Deeper layer =

Erector spinae, from medial to lateral:

• Spinalis, Longissimus, and Iliocostalis.

Spinalis:

• Originates on the thoracic vertebrae and the first two lumbar vertebrae (specifically¬, from their spinous processes).
• Inserts along the vertebral column as superiorly as the 2nd and 3rd cervical vertebrae. It sometimes has a cervical portion that inserts on the base of the skull.

Longissimus:

• Originates on the transverse processes of the lumbar, thoracic, and cervical vertebrae
• Inserts along the ribs and vertebral column (as superiorly as C2).
• It also sends a portion of muscle to attach to the mastoid processes on the skull.

Iliocostalis:

• Originates on the sacrum and ilia.
• Inserts along the ribs and vertebral column as superiorly as cervical vertebrae 4-7.

Additional information about the erector spinae:

• These muscles comprise many short segments of muscle fibers.
• The erector spinae muscles are prone to injury as a result of improper heavy lifting.
• When contracted bilaterally, the erector spinae muscles extend the back;
  Unilateral contraction produces lateral flexion of the back (moves the shoulder towards the lateral hip of the same side).
• The erector spinae muscles can be further subdivided according to vertical region:
  For example, the portion of iliocostalis that lies within the lumbar region is referred to as iliocostalis lumborum; the portion of spinalis that lies with the thoracic region is referred to as spinalis thoracis, and so on.

Superficial layer =

Serratus posterior superior:

• Originates on vertebrae C7-T3 (specifically, from their spinous processes and associated connective tissues), extends inferolaterally
• Inserts as thin strips of muscle, on ribs 2-5, lateral to the erector spinae.
• Elevates ribs 2-5 during forced inspiration.

Serratus posterior inferior:

• Originates on vertebrae T12-L3
• Inserts superolaterally on ribs 9-12 (specifically, to their inferior borders).
• Depresses ribs 9-12 during forced expiration.

• Serratus posterior superior and inferior muscles are mirror images of each other.
• The thoracolumbar fascia separates the erector spinae and serratus posterior superior and inferior muscles.

Key points:

• The superficial muscles of the back stabilize and also move the pectoral girdle and humerus.
• The muscles of the superficial back are symmetrically paired, and trapezius lies superficially over the others.

Trapezius:

• Originates on the skull (the external occipital protuberance, superior nuchal line) and the spines and connective tissues of C7-T12.
• Inserts on the lateral 1/3rd of the clavicle, and on the spine and acromion of the scapula.
• Because the fibers of trapezius are convergent, trapezius adducts, rotates, elevates, or depresses the scapula, depending on which fibers contract.

Levator scapulae:

• Originates on the transverse processes of vertebrae C1-C4.
• Inserts on the medial border of the scapula.
• It elevates and rotates the scapula.

Rhomboid minor:

• Originates on the spines of vertebrae C7-T1.
• Inserts on the scapula at the medial border, adjacent to its spine.
• It adducts the scapula.

Rhomboid major

• Originates on the spines of vertebrae T2-T5
• Inserts on the medial border of the scapula.
• It adducts the scapula.

Latissimus dorsi

• Originates on the spines of vertebrae T7-T12, the thoracolumbar fascia (which overlies the lumbar vertebrae and sacrum), ribs 9-12, and the iliac crest (of the ilium).
• Inserts on the floor of the bicipital groove of the humerus.
• Adducts, extends, and medially rotates the humerus, and also depresses the scapula.

Muscles of mastication are involved in chewing.

Temporalis:

• Originates on the temporal fossa of the temporal bone and inserts on the coronoid process and anterior ramus of the mandible; it elevates and retracts the mandible.

Masseter:

• Originates on the zygomatic bone and arch and inserts on the angle and lateral ramus of the mandible, which it elevates.

Medial pterygoid has two heads:

• The deep head originates on the medial aspect of the lateral pterygoid process of the sphenoid and inserts on the medial surface of the angle and ramus of the mandible.
• The superficial head originates on the tuberosity of the maxilla and inserts with the deep head on the mandibular angle.
• The medial pterygoid elevates the mandible and moves it laterally.

The lateral pterygoid also has two heads:

• The upper head originates from the infratemporal fossa of the sphenoid and inserts on the temporomandibular joint capsule and mandible.
• The lower head originates from the lateral surface of the lateral pterygoid plate and inserts with the upper head.
• The lateral pterygoid protrudes the mandible and moves it laterally.

Movements of the mandible at the temporomandibular joint:

• Protrusion moves the mandible anteriorly; the lateral pterygoid muscles produce this movement.
• Retraction moves the mandible posteriorly; the masseter produces this movement.
• Elevation moves the mandible superiorly towards the upper teeth; temporalis, masseter, and the medial pterygoid produce this movement.
• Depression moves the mandible inferiorly.

Orbicularis oris:

• Originates on the skin and other muscles around the mouth and inserts into the skin of the lips; it closes the lips.

Zygomaticus major:

• Originates on the lateral side of the zygomatic bone and inserts into the angle of the mouth; it draws the corner (aka, angle) of the lips laterally and superiorly.

Zygomaticus minor:

• Originates on the anterior aspect of the zygomatic bone and inserts into the skin of the upper lip, which it elevates.

Levator labii superioris:

• Originates on the maxilla (specifically, from the infraorbital margin), and inserts into the skin of the upper lip, which it elevates; it also dilates (flares) the nostrils.

Buccinator:

• Originates on the surfaces of the maxilla and mandible, and inserts into the lips; it compresses the cheeks.

Risorius:

• Originates on connective tissues on the lateral sides of the face and the skin of the cheeks and inserts into the angle of the mouth; it retracts the lips at the angle of the mouth.

Mentalis:

• Originates on the body of the mandible and inserts into the skin of the chin and lower lip, which it elevates and protrudes.

Depressor labii inferioris:

• Originates on the mandible (specifically, from the anterolateral body) and inserts into the skin of the lower lip, which it depresses.

Additional Information:

• These muscles blend into one another
• Cranial nerve VII innervates them.

Nasalis:

• Originates on the maxilla and inserts on the nasal cartilage at the midline; it flares the nostrils.

Corrugator supercilii muscles:

• Originates on the frontal bone (specifically, from the medial supraorbital margin) and insert on the medial half of the eyebrows and laterally on the supraorbital margin on the frontal bone; they depress the eyebrows (and pull them medially).

Orbicularis oculi:

• Originates on the medial margin of the orbit (specifically, from the maxillary, frontal, and lacrimal bones), and inserts into the rim and skin of the orbit and eyelids; it moves the eyelids and surrounding skin to squint and close the eyes.

Frontalis:

• Originates on the frontal bone and skin of the eyebrows, and inserts into the galea aponeurotica (aka, the epicranial aponeurosis); it elevates the eyebrows, which wrinkles the forehead horizontally.

Additional Information:

• Cranial nerve VII (the facial nerve) innervates these muscles.
• Bell’s palsy occurs when CN VII is damaged; Belly’s palsy results in asymmetrical paralysis of the muscles of facial expression.

KEY DEFINITIONS

• Foramina are holes that allow neurovascular structures to pass through the bone.
• Processes serve as muscle attachment sites.

Maxilla:

• Palatine process
  • Portion of the maxilla that extends posteriorly, towards the palatine.

• Incisive foramen
  • A small opening for neurovascular structures; its name reflects its location posterior to the incisors (the front teeth).
  • The nasopalatine nerve and sphenopalatine artery (and vein) pass through the incisive foramen to serve the palate and associated structures.

• Alveolar margin (aka, ridge)
  • The border of the maxilla where the upper teeth reside.

Sphenoid:

• Pterygoid process
  • Gives rise to the medial and lateral pterygoid plates, which are extensions where muscles of mastication (aka, chewing) attach.

• Foramen ovale
  • Petrosal nerve, mandibular nerve, and accessory meningeal artery.

• Foramen spinosum
  • Middle meningeal artery and meningeal branch of the mandibular nerve.

Temporal bone:

• Mandibular fossa
  • Shallow depression where the mandible (the bone of the lower jaw) articulates with the cranium.

• External auditory meatus
  • External opening of the ear.

• Styloid process
  • A pointy projection for attachment of muscles.

• Mastoid process
  • Larger, roughened cone-shaped projection for muscle attachment (sternocleidomastoid).

• Opening of the carotid canal
  • Internal carotid artery enters the cranium to supply the brain via this opening.

• Stylomastoid foramen
  • Lies between the styloid and mastoid processes.
  • CN VII (facial nerve).

Occipital bone:

• Foramen magnum
  • A large hole where the spinal cord enters the cranium to become the brainstem.
  • The brainstem, vertebral arteries, and CN XI (accessory nerve) pass through the foramen magnum;

• Occipital condyles
  • Lie on either side of the foramen magnum; these rounded surfaces articulate with the vertebral column.

• Hypoglossal canal
  • A small opening near the foramen magnum and the occipital condyles.
  • CN XII (hypoglossal nerve)
• External occipital crest
  • Extends from the posterior edge of the foramen magnum, and terminates at the external occipital protuberance

• External occipital protuberance
  • A roughened elevation; it serves as a muscle attachment site and is usually larger in adult males.

• Inferior and superior nuchal lines
  • Extend horizontally from the crest; they serve as muscle attachment sites.

• Foramen lacerum
  • Lies between the occipital, sphenoid, and temporal bones; it is irregularly shaped.
  • Greater petrosal nerve; it is a branch of the facial nerve (CN VII).

• Jugular foramen
  – Lies between occipital and temporal bones; it is named for the jugular vein, which exits through this space.
  • CNs IX (glossopharyngeal nerve), X (vagus), and XI (accessory), and the jugular vein

CRANIAL BONES

• Enclose and protect the brain

Frontal bone

• Comprises the forehead and the superior portion of the eye orbit

Parietal bone

• Comprises the superior portion of the lateral skull

Sphenoid bone

• Only small portion visible in lateral view
• Gives rise to pterygoid processes

Temporal bone

• Forms inferior boundary of lateral skull
• Squamous portion is area inferior to squamous suture
• Tympanic portion surrounds the external auditory meatus
• External auditory meatus is the external opening of the ear
• Mastoid process is the large bony projection near the occipital bone
• Styloid process is a long, pointy projection
• Mandibular fossa is a depressed area of the temporal bone that articulates with the mandible to form the temporomandibular joint

Occipital bone

• Comprises posterior and inferior portion of cranium

FACIAL BONES

• Form the face and protect the entrances to the oral and nasal cavities.

Zygomatic

• Contributes to the lateral eye orbits and cheeks

Nasal

• Forms the bony component of the external nose

Maxilla

• Contributes to the medial eye orbit, cheeks, and upper jaw
  *Anterior nasal spine is the projection of the maxillae that contributes to the inferior border of the opening of the nasal cavity.

Ethmoid

• Contributes to medial eye orbit

Lacrimal

• Contributes to medial eye orbit
• Fossa for the lacrimal sac, through which tears pass from the eye to the nasal cavity. This connection explains why your nose runs when you cry.

Mandible

• Bone of the lower jaw; it houses the lower teeth.
• Condyle (aka, head) articulates with the mandibular fossa of the temporal bone posteriorly
• Coronoid process provides an attachment site for temporalis, a powerful muscle of the jaw.
• Mandibular notch lies between condyle and coronoid process
• Ramus is the vertical portion
• Body is anterior and lateral base
• Angle lies between body and ramus, posteriorly
• Mental protuberance, which forms the chin

FOUR MAJOR SUTURES:

• Sutures are the immoveable joints between the skull bones.

Coronal suture

• Lies between the frontal bone, anteriorly, and the parietal bones, posteriorly;

Sagittal suture

• Lies between the right and left parietal bones;

Squamous suture

• Lies between the temporal and parietal bones.

Lambdoid suture

• Lies between the parietal and occipital bones.

Sutural bones (formerly called Wormian bones)

• Small, irregularly shaped bones that form between the major skull bones.

Pterion

• Where the temporal, sphenoid, parietal, and frontal bones meet
• Overlies the middle meningeal artery; thus, injury to the pterion is a common cause of intracranial epidural hematoma.

Additional Features:

Paranasal sinuses

• Spaces within the frontal, ethmoid, sphenoid, and maxilla; they are continuous with the nasal cavity (and are addressed in detail with the respiratory system).

Zygomatic arch

• The zygomatic process, which is a projection of the temporal bone
• The temporal process, which is a projection of the zygomatic bone

Key functions of the skull:

• Protects the brain and associated sensory organs.
• Provides attachment sites for the facial and neck muscles.

The skull itself comprises:

• 8 Cranial bones that enclose the brain
• 14 Facial bones that protect entryway to nasal and oral cavities, and provide muscle attachment sites.

The cranial bones include:

The unpaired bones:

Frontal

• Forms the forehead and superior rim of eye orbits.
• Supraorbital margin is the bony ridge framing the orbit superiorly
• Supraorbital notch/foramen provides passageway for neurovascular structures.

Occipital

• Contributes to the posterior and inferior surface and base of the skull.

Sphenoid

• A bat-shaped bone that spans the width of the skull.

Ethmoid

• An irregularly shaped bone that lies deep within the skull.
• Contributes superior and middle nasal conchae.

The paired bones:

Parietal

• Comprise the superior and lateral aspects of the skull.

Temporal

• Comprise the inferior portion of the lateral skull.

The facial bones include:

The unpaired bones:

Vomer

• Contributes to the nasal septum.

Mandible

• The bone of the lower jaw.
• Alveolar margin houses lower teeth.
• Mental foramen for neurovascular structures.

The paired:

Zygomatic

• Form the inferior margins of the orbits and the lateral cheekbones.

Lacrimal

• Contribute to the medial walls of the orbits.

Nasal

• Comprise the bridge of the nose.

Inferior nasal conchae

• Lie within the nasal cavity.

Palatine

• Contribute a small portion to the medial orbit wall (but chiefly contribute to the hard palate of the oral cavity).

Maxilla

• Fuse at the midline of the face to form the medial and inferior orbit walls, cheeks, and upper jaw.
• Infraorbital foramen for neurovascular structures.
• Alveolar margin is the border of the maxilla where the upper teeth are housed.

Spaces within the eye orbit:

• The superior orbital fissure is a wide space within the sphenoid (specifically, between the greater and lesser wings).
• The inferior orbital fissure is the space between the sphenoid, zygomatic, and maxillary bones.
• The optic foramen is the opening to the optic canal, through which the optic nerve (cranial nerve II) passes.

Features of the nasal cavity:

• Nasal septum separates the nasal cavity into right and left sides.
  • Comprises two different bones:
    Superiorly: Perpendicular plate of the ethmoid bone.
    Inferiorly: The vomer bone comprises the inferior portion.