Skeletal System: Axial Skeleton and Cranial Bones

The human adult body (after approximately 30 to 35 years of age) contains 206 bones. More bones are present before this age, but several merge over time.
The skeleton is divided into two main sections:
- Appendicular Skeleton: Generally comprises the arms and legs, along with the hip and shoulder bones that attach them to the torso.
- Axial Skeleton: Forms the long axis of the body, providing central support. It encases and protects the brain and spinal cord, and includes structures like the ribs and sternum.

The axial skeleton consists of 80 bones in total:
- Skull: 28 bones (8 cranial, 14 facial, 6 auditory ossicles)
- Hyoid bone: 1
- Vertebral column: 26 vertebrae
- Ribs: 24 (12 pairs)
- Sternum: 1
Functions:
- Provides the central support structure for the body.
- Protects the central nervous system (brain and spinal cord) by fully encasing them, offering robust protection.
- Protects organs within the ventral cavity (e.g., heart and lungs) via the rib cage.
- Serves as attachment sites for the lower limbs (though the hip and shoulder bones themselves are appendicular).

Bone: Refers to the entire, individual structure (e.g., the occipital bone).
Marking: A specific location or structure found on a particular bone (e.g., the foramen magnum on the occipital bone).
Analogy: A bone is like the entire country (e.g., The United States), while a marking is a specific place within that country (e.g., Connecticut).
Exam Importance: It is crucial to pay very careful attention to whether exam questions ask for the bone or the marking to ensure correct answers and full credit.

Number: 1 bone.
Location: Makes up the forehead.
Markings:
- Sinus: A space within the bone. Main job is mucus production, but also thought to aid in sound resonance and lighten the skull. Lighter skulls help balance the head on our relatively slender necks, which is beneficial for bipedal posture.

Number: 2 bones (right and left).
Location: Form the superior and lateral aspects of the skull. They are separated by the sagittal suture.
Markings: No specific markings mentioned for recall.

Location: Forms the back and bottom of the skull.
Markings:
- Foramen magnum: A large opening through which the spinal cord passes.
- External occipital protuberance: A prominent bump on the back of the skull where neck muscles attach. Due to prolonged periods of looking down at phones, this marking can become more bony and pronounced in many individuals due to increased mechanical stress.
- Occipital condyles: Flattened areas that form a joint with the first vertebra (C1, atlas).
Calvaria (Skull Cap): The collective term for the superior portion of the cranial bones (frontal, parietal, occipital, and parts of the temporal and sphenoid).
Clinical Relevance - 3D-Printed Skull: In 2014, Dr. Bon Verweij, a surgeon in The Netherlands, became the first person to successfully transplant a 3D-printed plastic skull into a patient whose skull was producing bony growths that pressed into her brain. This innovative procedure involved removing the diseased skull bone and replacing it with the 3D-printed substitute.

Number: 2 bones (right and left).
Location: Located on the sides of the head (temple area), encompassing the ear canal, and contributing to part of the chin bone.
Markings:
- Zygomatic process: Forms the posterior portion of the cheekbone.
- External acoustic meatus: The ear canal; also referred to as the auditory meatus or external auditory meatus.
- Mastoid process: A large, pointed projection inferior to the external acoustic meatus, serving as a site for muscle attachment. For example, the sternocleidomastoid muscle attaches here (the name indicates attachment to sternum, clavicle, and mastoid process).
- Styloid process: A slender, pointed projection, used for muscle attachment, particularly muscles that hold the hyoid bone in place.
- Mandibular fossa: An indentation where the mandible (lower jawbone) articulates.
- Carotid canal: A passageway located more anteriorly within the bone, allowing the carotid artery to pass through.
- Jugular foramen: Located on either side of the foramen magnum, serving as a passageway for the jugular vein.

Location: Found towards the bottom of the cranium, it has a distinctive butterfly shape when viewed from above. While mostly internal, a front portion makes up part of the temples.
Markings:
- Sella turcica: A saddle-shaped indentation where the pituitary gland sits. The pituitary gland is a crucial endocrine gland connected to the hypothalamus via the infundibulum.
- Greater wings: These form the front portion of the temple.
- Lesser wings: These are the pointy bits on either side when viewed from the top down, located superior and anterior to the greater wings.
- Sphenoid sinus: An air-filled cavity within the bone.
- Optic canal: A passageway through which the optic nerve travels from the eye to the brain.

Location: The smallest and most complex cranial bone. It is almost entirely inside the skull, with only a small portion visible inside the eye sockets near the tear ducts and contributing to the nasal cavity.
Markings: This bone has the most markings to remember due to its intricate structure.
- Cribriform plate: Flat, perforated plates on either side, containing numerous small holes. These holes allow olfactory nerves to pass from the nasal cavity up to the olfactory bulb in the brain, enabling the sense of smell.
- Crista galli: A central, pointed projection that serves as an anchor point for the meninges (protective layers around the brain) for stability. The meninges are unlike serous membranes found in other body cavities.
- Perpendicular plate: Forms the superior part of the nasal septum.
- Superior nasal conchae: Scroll-shaped projections that help filter inhaled air. These are not typically visible in an articulated (joined) skull.
- Middle nasal conchae: Similar scroll-shaped projections that also filter inhaled air. These are visible in an articulated skull.
- Ethmoid sinus: An air-filled cavity within the bone.

Definition: Immovable joints that connect the bones of the cranium.
Development: Sutures do not fully form until approximately age 24, allowing the brain and skull to grow and mature up to that point.
Key Sutures (on practical list):
- Sagittal suture: Runs down the midline, separating the two parietal bones.
- Coronal suture: Divides the frontal bone from the two parietal bones.
- Squamous suture: Located between the parietal bone and the temporal bone on each side.
- Lambdoid suture: Separates the occipital bone from the two parietal bones.
Important Note: While