The Skeletal System: Overview

Divisions of the Skeleton

The human skeleton is divided into two main regions: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central supporting axis of the body, while the appendicular skeleton includes the limbs and girdles.

• Axial Skeleton: Skull, vertebral column, thoracic cage (ribs and sternum)

• Appendicular Skeleton: Upper and lower limbs, pectoral and pelvic girdles

Anatomical Features of Bones

Bone Surface Markings

Bones exhibit a variety of markings that serve as sites for muscle attachment, passageways for nerves and blood vessels, and articulation points for joints. Understanding these features is essential for identifying bone functions and relationships.

• Projection: Outward bulge of bone, often due to muscle pull or joint modification

• Depression: Bowl- or groove-like cut-out, serving as passageways or joint surfaces

• Opening: Hole or canal for passage of blood vessels and nerves

Axial Skeleton: Structure and Function

Components and Functions

The axial skeleton consists of 80 bones divided into three major regions: the skull, vertebral column, and thoracic cage. It forms the longitudinal axis of the body, supports the head, neck, and trunk, and protects vital organs such as the brain, spinal cord, and thoracic organs.

• Skull: Protects the brain and forms the structure of the face

• Vertebral Column: Supports the body and protects the spinal cord

• Thoracic Cage: Protects the heart and lungs

The Skull

Structure and Sutures

The skull is the most complex part of the skeleton, composed of 22 bones. Most are joined by immovable sutures, which are fibrous joints that appear as seams on the cranial surface. Sutures are synarthroses, allowing minimal movement for compliance and elasticity.

• Cranial Bones: Form the cranium, protecting the brain

• Facial Bones: Form the structure of the face

• Sutures: Coronal, sagittal, lambdoid, and squamous

Fontanelles in Newborns

Fontanelles are "soft spots" on the skull of newborns where bones have not yet fused. Premature fusion is known as craniosynostosis. These areas allow for growth and flexibility during birth and early development.

Major Sutures of the Skull

• Sagittal Suture: Between parietal bones

• Coronal Suture: Separates frontal from parietal bones

• Lambdoid Suture: Joins occipital with parietal bones

• Squamous Suture: Joins temporal and parietal bones

Cranial Bones

Frontal Bone

The frontal bone forms the anterior portion of the cranium, including the forehead. The supraorbital margin is pierced by the supraorbital foramen, allowing passage of the supraorbital artery and nerve. The glabella is the smooth area between the orbits.

Parietal Bones

The right and left parietal bones form most of the cranial roof and part of its walls. Each is bordered by four sutures that join it to neighboring bones.

Temporal Bones

The temporal bones form the lower wall and part of the floor of the cranial cavity. They include the squamous, tympanic, mastoid, and petrous parts, each with distinct features and functions.

Occipital Bone

The occipital bone forms the rear of the skull and much of its base. Its most prominent feature is the foramen magnum, through which the spinal cord passes. Nuchal lines are sites of muscle attachment.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone that articulates with all other cranial bones. It contains sphenoidal sinuses and the sella turcica, which houses the pituitary gland. The bone has greater and lesser wings, pterygoid processes, and several foramina for nerve and vessel passage.

Ethmoid Bone

The ethmoid bone is located between the orbital cavities and forms the roof of the nasal cavity. Its perpendicular plate forms the superior part of the nasal septum, and the cribriform plate allows passage of olfactory nerves.

Facial Bones

Overview and Functions

Facial bones do not directly contact the brain. They support the teeth, give shape to the face, form part of the orbital and nasal cavities, and provide attachment for muscles of facial expression and mastication.

Maxillae

The maxillae form the upper jaw and the hard palate, which is the roof of the mouth and the floor of the nasal cavity. Each tooth root is inserted into a deep socket called an alveolus.

Palatine Bones

The palatine bones are L-shaped, forming the posterior one-third of the hard palate and part of the nasal cavity and orbits.

Zygomatic Bones

The zygomatic bones form the cheekbones and the inferolateral margins of the orbits. They articulate with the zygomatic processes of the temporal, frontal, and maxillary bones.

Lacrimal Bones

The lacrimal bones are the smallest bones of the skull, forming part of the medial wall of each orbit. The lacrimal fossa houses the lacrimal sac, which drains tears into the nasal cavity.

Nasal Bones

The nasal bones form the bridge of the nose and support cartilages that shape the lower portion of the nose.

Nasal Conchae

There are three conchae in the nasal cavity: superior and middle (parts of the ethmoid bone), and inferior (a separate facial bone). The conchae increase turbulence of airflow in the nasal cavity.

Vomer

The vomer forms the inferior portion of the nasal septum, separating the left and right nasal cavities. Its name refers to its resemblance to a plow blade.

Mandible

The mandible is the largest and strongest bone of the face, forming the lower jaw. It consists of a body (chin) and two upright rami. Key features include the mandibular angle, coronoid process, condylar process, mandibular notch, alveolar process, and foramina for nerves and blood vessels.

Bones Associated with the Skull

Hyoid Bone

The hyoid bone does not articulate with any other bone. It supports the tongue and provides attachment sites for muscles of the neck and pharynx, helping to keep the larynx open.

Auditory Ossicles

The auditory ossicles (malleus, incus, stapes) are tiny bones in the middle ear associated with hearing.

Orbits and Nasal Cavity

Orbits

The orbits are cavities that encase the eyes and lacrimal glands, serving as sites of attachment for eye muscles. They are formed by parts of seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid.

Nasal Cavity

The nasal cavity is formed by several bones. The roof is made by the cribriform plates of the ethmoid, lateral walls by the conchae and perpendicular plates, and the floor by the palatine and maxillary bones. The nasal septum is formed by the vomer and perpendicular plate of the ethmoid, with the anterior portion formed by septal cartilage.

Paranasal Sinuses

Structure and Function

The paranasal sinuses are air cavities situated around the nasal cavity. They help circulate air in the respiratory system and are paired and bilateral. The sinuses include the frontal, ethmoidal, sphenoidal, and maxillary sinuses.

Summary Table: Cranial and Facial Bones

Additional info: Academic context was added to clarify bone functions, anatomical features, and relationships. Tables were reconstructed and expanded for completeness. Images were included only when directly relevant to the explanation of the paragraph.