Chapter 6: The Skeletal System - Axial Division

The Axial Division of the Skeletal System

I. Introduction to the Skeletal System

• The human skeleton is divided into two main sections:

  • Axial skeleton

  • Appendicular skeleton

• An adult human skeleton consists of $206$ separate bones.

  • The axial skeleton comprises $80$ bones, forming the central axis of the body.

  • The appendicular skeleton consists of $126$ bones.

A. Components of the Axial & Appendicular Skeletons

• Axial Skeleton Components:

  • Skull bones

  • Auditory ossicles

  • Hyoid bone

  • Vertebral column

  • Thoracic cage (ribs and sternum)

• Appendicular Skeleton Components:

  • Limbs (upper and lower)

  • Pectoral girdle (shoulder)

  • Pelvic girdle (pelvis)

• Detailed Bone Count of the Axial Skeleton:

  • Skull and Associated Bones: $29$

    • Cranium: $8$

    • Face: $14$

    • Auditory ossicles: $6$ (enclosed in temporal bones)

    • Hyoid: $1$

  • Thoracic Cage: $25$

    • Sternum: $1$

    • Ribs: $24$

  • Vertebral Column: $26$

    • Vertebrae: $24$

    • Sacrum: $1$

    • Coccyx: $1$

  • Total Axial Bones: $80$

B. Functions of the Axial Skeleton

• Provides a framework that supports and protects vital organs located in the dorsal (cranial and vertebral cavities) and ventral (thoracic, abdominal, pelvic cavities) body cavities.

• Protects special sense organs crucial for taste, smell, hearing, balance, and vision.

• Serves as attachment sites for muscles that perform several critical functions:

  • Adjust the posture of the head, neck, and trunk.

  • Facilitate movement of the thoracic cage for respiration.

  • Stabilize the appendicular skeleton.

II. The Skull and Associated Bones

• The skull is subdivided into three main parts:

  • Face ($14$ bones)

  • Cranium ($8$ bones)

  • Associated bones ($7$ bones)

A. Bones of the Cranium (8 bones)

In the adult skull, the cranial bones form the braincase and major portions of the face.

1. Occipital Bone (1 single bone)

• Most posterior bone of the skull.

• Lambdoid suture: Connects the occipital bone with the two parietal bones superiorly.

• External occipital protuberance (EOP): A noticeable bulge located about midway on the posterior aspect of the occipital bone.

• Foramen magnum: A large opening at the base of the occipital bone, allowing the spinal cord to connect with the brain.

• Occipital condyles: Smooth, hyaline cartilage-covered surfaces on either side of the foramen magnum, which articulate with the first cervical vertebra (atlas, C1).

• Hypoglossal canals: Openings located superior to the occipital condyles, through which the hypoglossal nerve (CN XII), innervating the tongue, passes.

• Condyloid foramen: Situated within the condyloid fossa.

• Other internal features: Jugular notch, groove for sigmoid sinus, internal occipital crest, fossae for cerebellum and cerebrum, internal occipital protuberance, superior and inferior nuchal lines.

2. Parietal Bones (2 paired bones)

• Form a large portion of the superior and lateral walls of the cranium.

• Sagittal suture: Connects the left and right parietal bones on the superior midline of the skull.

• Coronal suture: Connects the frontal bone with the two parietal bones anteriorly.

• Squamous suture: Connects each parietal bone with the temporal bone laterally.

• Their internal surface often retains impressions of the cranial blood vessels.

3. Frontal Bone (1 single bone)

• Most anterior bone of the skullcap, forming the forehead and superior part of the eye sockets.

• Coronal suture: Connects the frontal bone with the two parietal bones posteriorly.

• Supra-orbital foramen: An opening or notch above each eye socket (supra-orbital margin), through which the ophthalmic branch of the trigeminal nerve (V1) passes.

• An internal ridge, the frontal crest, is positioned anterior to the cribriform plate of the ethmoid bone.

• Contains a notch that accommodates the ethmoid bone.

4. Temporal Bones (2 paired bones)

• Form the inferior and lateral walls of the cranium, contributing to the cranial floor.

• Squamous part (squama): The relatively flat, superior portion forming part of the lateral cranial wall.

• Squamous suture: Connects the squamous part of the temporal bone to the parietal bone.

• External acoustic meatus: The opening of the ear canal.

• Mastoid process: A prominent, rough projection posterior to the ear, containing mastoid sinuses. It serves as an attachment site for various neck muscles.

• Styloid process: A long, slender projection from the inferior surface of the temporal bone, serving as an attachment point for neck muscles and ligaments.

• Zygomatic process: An extension that articulates with the temporal process of the zygomatic bone, forming the posterior part of the zygomatic arch (cheekbone).

• Mandibular fossa: A depression on the inferior surface that articulates with the mandibular condyle (head) of the mandible, forming the temporomandibular joint (TMJ).

• Petrous portion: A hard, rock-like internal projection forming an oblique ridge between the temporal bone and the occipital bone. It houses the delicate auditory sense organs for hearing and balance, including the three auditory ossicles per ear.

• Internal acoustic meatus: A canal within the petrous portion, transmitting facial (CN VII) and vestibulocochlear nerves (CN VIII) to the inner ear.

  • *Note: Petrous means