💀 Skeletal System Lesson

PEST OF 6: the six cranial bones

Six bones form the protective vault around the brain — two paired, four unpaired — and one of them is called the "keystone" for a specific structural reason.

Parietal

Ethmoid

Sphenoid

Temporal

Occipital

Frontal

📖 Full Breakdown

Six bones, two paired and four unpaired, forming the cranial vault

The sphenoid earns a special nickname because of its unique structural relationship to every other cranial bone.

Parietal (paired)

Top and sides of the skull

One of only two paired bones in this group.

Ethmoid (unpaired)

Roof of the nasal cavity

Contains the cribriform plate, through which CN I (Olfactory) passes — connecting this cranial bone directly to the sense of smell.

Sphenoid (unpaired)

The "keystone" of the cranial floor

Called the keystone because it articulates with EVERY other cranial bone — a unique structural role that no other cranial bone shares. Also houses the pituitary gland within the sella turcica.

Temporal (paired)

Sides of the skull

Contains ear structures and the mastoid process.

Occipital (unpaired)

Back of the skull

Contains the foramen magnum, where the spinal cord exits the skull to become continuous with the vertebral column.

Frontal (unpaired)

Forehead and orbital roofs

Houses the frontal sinuses.

🩺 Clinical / Exam Application

A traumatic injury to the base of the skull affects the sphenoid bone specifically, and the patient develops hormonal abnormalities. Because the sphenoid houses the pituitary gland within its sella turcica — a specific bony depression shaped to cradle this critical endocrine gland — direct injury to this particular cranial bone can physically affect pituitary function, connecting skeletal trauma directly to endocrine disruption in a way injury to most other cranial bones would not.

⚠️ Exam Alert

The sphenoid's nickname as the "keystone" of the cranial floor is frequently tested — know specifically that it articulates with all other cranial bones, a unique structural distinction, and that it houses the pituitary gland in the sella turcica.

🚧 Common Trap

Don't assume all six cranial bones are paired or unpaired uniformly. Only parietal and temporal are paired (2 each); ethmoid, sphenoid, occipital, and frontal are each single, unpaired bones — mixing up which bones come in pairs is a common error.

✅ Quick Check

Why does the sphenoid bone specifically get called the "keystone" of the cranial floor, and what important endocrine structure does it house?

📝 Exam Prep

Common Exam Questions

❓ What are the six cranial bones?

✅ Parietal (paired), Ethmoid, Sphenoid, Temporal (paired), Occipital, and Frontal. Together they form the protective vault surrounding the brain.

❓ Why is the sphenoid bone called the "keystone" of the cranial floor?

✅ The sphenoid articulates with every other cranial bone, a unique structural relationship not shared by any other single cranial bone. It also houses the pituitary gland within its sella turcica.