Skull and Facial Muscles - Anatomy & Physiology

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Introduction

Deer Skull - Copyright nabrown RVC

The skull is divided into three components- the neurocranium, the dermatocranium and the viscerocranium. The skull also includes the hyoid apparatus, mandible, ossicles of the middle ear and the cartilage of the larynx, nose and ear. The skull protects the brain and head against injury and supports the structures of the face. In some animals the skull is also used for defensive actions, for example in horned ungulates such as red deer stags.

The neurocranium develops from the neural crest and mesoderm and undergoes endochondral ossification. It lies ventral to the brain.

The dermatocranium lies dorsal to the brain and develops from the neural crest and mesoderm. It undergoes intramembranous ossification.

The viscerocranium is the pharyngeal skeleton. It is derived only from the neural crest and undergoes endochondral and intramembranous ossification.

The shape and size of the skull varies widely, not only between species but also with age, breed and sex of similar species.

Structure

• The skull is made of many smaller bones

• Most of the skull bones are paired

• Cartilage or fibrous tissue separates the bones of the skull in the young animal

• Once growth has ceased, the sutures begin to ossify

Function

• Protection of brain

• Support facial muscles by providing origin and insertion sites

• Foramen provide entry and exit places for the vasculature and nervous system

• Defense

Bones of the Skull

Occipital Bone

• Forms the nuchal wall

• Forms the foramen magnum

• Basilar part (pars basilaris)
  • Caudal base of the cranium
  • Rostral to foramen magnum
  • Joined by cartilagenous suture to basisphenoid bone
  • Muscular tubercules on ventral surface where the flexors of the head and neck attach
  • Caudal cranial fossa enclose the pons and medulla oblongata

• Squamous part (pars squamosa)
  • Dorsal to lateral parts and occipital condyles
  • Nuchal crest present
    • Easily palpable
    • Landmark for collection of cerebrospinal fluid (CSF)
  • External occipital protuberances present which provide muscle attachment sites for the nuchal ligament

• Lateral parts (partes laterales)
  • Lateral borders of foramen magnum
  • Occipital condyles present which articulate with the atlas to form the atlanto-occipital joint
  • Paracondylar process present which provides muscle attachment sights for muscles of the head
  • Hypoglossal canal

Sphenoid Bone

• Forms the base of the neurocranium

• Composed of a body and wings

• Bones separated by cartilage which ossifies with age

• Presphenoid (os praespenoidale)
  • Rostrally
  • Rostral caudal fossa
  • Hollow body with sphenoid sinuses located inside
  • Sphenoid rostrum present
  • Optic chiasma
  • Optic Canal

• Basisphenoid (os basispenoidalis)
  • Caudally
  • Median cranial fossa
  • The wings oppose the temporal bone, maxilla, orbit and the brain
  • Wings form the oval foramen and other foramen (see species differences) carotid notch, oval foramen and spinous notch (in the horse)
  • Pterygoid processes present
  • Alar canal (rostral and caudal alar foramen)

Temporal Bone

• Composed of squamous, petrosal and tympanic parts

• Lateral wall of the cranial cavity

• Articulates with the frontal, pareital and sphenoid bones

• Zygomatic process
  • On squamous part
  • Joins the temporal process of the zygomatic bone to form the zygomatic arch
  • Forms the articulating surface of the temperomandibular joint

• Articular tubercle present

• Mandibular fossa present

• Occipital process and retrotympanic process surrounds the external acoustic meatus

• Petrosal part encloses the inner ear
  • Internal acoustic meatus present
  • Forms mastoid process ventrally
  • Styloid process present which attaches the hyoid apparatus
  • Stylomastoid foramen present

• Tympanic part is the ventral section of the temporal bone
  • Tympanic bulla present
  • Tympanic membrane separates tympanic cavity from external acoustic meatus
  • Encloses auditory ossicle dorsally
  • Musculotubal canal where tensors of the soft palate originate

Foramen

• Jugular Foramen
  • Located either side of basilar part of occipital bone
  • Adjacent to tympanic bulla

• Foramen Magnum
  • Formed by occipital bones

• Hypoglossal Canal
  • Between paracondylar and condylar processes on lateral part of occipital bone

• Optic Chiasma
  • Runs in transverse depression behind sphenoid rostrum on presphenoid bone

• Optic Canal
  • Ends from optic chiasma over wings of the presphenoid bones

• Oval Foramen
  • Caudal wing of the basisphenoid bones

• Alar Canal
  • Rostral border of the basisphenoid bone at the base of the pterygoid processes

• Internal Acoustic Meatus
  • Medial surface of the petrosal part of the temporal bone

• Stylomastoid Foramen
  • Situated on the petrosal part of the temporal bone

Species Differences

Brachycephalic skull (Pug) - Copyright RVC

Canine

• Dogs have different skull lengths depending on breed
  • Mesocephalic dogs have average conformation
  • Dolichocephalic dogs have longer skull lengths
  • Brachycephalic dogs have shorter skull lengths

• 2 halves of the mandible do not fuse allowing some movement

• External sagittal crest arises from nuchal crest

• Wings of the basisphenoid bones form the oval foramen, spinous foramen and carotid canal

• No foramen lacernum

• Styloid process absent

Feline skull (Lion) - Copyright RVC

Feline

• The mandible appears globular in shape

• Large orbits with complete bony margins

• Large tympanic bulla which can be palpated

• 2 halves of the mandible do not fuse allowing some movement

• Weak external sagittal crest arises from nuchal crest

• Wings of the basisphenoid bones form the oval foramen, spinous foramen and carotid canal

• No foramen lacernum

• Styloid process absent

Horse Skeleton - Copyright Nottingham

Equine

• Weak external sagittal crest arises from nuchal crest

• Long skull length

• Orbit placed more laterally with a complete bony rim

• Strong zygomatic arch which continues on to form the facial crest

• Deep nasoincisive notch

• Prominent hamular process

• Large mandible
  • Vascular notch on mandible
  • High ramus

• Wings of the basisphenoid bones form the oval foramen, spinous notch and carotid notch

• Foramen lacernum present

Sheep skull - Copyright RVC

Ruminant

• Skull is short and wide

• Cornual process on frontal bone

• Nuchal crest reduced to nuchal line

• Prominent temporal line

• Elevated orbital ring which is complete

• No facial crest

• Prominent tympanic bullae

• Nasoincisive notch present

• Wings of the basisphenoid bones form the oval foramen

• No foramen lacernum

• Petrosal and tympanic parts of the temporal bone fused to the squamous part

Pig skull showing tusks - Copyright RVC

Porcine

• Thick nucal crests

• Prominent temporal line

• Orbit is incomplete and small

• Strong and deep zygomatic arch

• Large tympanic bullae

• High caudal part of the skull

• Styloid process absent

Bird skull - Copyright RVC

Avian

• Pneumatised skull bones
  • Spaces in skull bones which connect to airways in the head rather than the air sacs

• Large orbits

• Skull plates are separated by spongy bone

• A single occipital condyle articulates with the atlas allowing more rotation of the head

• In parrots, the nasal bone and frontal bone are joined by a flexible cartliage structure allowing greater jaw opening which is called the craniofacial hinge. This allows kinesis to occur.

• Thin jugal arch (equivalent to zygomatic arch)

• Middle ear contains only the columella (equivalent to the stapes)

Links

Deglutition

Mastication