Anatomy of the cranial bones

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  • 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 babys 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