ANATOMY OF THE CRANIAL BONES

The skull consists of 8 cranial bones and 14 facial bones. The eight bones of the cranium articulate firmly with one another to enclose and protect the brain and sensory organs. The 14 facial bones form the framework for the facial region and support the teeth.The facial bones,with the exception of the mandible (“jawbone”),are also firmly interlocked with one another and the cranial bones.The skull has several cavities. The cranial cavity is the largest. The nasal cavity is formed by both cranial and facial bones and is partitioned into two chambers, or nasal fossae, by a nasal septum of bone and cartilage. Four sets of paranasal sinuses, located within the bones surrounding the nasal area, communicate via ducts into the nasal cavity. Middle- and inner-ear cavities are positioned inferior to the cranial cavity and house the organs of

hearing and balance. The two orbits for the eyeballs are formed by facial and cranial bones. The oral, or buccal cavity (mouth), which is only partially formed by bone, is completely within the facial region.

 During fetal development and infancy, the bones of the cranium are separated by fibrous unions. There are also six large areas of connective tissue membrane that cover the gaps between the developing bones. These membranous sheets are called fontanelles, meaning “little fountains.” The name derives from the fact that a baby’s pulse can be felt surging in these “soft spots” on the skull. The fontanelles permit the skull to undergo changes in shape, called molding, during parturition (childbirth), and they accommodate the rapid growth of the brain during infancy. Ossification of the fontanelles is normally complete by 20 to 24 months of age. The following fontanelles are present:

1. Anterior (frontal) fontanelle,fonticulus anterior. The anterior fontanel is diamond-shaped and is the most prominent. It is located

on the anteromedian portion of the skull.

2. Posterior (occipital) fontanelle,fonticulus posterior. The posterior fontanel is positioned at the back of the skull on the median line. It is also diamond-shaped, but smaller than the anterior

fontanelle.

3. Anterolateral (sphenoid) fontanelles,fonticulus sphenoidalis. The paired anterolateral fontanels are found on both sides of the skull, directly lateral to the anterior fontanelle. They are relatively small and irregularly shaped.

4. Posterolateral (mastoid) fontanelles,fonticulus mastoideus. The paired posterolateral fontanelles, also irregularly shaped, are located on the posterolateral sides of the skull.

    A prominent sagittal suture extends the anteroposterior median length of the skull between the anterior and posterior fontanelles. A coronal suture extends from the anterior fontanel to the anterolateral fontanel. A lambdoid suture extends from the posterior fontanel to the posterolateral fontanelle. A squamous suture connects the posterolateral fontanelle to the anterolateral fontanelle.

Clinical application:During normal childbirth, the fetal skull comes under tremendous pressure. Bones may even shift, altering the shape of the skull. A common occurrence during molding of the fetal skull is for the occipital bone to be repositioned under the two parietal bones. In addition, one parietal bone may shift so as to overlap the other. This makes delivery easier for the mother. If a baby is born breech (buttocks first), these shifts do not occur. Delivery becomes much more difficult, often requiring the use of forceps.

                             CRANIAL BONES

 The cranial bones enclose and protect the brain and associated sensory organs. They consist of one frontal, two parietals, two temporals, one occipital, one sphenoid, and one ethmoid.

                             THE FRONTAL BONE

 The frontal bone forms the anterior roof of the cranium,the forehead,the roof of the nasal cavity and the superior arches of the orbits,which contain the eyeballs.  The frontal bone develops in two halves that grow together.

Generally,they are completely fused by age 5 or 6. A suture sometimes persists between these two portions and is referred to as a metopic suture.This makes them unpaired.

   The frontal bone has three parts:

1.Squamous part,squama frontalis:a vertical convex part of the bone,which consists of the external,internal and temporal surfaces. The squamous part bears the following structures:

• Frontal tuber,tuber frontale: a paired,most prominent elevation of the squamous part.
• Superciliary arch,arcus: a paired, prominent, rounded arch situated below and medial to the frontal tuber.
• Glabella,glabella: an unpaired area between the superciliary arches and above the base of the nose.
• Supra-orbital margin,margo supraorbitalis: a paired,prominent area on the border with the orbital part.
• Supra-orbital notch,incisura supraorbitalis:resides medially on the supra-orbital margin .
• The frontal notch,incisura frontalis:a shallow notch that resides medially from the supra-orbital notch.
• Zygomatic process,processus zygomaticus: a directed downward,lateral portion of the supra-orbital margin.
• Temporal line,linea temporalis: arches upward and backward from the zygomatic process.
• Frontal crest,crista frontalis: unpaired, located in the middle of the of the inferior portion of the bone.

2.Orbital part,pars orbitalis:o horizontal part,which contains the orbital and cerebral surfaces.It bears the following parts:

• Fossa for lacrimal gland,fossa glandulae lacrimalis:resides near the zygomatic process in the upper,lateral angle of the orbit.
• Trochlear fovea,fovea trochlearis:a small depression located medially.
• Trochlear spine,spina trochlearis:lies near the trochlear fovea.
• Cerebral yokes,juga cerebralia:also reside on the cerebral surface between the impressions for cerebral gyri.

3.Nasal part,pars nasalis:an unpaired part located in the middle between two orbital parts surrounding the ethmoidal notch.It also has the following parts:

• Nasal spine,spina nasalis which lies in the middle.
• opening of the frontal sinus,apertura sinus frontalis: a paired orifice leading into the frontal sinus.
• Ethmoidal notch,incisura ethmoidalis resides in the middle and separates orbital parts.
• Frontal sinus,sinus frontalis: a sizeable air cavity within the lower portion of the squamous part.The frontal sinus is separated by a bony septum into two cavities, which communicate with the nasal cavity.
  

                                                     


                                                        THE PARIETAL BONE

 The two parietal bones form the upper sides and roof of the cranium.The coronal suture separates the 
frontal bone from the parietal bones, and the sagittal suture along the superior midline separates the right and left parietals from each other.

                  Borders of parietal bone

• sagittal border,margo sagittalis: the upper margin of the bone,which articulates with the opposite parietal bone to form the sagittal suture.
• Frontal border,margo frontalis: a directed forward border,which articulates with frontal bone to form the coronal suture.
• Occipital border,margo occipitalis: a directed backward border, which articulates with the occipital bone forming the lambdoid suture.
• Squamous border,margo squamosus: the fourth,oblique,directed downward border of the parietal bone. It overlaps with the squamous part of the temporal bone.

                            Angles of the parietal bone

• Frontal angle,angulus frontalis the anterior,upper,straight angle,which articulates with the frontal bone.
• Sphenoidal angle,angulus sphenoidalis the anterior,lower,sharper angle,which articulates with the greater wing of the sphenoid bone.
• Occipital angle,angulus occipitalis the posterior,lower,rounded angle,which articulates with the occipital bone.
• Mastoid angle,angulus mastoideus: the posterior,upper,blunt angle,which articulates with the mastoid process of the temporal bone.
  

                          

                               TEMPORAL BONE

The two temporal bones form the lower sides of the cranium. Each temporal bone is joined to its adjacent parietal bone by the squamous suture. Structurally, each temporal bone has four parts.

1. Squamous part,pars squamosa. The squamous part is the flattened plate of bone at the sides of the skull. Projecting forward is a zygomatic  process that forms the posterior portion of the zygomatic arch. On the inferior surface of the squamous part is the cuplike mandibular fossa, which forms a joint with the condyle of the mandible. This articulation is the temporomandibular joint.

2.Tympanic part,pars tympanica. The tympanic part of the temporal bone contains the external acoustic meatus, or ear canal, which is posterior to the mandibular fossa. A thin, pointed styloid process projects inferiorly from the tympanic part.External acoustic opening which resides in the centre where the parts of the temporal bone articulate.A tympanosquamous fissure separates the tympanic part from the squamous part.It is split into two parts by the plate of the petrous part:petrosquamous fissure and petrotympanic fissure.The tympanomastoid fissure separates the tympanic part from the mastoid process.

                  The Tympanic Cavity,cavitas tympani

The tympanic cavity is the air filled cavity of irregular shape situated within the petrous part of the temporal bone.It is invested with the mucous membrane.

   walls of the tympanic cavity:The tympanic cavity has six walls,namely;

• The tegmental wall,paries tegmentalis is the wall related tot he tegmen tympani.
• The jugular wall,paries jugularies related to the jugular fossa.
• The labyrinthine wall,paries labyrinthicum is the medial wall related to the bony labyrinth.
• The membranous wall,paries membranaceus is a small lateral wall scaled with the tympanic membrane.
• The carotid wall,paries caroticus is the wall related to the carotid canal.The tympanic opening of the auditory tube and the opening of the caroticotympanic caniculi can be distinguished on this wall.
• The mastoid wall,paries mastoideus is the posterior wall related to the mastoid process of the temporal bone.
  

3. Mastoid part,pars mastoidea. The mastoid process, a rounded projection posterior to the external acoustic meatus, accounts for the mass of the mastoid part. The mastoid foramen  is directly posterior to the mastoid process. The stylomastoid foramen, located between the mastoid and styloid processes , provides the passage for part of the facial nerve.

4. Petrous part(pyramid),pars tetrosa. The petrous part can be seen in the floor of the cranium. The structures of the middle ear and inner ear are housed in this dense part of the temporal bone. The carotid canal and the jugular foramen border on the medial side of the petrous part at the junction of the temporal and occipital bones. The carotid canal allows blood into the brain via the internal carotid artery, and the jugular foramen lets blood drain from the brain via the internal jugular vein. Three cranial nerves also pass through the jugular foramen.The following surfaces are distinguished in the pyramid:

• Anterior surface,facies anterior: faces forward,upward and laterally.
• Posterior surface,facies posterior:faces backward and medially.
• Inferior surface,facies inferior:faces downward towards the area of the external surface of the cranial base.

The three surfaces of the petrous part are bounded by three borders:

• Anterior border of the petrous part,margo anterior partis pertrosae:the shortest border,which connects the squama with the apex of the pyramid.
• Superior border of the petrous part,margo superior patris pertrosae:saparates the anterior and posterior surfaces of the pyramid.
• Posterior border of the petrous part,margo posterior patris petrosae:separates the posterior and inferior surfaces of the pyramid.
  

              Canals of the temporal bone

• Facial canal,canalis nervi facialis:It transmits the facial nerve.The facial canal begins at the bottom of the internal acoustic meatus and ends in the stylomastoiss foramen.
• Canal for chorda tympani,canaliculis chordae tympani: It gives passage to a thin nerve(chorda tympani).
• Carotid canal,canalis caroticus: It transmits the internal carotid artery.The carotid canal begins on the inferior surface of the pyramid.
• Caroticotympanic canal,canaliculus caroticotympanici:begin on the posterior of the carotid canal and enter the tympanic cavity penetrating its anterior wall.Its gives passage to the corresponding nerves and vessels.
• Musculotubal canal,canalis musculotubarius:It exits the tympanic cavity.The musculotubal canal resides in the angle between the anterior margin of the pyramid and the squamous part of the temporal bone. A horizontal septum divides the musculotubal canal into two:
• Canal for tensor tympani,semicanalis m. tensor tympani,situated above.
• Canal for auditory tube,semicanalis tubae auditivae,resides below the previous canal and transmits the auditory(Eustachian) tube,which establishes the communication between the tympanic cavity and the nasal part of the pharynx. 
• Tympanic canal,canaliculis tympanicus:begins on the inferior surface of the pyramid in the petrous fossula and runs vertically upward penetrating the inferior wall of the tympanic cavity.It houses the tympanic nerve.
• Mastoid canal,canaliculis mastoideus:begins at the bottom of the jugular fossa,crosses the facial canal in its lower portion and opens into the tympanomastoid fissure.It gives passage to the auricular branch of the vagus nerve.

    Clinical application:The mastoid process of the temporal bone can be easily palpated as a bony knob immediately behind the earlobe. This process contains a number of small air-filled spaces called mastoid cells that can become infected in mastoiditis, as a result, for example,of a prolonged middle-ear infection.

                               OCCIPITAL BONE

The occipital bone forms the posterior and most of the base of the skull. It articulates with the parietal bones at the lambdoid suture. The foramen magnum is the large hole in the occipital bone through which the spinal cord passes to attach to the brain stem. On each side of the foramen magnum are the occipital condyles, which articulate with the first vertebra (the atlas) of the vertebral column.  At the anterolateral edge of the occipital condyle is the hypoglossal canal, through which the hypoglossal nerve passes. A condyloid  canal lies posterior to the occipital condyle . The external occipital protuberance is a prominent posterior projection on the occipital bone that can be felt as a definite bump just under the skin. The superior nuchal  line is a ridge of bone extending laterally from the occipital protuberance to the mastoid part of the temporal bone. Sutural bones are small clusters of irregularly shaped bones that frequently occur along the lambdoid suture.

 The occipital bone has the following parts:

• Basilar part,pars basilaris, an unpaired portion located in front of the foramen magnum.
• Lateral part,pars lateralis, a paired part situated on each side of the foramen magnum.
• Squamous part,squama occipitalis, an unpaired larger portion of the bone located the foramen.

 

                                     

                                        SPHENOID BONE

 The sphenoid bone forms part of the anterior base of the cranium. This bone has a somewhat mothlike shape. It consists of a body and laterally projecting greater and lesser wings that form part of the orbit. The wedgelike body contains the sphenoidal sinuses and a prominent saddlelike depression, the sella turcica. Commonly called “Turk’s saddle,” the sella turcica houses the pituitary gland. A pair of pterygoid processes project inferiorly from the sphenoid bone and help form the lateral walls of the nasal cavity.

Several foramina are associated with the sphenoid bone:

1. The optic canal is a large opening through the lesser wing into the back of the orbit that provides passage for the optic nerve and the ophthalmic artery.

2. The superior orbital fissure is a triangular opening between the wings of the sphenoid bone that provides passage for the ophthalmic nerve, a branch of the trigeminal nerve, and for the oculomotor, trochlear, and abducens nerves.

3. The foramen ovale is an opening at the base of the lateral pterygoid plate,through which the mandibular nerve passes.

4. The foramen spinosum is a small opening at the posterior angle of the sphenoid bone that provides passage for the middle meningeal vessels.

5. The foramen lacerum is an opening between the sphenoid and the petrous part of the temporal bone, through which the internal carotid artery and the meningeal branch of the ascending pharyngeal artery pass.

6. The foramen rotundum is an opening just posterior to the superior orbital fissure, at the junction of the anterior and medial portions of the sphenoid bone. The maxillary nerve passes through this foramen.

Clinical application:Located on the inferior side of the cranium, the sphenoid bone would seem to be well protected from trauma. Actually, just the opposite is true—and in fact the sphenoid is the most frequently fractured bone of the cranium. It has several broad, thin, platelike extensions that are perforated by numerous foramina. A blow to almost any portion of the skull causes the buoyed, fluid-filled brain to rebound against the vulnerable sphenoid bone, often causing it to fracture.

                                 THE ETHMOID BONE

The ethmoid bone is located in the anterior portion of the floor of the cranium between the orbits, where it forms the roof of

the nasal cavity. An inferior projection of the ethmoid bone, called the perpendicular plate, forms the superior part of the nasal septum that separates the nasal cavity into two chambers. Each chamber of the nasal cavity is referred to as a nasal fossa. Flanking the perpendicular plate on each side is a large but delicate mass of bone riddled with ethmoidal air cells, collectively constituting the ethmoid sinus. A spine of the perpendicular plate, the crista galli,projects superiorly into the cranial cavity and serves as an attachment for the meninges covering the brain. On both lateral walls of the nasal cavity are two scroll-shaped plates of the ethmoid

bone, the superior and middle nasal conchae,also known as turbinates. At right angles to the perpendicular plate, within the floor of the cranium, is the cribriform plate, which has numerous cribriform foramina for the passage of olfactory nerves from the epithelial lining of the nasal cavity. 

Clinical application:The moist,warm vascular lining within the nasal cavity is susceptible to infections,particularly if a person is not in good health. Infections of the nasal cavity can spread to several surrounding areas. The paranasal sinuses connect to the nasal cavity and are especially prone to infection. The eyes may become reddened and swollen during a nasal infection because of the connection of the nasolacrimal duct,through which tears drain from the anterior surface of the eye to the nasal cavity. Organisms may spread via the auditory tube from the nasopharynx to the middle ear. With prolonged nasal infections,organisms may even ascend to the meninges covering the brain via the sheaths of the olfactory nerves and pass through the cribriform plate to cause meningitis.

                                   Kamal Umar Labaran

                     Donetsk National Medical University.