Human Anatomy · Axial Skeleton

• The skull has 22 bones: 8 cranial bones (the braincase) and 14 facial bones. The hyoid is associated with the skull but counted separately.
• The calvaria is the domed skullcap; the cranial base is the floor, shaped into anterior, middle, and posterior cranial fossae that cradle the brain.
• Most skull bones meet at immovable fibrous joints called sutures; only the mandible moves freely.

• Frontal (single): the forehead, the roofs of the orbits, the floor of the anterior cranial fossa.
• Parietal (paired): the roof and upper sides; they meet at the sagittal suture and the frontal bone at the coronal suture.
• Occipital (single): the back and base. Key markings: the foramen magnum (spinal cord), the occipital condyles (meet the atlas, the "yes" joint), the external occipital protuberance, and the hypoglossal canal (CN XII).
• Foramen magnum contents mnemonic: "VAMPires Sing AT Midnight" (vertebral arteries, spinal accessory nerve, alar & apical ligaments, the medulla).

The temporal bone packs many testable markings.

• Zygomatic process: joins the cheekbone to form the zygomatic arch.
• Mandibular fossa: the socket of the temporomandibular joint.
• External acoustic meatus (ear canal); mastoid process (air-filled, muscle attachment); styloid process (anchors tongue/neck muscles).
• Petrous part: dense wedge housing the middle and inner ear; the internal acoustic meatus carries the hearing and facial nerves; the stylomastoid foramen is where the facial nerve exits.

• Sphenoid, the keystone that articulates with every other cranial bone: the body holds the sphenoid sinus and the sella turcica (the saddle cradling the pituitary); the greater and lesser wings and pterygoid processes extend out. It carries the optic canal, superior orbital fissure, and foramen ovale.
• Ethmoid, the deepest bone: the crista galli (anchors the brain's dura), the cribriform plate (the olfactory nerve filaments pass through), the perpendicular plate (upper nasal septum), the superior and middle conchae, and the ethmoid sinuses.

• Six paired: maxillae (upper jaw, anterior hard palate, hold upper teeth), palatine (posterior hard palate), zygomatic (cheekbones), nasal (bridge), lacrimal (medial orbit, tear-duct groove), inferior nasal conchae.
• Two single: vomer (lower nasal septum) and mandible (lower jaw, the only freely movable skull bone).
• Mandible markings: body, ramus, angle, condylar process (the jaw joint), coronoid process (temporalis muscle), and the mental foramen (nerve to the chin and lip). The infraorbital foramen of the maxilla passes the nerve to the face.

• Sutures (immovable fibrous joints): coronal (frontal to parietals), sagittal (between parietals), lambdoid (parietals to occipital), squamous (parietal to temporal).
• The pterion, the H-shaped meeting of frontal, parietal, temporal, and sphenoid, is the thinnest part of the skull; the middle meningeal artery runs just beneath it, so a blow there can bleed into the skull.
• Fontanelles are fibrous gaps in the infant skull that allow molding at birth and growth. The anterior fontanelle is largest and closes by about 18 to 24 months; the posterior closes by about 2 months.

• The orbit is built from seven bones (frontal, sphenoid, zygomatic, maxilla, lacrimal, ethmoid, palatine).
• The nasal septum = perpendicular plate of the ethmoid + the vomer; the hard palate = maxillae + palatine bones.
• The paranasal sinuses (frontal, ethmoid, sphenoid, maxillary) lighten the skull and drain into the nasal cavity, so a cold can become sinusitis; the maxillary sinus sits so near the upper teeth that a tooth infection can reach it.
• The hyoid bone, slung from the styloid processes, articulates with no bone and anchors the tongue.

A foramen is an opening for a nerve or vessel. Know the big ones by what passes through.

• Foramen magnum (occipital): spinal cord and vertebral arteries.
• Optic canal (sphenoid): optic nerve (CN II). Superior orbital fissure: the eye-movement nerves. Foramen ovale: trigeminal branch to the lower jaw.
• Cribriform foramina (ethmoid): olfactory nerve (CN I). Internal acoustic meatus: hearing and facial nerves.
• Carotid canal: internal carotid artery. Jugular foramen: internal jugular vein. Mental foramen: nerve to chin and lip.

• The column starts as 33 vertebrae and, after the sacral and coccygeal bones fuse, ends as 26 bones: 24 separate vertebrae plus the sacrum and coccyx.
• Regions, superior to inferior: cervical (7), thoracic (12), lumbar (5), the sacrum (5 fused), and the coccyx (4 fused).
• Four front-to-back curvatures act like a spring. Primary curves (thoracic, sacral) are present at birth; secondary curves (cervical, lumbar) develop as a baby lifts its head and begins to walk.
• Abnormal curves (beyond the notes): exaggerated thoracic kyphosis, lumbar lordosis, and lateral scoliosis.

• Body: the weight-bearing block in front.
• Vertebral arch: two pedicles and two laminae enclosing the vertebral foramen. Stacked, these form the vertebral canal for the spinal cord.
• Processes: one spinous, two transverse, and paired superior and inferior articular processes with facets that link neighboring vertebrae.
• Intervertebral foramina: the side gaps where spinal nerves exit. Between bodies sit the intervertebral discs: a tough outer anulus fibrosus around a gel-like nucleus pulposus.

• Atlas (C1): a ring with no body and no spinous process; it holds up the skull at the atlanto-occipital joint, the "yes" nod.
• Axis (C2): bears the dens (odontoid process), the peg the atlas pivots around at the atlanto-axial joint, the "no" rotation.
• Vertebra prominens (C7): a long spinous process you can feel at the base of the neck.
• Sacrum: five fused vertebrae, a triangle wedged between the hip bones. Coccyx: four fused vertebrae, the tailbone.

• Sternum (three fused parts): the manubrium (meets the clavicles and first ribs), the body, and the xiphoid process. The jugular notch is the dip you feel at the top; the sternal angle is the ridge at the manubrium-body junction, level with the second rib, a key landmark.
• Ribs: the head articulates with vertebral bodies, the tubercle with a transverse process, and the costal cartilage connects a rib toward the sternum.