Skeletal System Notes: Skull, Sutures, Fontanels, and Hyoid

The skeleton is divided into two major regions:
- Axial skeleton: $80$ named bones (skull, vertebral column, thoracic cage, hyoid bone).
- Appendicular skeleton: $126$ named bones (pectoral girdles, pelvic girdle, upper and lower limbs).
Pectoral girdles: there are two, located in the shoulders; they do not touch each other in the front or back.
Pelvic girdle: there is only one, formed by the hip bones that join at the front.
Total bones in a typical adult skeleton: $206$ .
Variation in numbers:
- Sesamoid bones: extra bones that form within tendons (e.g., the patella).
- Sutural bones: extra bones that develop along skull sutures.
In a baby, there are about $270$ bones.
These bones fuse to form the typical adult skeleton; numbers decrease with development.

This video covers the bones of the cranium and the facial bones, and the hyoid bone.
The skull comprises 8 cranial bones: two parietals, two temporals, frontal, occipital, sphenoid, and ethmoid.
In addition to protecting the brain, the skull houses the ear ossicles, and provides attachment sites for jaw, neck, and facial muscles.

Frontal bone: develops as two separate bones; there is a frontal suture between them.
- The frontal suture is typically gone by age ~6.
Parietal bones: there are two (left and right).
Temporal bones: there are two (left and right).
- External auditory meatus: entrance to the ear canal.
- Mastoid process: attachment site for the sternocleidomastoid muscle.
- Styloid process: attachment point for ligaments that connect to the hyoid bone.
Occipital bone: located at the back/base of the skull.
Sphenoid bone: located in the middle of the skull; contains the lesser wing (top) and greater wings (lower).
Ethmoid bone: located above the sphenoid; contains the cribriform plates and the olfactory foramina; the perpendicular plate contributes to the nasal septum.

For the inferior view, the foramen magnum is the large opening in the occipital bone through which the spinal cord passes.
Occipital condyles: rounded processes that articulate with the atlas (the first cervical vertebra).
Articulation: a joint where two bones come together.
Atlas (C1) articulates with the occipital bone; movement between them allows flexion and extension (nodding “yes”).

Cribriform plates: horizontal bony plates with numerous tiny holes (olfactory foramina).
Olfactory nerves pass through the olfactory foramina.
Crista galli: a vertical ridge on the cribriform plate; serves as an attachment for the falx cerebri (dura mater).

Nasal septum upper part formed by the perpendicular plate of the ethmoid; lower part by the vomer.
Vomer: a single bone forming the inferior portion of the nasal septum.

Mandible (lower jaw).
Vomer (also part of nasal septum).
Maxillae (two): upper jaws; contain the upper teeth.
Zygomatic bones (two): cheekbones.
Nasal bones (two).
Lacrimal bones (two).
Palatine bones (two): form part of the posterior hard palate.
Inferior nasal conchae (two).
Note: the orbit (eye socket) is formed by both cranial and facial bones; its contents include elements from both bone groups.

Upper jaw is formed by the maxilla; contains the upper teeth.
Palatine bones contribute to the posterior part of the hard palate.
The maxilla and palatine bones should fuse along the midline to form a complete palate.
If the sutures don’t fuse properly, conditions like cleft palate and cleft lip can occur.

Fontanels are gaps between cranial bones in a developing fetus/newborn; they are dense connective tissue membranes (not bone yet).
Purposes:
- Allow fetal skull to deform slightly to pass through the birth canal.
- Accommodate rapid brain growth during infancy.
Fontanels eventually ossify (fuse) within the first year.
The largest fontanel is the anterior fontanelle; it may take up to two years to ossify.
The skull typically reaches adult size around $8-9$ years old.
At birth, the fetal skull has several fontanels:
- Anterior fontanelle (largest) on top of the skull.
- Anterolateral fontanelles (sphenoidal and mastoid fontanelles).
- Posterior fontanelle at the back (occipital region).

The hyoid bone hangs inferior to the jaw and does not articulate with any other bone.
It is suspended by ligaments from the styloid processes of the temporal bones and is connected to the larynx.
Functions:
- Supports the tongue.
- Provides attachment for tongue, neck, and pharyngeal muscles.

The skull’s structure combines protection (brain, sensory organs) with openings for nerves and vessels (foramina, canals).
Sutures allow growth and gradual fusion of skull bones; their timing affects fontanelle size and persistence.
The facial bones protect sense organs (sight, smell, taste) and contribute to the pathways of digestion and respiration.
Understanding the relationships among the cranial nerves, foramina, and sutures helps interpret imaging and clinical conditions (e.g., cleft palate, nerve entrapment).
The hyoid bone’s unique articulation (no direct bone-to-bone joints) underscores its role in speech and swallowing mechanics.