Human Skeletal Anatomy: Skull, Spine, and Thorax Bones

Cranial Bones

Sphenoid Bone

The sphenoid bone, shaped like a wedge in the middle of the skull base, presents a body, two lesser wings, two greater wings, and two pterygoid processes. Its upper body contains a cavity that houses the pituitary gland. It participates in the formation of the orbital cavity and contains the sphenoid sinus within its body.

Occipital Bone

It features a large opening, the foramen magnum, which allows for the continuity of the brainstem with the spinal cord. On both sides of the foramen magnum are the occipital condyles, which articulate with the atlas.

Temporal Bone

It has three main parts: the squamous part (forming part of the cranial vault), the petrous part (housing portions of the ear canal, middle ear, inner ear, and the organ of balance), and the mastoid part.

Parietal Bone

It is a classic example of a flat bone, articulating with its counterpart on the opposite side, as well as with the frontal, occipital, and temporal bones (squamous part), and the greater wing of the sphenoid. It is a prominent component of the cranial vault.

Base of the Skull

The base of the skull presents three well-defined compartments, known as the cranial fossae:

• The anterior cranial fossa extends to the posterior border of the lesser wings of the sphenoid bone.
• The middle cranial fossa ranges from the lesser wings of the sphenoid to the upper border of the petrous part of the temporal bone.
• The posterior cranial fossa extends posteriorly from the petrous part of the temporal bone.

The anterior cranial fossa houses the frontal lobes of the brain. The middle cranial fossa contains the temporal lobes and the pituitary gland. The posterior cranial fossa accommodates the cerebellum and the occipital lobes. The base of the skull features several foramina (holes) that facilitate the connection of the brain with the extracranial structures, crucial for neurological functions, blood supply, and venous drainage. These include the optic foramen, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, foramen lacerum, carotid canal, and hypoglossal canal (anterior condylar foramen).

Bones of the Trunk and Neck

This section covers the spine, sternum, ribs, and hyoid bone.

Regional Morphological Variants of Vertebrae

Atlas (C1 Vertebra)

The Atlas (C1 vertebra) is the first cervical vertebra and is considered atypical because it lacks a vertebral body.

Structure of the Atlas

• Two lateral masses joined by two arches: one anterior and one posterior.
• Each lateral mass has six articular facets.
• On its superior surface, there is a glenoid articular facet that articulates with the occipital condyles.
• On its inferior surface, it has an articular facet for the axis (C2 vertebra).

Axis (C2 Vertebra)

The Axis (C2 vertebra) is the second cervical vertebra, notable for a prominent projection called the odontoid process (or dens), which consists of an apex, neck, and base.

Structure of the Axis

• Vertebral Body: Cuboid in shape with six sides.
• Dens (Odontoid Process): A superior projection from the body.
• Vertebral Foramen: Heart-shaped, resembling a playing card suit.
• Transverse Processes: Project laterally from the body and contain transverse foramina, through which the vertebral artery passes.
• Articular Processes: Two superior articular processes located on either side of the dens.

Sternum

The sternum is a flat, long, median, and symmetrical bone located in the anterior chest. It has an anterior (outer) surface and a posterior (inner) surface. It articulates with the clavicles (collarbones) and the first seven pairs of costal cartilages. Anatomically, it is divided into three distinct parts:

• Manubrium: The uppermost part, also known as the handle.
• Body of the Sternum: The central and largest part.
• Xiphoid Process: The smallest, most inferior part, often cartilaginous.