The Skull Its bones can be divided into : 1. NEUROCRANIUM It forms a protective case around the brain. 2. VISCEROCRANIUM It forms the skeleton of the face.

NEUROCRANIUM Is divided into two portions: (A) Membranous part : It consists of Flat bones (Calvaria) or Cranial Vault, which surround the brain.

NEUROCRANIUM (B) Cartilaginous part Chondrocranium It forms the bones of the base of the skull.

MEMBRANOUS NEUROCRANIUM Intra membranous ossification of mesenchyme at the top and most of the sides of the brain forms a number of Flat, membranous bones.

MEMBRANOUS NEUROCRANIUM The mesenchymal cells are derived mostly from the Neural Crest and partly from the Para Axial mesoderm.

MEMBRANOUS NEUROCRANIUM At birth, the neural crest cells form condensed Fibrous joints between the flat bones of the skull (Sutures). At the meeting of more than two sutures, they become wide to form Fontanelles.

MEMBRANOUS NEUROCRANIUM The fontanelles are (6) in number. The Anterior Fontanelle is the most prominent. Its palpation gives an idea about the intracranial pressure and the process of ossification of the skull.

MOLDING It is adaptation of the shape of the fetal skull to the pelvic cavity during birth. It depends on the softness of the bones and their loose connections at the sutures to allow the overlapping of the bones of the skull.

MOLDING 1. The Frontal bone becomes flat. 2. The Occipital bone is drawn out. 3. Slight overriding of one Parietal bone on the other. Restoration of the normal shape is within few days after birth.

CHONDROCRANIUM It is formed by fusion of several separate cartilages.

(1) PARACHORDAL CARTILAGE (BASAL) PLATE It is the cartilage in front of the rostral limit of the notochord (center of sella turcica). It is derived from the Neural Crest cells.

PARACHORDAL (BASAL) PLATE It fuses with the cartilages derived from the Sclerotomes of the occipital somites.

PARACHORDAL PLATE It forms the Base of the Occipital bone. It extends around the cranial end of the spinal cord to form the boundaries of the foramen magnum.

CARTILAGENOUS NEUROCRANIUM Ossification begins in the: Occipital bone . Basisphenoid (body of sphenoid) bone. Ethmoid bone. In that order.

(2) HYPOPHYSEAL CARTILAGE It is formed around the developing pituitary gland,rostral to the occipital basal plate. Its cartilage fuses to form the Body of the sphenoid bone.

CARTILAGENOUS NEUROCRANIUM TRABECULAE CRANII They fuse to form the body of the ethmoid. ALA ORBITALIS It forms the lesser wing of the sphenoid. ALA TEMPORALIS It forms the greater wing of the sphenoid.

CARTILAGENOUS NEUROCRANIUM OTIC CAPSULES They surround the otic vesicles (primordia of the internal ear) and will form the petrous and mastoid parts of the temporal bone.

CARTILAGENOUS NEUROCRANIUM NASAL CAPSULES They surround the nasal sacs and contribute to the formation of the ethmoid bone.

CARTILAGENOUS NEUROCRANIUM An elongated median plate of cartilage is formed between the nasal region and the anterior border of the foramen magnum.

VISCEROCRANIUM The mesenchyme forming the bones of the face including the nasal and lacrimal bones is derived from the Neural Crest cells that migrate into the first two pharyngeal arches.

CARTILAGENOUS VISCEROCRANIUM The Dorsal end of the first arch (Meckel’ cartilage) forms the bony ossicles of the middle ear: Malleus and Incus . Its remnant is the Sphenomandibular ligament. .

CARTILAGENOUS VISCEROCRANIUM The Dorsal end of the Second arch (Reichert’ cartilage) forms : 1. Stapes of the middle ear. 2. Styloid process of the temporal bone.

CARTILAGENOUS VISCEROCRANIUM The Ventral end of the 2nd arch forms : 1.Lesser horn 2. Superior part of the body of hyoid bone. The Ventral part of the 3rd arch forms: 1. Greater horn. 2.Inferior part of the body of hyoid.

CARTILAGENOUS VISCEROCRANIUM The 4th and 6th arches fuse to form the Laryngeal cartilages Except the Epiglottis.

MEMBRANOUS VISCEROCRANIUM Intramembranous ossification of the Maxillary prominence (dorsal portion of the first arch ) gives the following bones: 1. Squamotemporal (becomes part of the neurocranium) . 2. Maxilla. 3. Zygomatic.

MEMBRANOUS VISCEROCRANIUM Intramembranous ossification of the Mandibular prominence (ventral portion of the first arch) forms the Mandible. Endochondral ossification occurs in the mandibular condyles and the median plane

NEW BORN SKULL It is large in proportion to the rest of the skeleton. The small face compared to the calvaria is due to: 1. Small size of the jaws. 2. Absence of the Para nasal sinuses. 3. Underdeveloped facial bones at birth.

POST NATAL GROWTH OF THE SKULL The greatest increase of size of the calvaria is during the first two years. This is the period of most rapid increase of size of the brain. The calvaria normally increases in capacity until about 16 years of age. A slight increase of its size for 3 to 4 years because of increase thickening of the bone.

CRANIOSYNOSTOSIS It is premature closure of the skull sutures. Its cause is unknown but genetic factors are important it is more common in males.

SCAPHOCEPHALY Premature closure of the Sagittal suture. The skull becomes long, narrow and wedge- shaped.

OXYCEPHALY Premature closure of the Coronal suture. The skull is high and tower like.

PLAGIOCEPHALY Premature closure of the Coronal or Lambdoid sutures. The skull is twisted and asymmetric.

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