Anatomy & Physiology Study Notes: The Human Skeleton and Skull

Introduction to the Skeleton

The human skeleton is a complex framework of bones that provides structure, protection, and movement for the body. It consists of 206 named bones in the adult, though the exact number can vary with age and individual development. Infants have more bones, many of which fuse during growth. Some individuals may also have extra sesamoid bones that do not fuse or fuse irregularly.

• Number of Bones: 206 in adults; more in infancy due to unfused bones.

• Variation: Number varies by age and individual.

• Sesamoid Bones: Small bones that may not fuse or may fuse irregularly.

Axial Skeleton (The Trunk of the Body)

Overview

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and rib cage (ribs and sternum). It is primarily responsible for supporting and protecting the organs of the head, neck, and trunk.

• Components: Skull, spinal column, rib cage, sternum

• Total Bones: 80 bones

• Vertebrae: 24 vertebrae (grouped), plus the sacrum and coccyx

• Ribs: 24 ribs (12 pairs)

• Function: Provides structure and protects internal organs

Appendicular Skeleton (The Appendages)

Overview

The appendicular skeleton includes the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton. It is essential for movement and manipulation of the environment.

• Total Bones: 126 bones

• Girdles: Pectoral (shoulder) and pelvic (hip) girdles

• Function: Facilitates movement and supports limb attachment

• Examples: Shoulder blade (scapula), collar bone (clavicle), hip bones

The Skull

Structure and Function

The skull is a complex bony structure that houses and protects the brain, forms the structure of the face, and provides cavities for the sense organs. It is composed of cranial bones, facial bones, and associated bones.

• Cranial Bones: Form the bony house for the brain

• Facial Bones: Form the structure of the face

• Associated Bones: Include bones for hearing (ossicles) and swallowing

Functions of the Skull

• Protects the brain (acts as a helmet)

• Creates facial structures

• Provides openings for air and food

• Anchors teeth

• Houses sense organs (eyes, ears, nose)

Cranial Bones

The cranial bones are joined by sutures, which are immovable joints. There are 8 cranial bones:

• Frontal (1): Forehead bone

• Parietal (2): Upper sides of the head

• Occipital (1): Back of the head; contains the foramen magnum (spinal cord opening)

• Temporal (2): Sides of the head; contains the external auditory meatus (ear canal)

• Sphenoid (1): Butterfly-shaped bone at the base of the skull; contains the sella turcica (houses the pituitary gland)

• Ethmoid (1): Located between the eye sockets and nasal cavity; forms part of the nasal septum and cranial floor

• Cribriform Plate and Crista Galli: Part of the ethmoid bone; crista galli projects into the cranial cavity

Facial Bones

The facial skeleton is made up of 14 bones, most of which are paired. These bones form the structure of the face and provide cavities for the sense organs.

• Maxilla (2): Upper jaw; forms most of the nasal cavity and orbital cavity

• Mandible (1): Lower jaw; only freely movable bone of the skull

• Zygomatic (2): Cheek bones; form the zygomatic arch with the temporal bone

• Nasal (2): Bridge of the nose

• Lacrimal (2): Inner wall of the eye sockets

• Inferior Nasal Conchae (2): Lateral walls of the nasal cavity

• Vomer (1): Forms part of the nasal septum

• Palatine (2): Posterior part of the hard palate (roof of the mouth)

Cavities of the Skull

The cranial and facial bones together form several important cavities:

• Nasal Cavity: Formed by the maxilla, nasal, sphenoid, vomer, inferior nasal conchae, palatine, and ethmoid bones

• Orbital Cavity: Eye socket; formed by maxilla, lacrimal, ethmoid, sphenoid, palatine, frontal, and zygomatic bones

• Septum: Separates the right and left sides of the nose; made up of cartilage, ethmoid, and vomer bones

Sinuses

Structure and Function

Sinuses are air-filled cavities within certain skull bones, lined with mucosa and connected to the nasal cavity. They serve several important functions:

• Lighten the weight of the skull

• Warm and humidify inhaled air

• Enhance vocal resonance (contribute to the quality of the voice)

Paranasal Sinuses

The paranasal sinuses are found in the following bones:

Bone	Sinus Name
Frontal bone	Frontal sinus
Ethmoid bone	Ethmoid air cells
Maxilla	Maxillary sinus
Sphenoid bone	Sphenoidal sinus

Mnemonic: "FEMS" (Frontal, Ethmoid, Maxillary, Sphenoid) helps remember the locations of the paranasal sinuses.

Summary Table: Major Bones of the Skull

Bone	Location	Key Function
Frontal	Forehead	Protects frontal lobe of brain
Parietal	Upper sides of skull	Protects parietal lobes
Occipital	Back of skull	Protects occipital lobe; foramen magnum for spinal cord
Temporal	Sides of skull	Houses ear canal; protects temporal lobes
Sphenoid	Base of skull	Houses pituitary gland
Ethmoid	Between eyes, nasal cavity	Forms part of nasal septum and cranial floor
Maxilla	Upper jaw	Holds upper teeth; forms part of nasal/orbital cavities
Mandible	Lower jaw	Holds lower teeth; only movable skull bone
Zygomatic	Cheek	Forms cheekbone; part of zygomatic arch