|Chapter 1 Human Anatomy Body Parts||Chapter 2 Human organ systems||Chapter 3 Human Skeleton||Chapter 4|
|Chapter 5 Ear||Chapter 6 Throat||Chapter 7 Shoulder Girdle||Chapter 8 Hand - Finger|
A skull, or cranium, is a bony structure of Craniates, which serves as the general framework for a head. The skull supports the structures of the face and protects the brain against injury.
In humans, the adult skull is normally made up of 28 bones. Except for the mandible, all of the bones of the skull are joined together by sutures, rigid articulations permitting very little movement.
Eight bones form the neurocranium (braincase), a protective vault of bone surrounding the brain and medulla oblongata. Fourteen bones form the splanchnocranium, the bones supporting the face. Encased within the temporal bones are the six ear ossicles of the middle ear. The hyoid bone, supporting the larynx, is usually not considered as part of the skull, as it does not articulate with any other bones.
Development of the skull
The skull is a complex structure; its bones are formed both by intramembranous and endochondral ossification. The bones of the splanchnocranium and the sides and roof of the neurocranium are formed by intramembranous (or dermal) ossification, while the bones supporting the brain (the occipital, sphenoid, temporal, and ethmoid) are largely formed by endochondral ossification.
At birth, the human skull is made up of 45 separate bony elements. As growth occurs, many of these bony elements gradually fuse together into solid bone (for example, the frontal bones). The bones of the roof of the skull are initially separated by regions of dense connective tissue called "sutures". There are five sutures: the frontal suture, sagittal suture, lambdoid suture, coronal suture, and squamosal suture. At birth these regions are fibrous and moveable, necessary for birth and later growth. This growth can put a large amount of tension on the "obstetrical hinge," which is where the squamous and lateral parts of the occipital bone meet. A possible complication of this tension is rupture of the great cerebral vein of Galen. Larger regions of connective tissue where multiple sutures meet are called fontanelles. The six fontanelles are: the anterior fontanelle, the posterior fontanelle, the two sphenoid fontanelles, and the two mastoid fontanelles. As growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone. The posterior fontanelle usually closes by eight weeks, but the anterior fontanelle can remain up to eighteen months. The anterior fontanelle is located at the junction of the frontal and parietal bones; it is a "soft spot" on a baby's forehead. Careful observation will show that you can count a baby's heart rate by observing his or her pulse pulsing softly through the anterior fontanelle.
If the brain is bruised or injured it can be extremely serious. Normally the skull protects the brain from damage through its hard unyieldingness, but in some cases of head injury, there can be raised intracranial pressure through mechanisms such as a subdural hematoma. In these cases the raised intracranial pressure can cause herniation of the brain out of the foramen magnum ('coning') because there is no space for the brain to expand; this can result in significant brain damage or death unless an urgent operation is performed to relieve the pressure. This is why patients with concussion must be watched extremely carefully.
The skull also contains the sinus cavities. The meninges are the membranes that separate the brain from the skull.
Bones of the human skull
Parietal bone (2)
Temporal bone (2)
Palatine bone (2)
Zygomatic bone (2)
Nasal bone (2)
Lacrimal bone (2)
Inferior nasal conchae (2)
In addition to the usual centers of ossification of the cranium, others may occur, giving rise to irregular isolated bones termed sutural or Wormian bones.
Notable sutures in the skull
Most sutures are named for the bones they articulate, but some have special names of their own.
Sagittal suture - along the midline, between parietal bones
Coronal suture - between the frontal and parietal bones
Lambdoidal suture - between the parietals and the occipital bone
Squamosal suture - between the parietal and the temporal bone
Metopic suture - between the two frontal bones, prior to the fusion of the two into a single bone
Foramina of skull base
The following is a list of holes, or foramina, in the base of the skull and what goes through each of them.
One must be only well versed with these names as they often come across during dictations.
Arranged from anterior to posterior:
Foramen caecum - emissary veins to superior sagittal sinus
Foramina of cribriform plate - olfactory nerve bundles
Posterior ethmoidal foramen - posterior ethmoidal artery, vein and nerve
Optic canal - optic nerve (II), ophthalmic artery
Superior orbital fissure
Oculomotor nerve (III)
Trochlear nerve (IV)
Lacrimal, frontal and nasociliary branches of ophthalmic nerve (V1)
Abducens nerve (VI)
Superior ophthalmic vein
Foramen rotundum - maxillary nerve (V2)
Mandibular nerve (V3)
Accessory meningeal artery
Lesser petrosal nerve (occasionally)
Middle meningeal artery and vein
Meningeal branch of mandibular nerve
Internal carotid artery
Internal carotid nerve plexus
Hiatus of canal of lesser petrosal nerve
Hiatus of canal of greater petrosal nerve
Internal acoustic meatus
Facial nerve (vii)
Vestibulocochlear nerve (VIII)
Inferior petrosal sinus
Glossopharyngeal nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Posterior meningeal artery
Internal jugular vein
Hypoglossal canal - hypoglossal nerve (XII)
Meningeal branches of vertebral arteries
Spinal roots of accessory nerves