma Formation: A broken bone bleeds; within hours, the blood clots at the site to form a hematoma. 2. Tissue Removal: Osteoclasts remove dead bone tissue in the area. 3. Callus Formation: Fibroblasts penetrate the area to build a fibrocartilaginous callus that bridges the break. 4. Transformation: Over many weeks, osteoblasts transform the cartilaginous callus into hard bone. 5. Remodeling: Continued mechanical stress and remodeling shape the new bone. This process can take months or years to regain original strength and internal structure.
Bone tissue grows significantly slower than skin; while a cut might heal in a couple of days, bones require weeks or months of stabilization (usually in a cast) to prevent re-breaking during the repair phase.

Bones serve as a primary storage site for calcium, which is utilized by the thyroid glands and the circulatory system.
Osteoporosis: An imbalance where osteoclasts remove bone tissue faster than osteoblasts can replace it. * Results in a decrease in overall bone tissue density. * Leads to weakened, brittle bones that are easy to break, as well as increased pain and tenderness. * More common in women and older individuals because bone growth is naturally slow.
Prevention: High calcium intake early in life (20s and 30s) through diet or supplements is recommended.

The skull consists of cranial bones (surrounds the brain) and facial bones.
Cranial Bones ( $8$ total):
1. Occipital Bone: Located at the back of the head.
* Foramen Magnum: The large hole at the base of the skull where the spinal cord exits. It is the second largest foramen in the body.
* Occipital Condyles: Rounded projections on either side of the foramen magnum that articulate with the spine to allow for nodding.
2. Parietal Bones: Located on the top and sides of the skull.
3. Temporal Bones: Located at the sides of the head, near the ear region.
* External Auditory/Acoustic Meatus: The ear canal.
* Mastoid Process: Large rounded projection posterior to the ear canal; contains the mastoid sinuses.
* Styloid Process: A sharp, pointy projection deep to the mastoid process.
* Zygomatic Process (Zygomatic Arch): A part of the temporal bone that jets forward to meet the zygomatic (cheek) bone.
4. Frontal Bone: Makes up the forehead.
* Frontal Sinus: An air-filled cavity located in the forehead region; inflammation here causes sinus pressure.
5. Sphenoid Bone: A butterfly-shaped bone visible underneath the skull and at the temples; it forms the back of the eye socket.
6. Ethmoid Bone: Tucked inside the skull; it forms the top half of the nose.