The skull is a bony structure that forms the head in vertebrates. It provides a protective cavity for the brain; the skull is composed of two parts: the mandible. In the human, these two parts are the neurocranium and the viscerocranium or facial skeleton that includes the mandible as its largest bone; the skull forms the anterior most portion of the skeleton and is a product of cephalisation—housing the brain, several sensory structures such as the eyes, ears and mouth. In humans these sensory structures are part of the facial skeleton. Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, fixing the position of the ears to enable sound localisation of the direction and distance of sounds. In some animals such as horned ungulates, the skull has a defensive function by providing the mount for the horns; the English word "skull" is derived from Old Norse "skulle", while the Latin word cranium comes from the Greek root κρανίον.
The skull is made up of a number of fused flat bones, contains many foramina, fossae and several cavities or sinuses. In zoology there are openings in the skull called fenestrae. For details and the constituent bones, see Neurocranium and Facial skeleton The human skull is the bony structure that forms the head in the human skeleton, it forms a cavity for the brain. Like the skulls of other vertebrates, it protects the brain from injury; the skull consists of two parts, of different embryological origin—the neurocranium and the facial skeleton. The neurocranium forms the protective cranial cavity that surrounds and houses the brain and brainstem; the upper areas of the cranial bones form the calvaria. The membranous viscerocranium includes the mandible; the facial skeleton is formed by the bones supporting the face Except for the mandible, all of the bones of the skull are joined together by sutures—synarthrodial joints formed by bony ossification, with Sharpey's fibres permitting some flexibility.
Sometimes there can be extra bone pieces within the suture known as sutural bones. Most these are found in the course of the lambdoid suture; the human skull is considered to consist of twenty-two bones—eight cranial bones and fourteen facial skeleton bones. In the neurocranium these are the occipital bone, two temporal bones, two parietal bones, the sphenoid and frontal bones; the bones of the facial skeleton are the vomer, two inferior nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, two lacrimal bones. Some sources count the maxilla as having two bones; some of these bones—the occipital, frontal, in the neurocranium, the nasal and vomer, in the facial skeleton are flat bones. The skull contains sinuses, air-filled cavities known as paranasal sinuses, numerous foramina; the sinuses are lined with respiratory epithelium. Their known functions are the lessening of the weight of the skull, the aiding of resonance to the voice and the warming and moistening of the air drawn into the nasal cavity.
The foramina are openings in the skull. The largest of these is the foramen magnum that allows the passage of the spinal cord as well as nerves and blood vessels; the many processes of the skull include the zygomatic processes. The skull is a complex structure; the skull roof bones, comprising the bones of the facial skeleton and the sides and roof of the neurocranium, are dermal bones formed by intramembranous ossification, though the temporal bones are formed by endochondral ossification. The endocranium, the bones supporting the brain are formed by endochondral ossification, thus frontal and parietal bones are purely membranous. The geometry of the skull base and its fossae, the anterior and posterior cranial fossae changes rapidly; the anterior cranial fossa changes during the first trimester of pregnancy and skull defects can develop during this time. At birth, the human skull is made up of 44 separate bony elements. During development, many of these bony elements fuse together into solid bone.
The bones of the roof of the skull are separated by regions of dense connective tissue called fontanelles. There are six fontanelles: one anterior, one posterior, two sphenoid, two mastoid. At birth these regions are fibrous and moveable, necessary for birth and growth; this growth can put a large amount of tension on the "obstetrical hinge", where the squamous and lateral parts of the occipital bone meet. A possible complication of this tension is rupture of the great cerebral vein; as growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone creating sutures. The five sutures are the two squamous sutures, one coronal, one lambdoid, one sagittal suture; the posterior fontanelle closes by eight weeks, but the anterior fontanel can remain open up to eighteen months. The anterior fontanelle is located at the junction of the parietal bones. Careful observation will show that you can count a baby's heart
External occipital crest
The external occipital crest is part of the external surface of the squamous part of the occipital bone. It is a ridge along the midline, beginning at the external occipital protuberance and descending to the foramen magnum, that gives attachment to the nuchal ligament, it is called the median nuchal line
The foramen magnum is a large oval opening in the occipital bone of the skull in humans and various other animals. It is circular openings in the base of the skull; the spinal cord, an extension of the medulla, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments, it transmits the spinal component of the accessory nerve into the skull. The opisthion is the midpoint on the posterior margin of the foramen magnum and is a cephalometric landmark. Another landmark is the basion located at the midpoint on the anterior margin of the foramen magnum; the foramen magnum is a important feature in bipedal mammals. One of the attributes of a bipedal animal’s foramen magnum is a forward shift of the anterior border. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion.
The forward shift of the foramen magnum is apparent in bipedal hominins, including modern humans, Australopithecus africanus, Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was bipedal, may be the earliest known bipedal ape; the discovery of this feature has given scientists another form of identifying bipedal mammals. The alar ligament, attached on each side to the tubercle of occipital condyle on each side of Foramen magnum divides it into anterior smaller compartment and posterior larger compartment. Thus, in humans, the neck muscles do not need to be as robust. Comparisons of the position of the foramen magnum in early hominid species are useful to determine how comfortable a particular species was when walking on two limbs rather than four. Posterior cranial fossa This article incorporates text in the public domain from page 129 of the 20th edition of Gray's Anatomy "Anatomy diagram: 34257.000-1".
Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01. Diagram 1 Diagram 2 3D animation showing position of basion on YouTube
The glabella, in humans, is the skin between the eyebrows and above the nose. It refers to the underlying bone, depressed, joins the two brow ridges, it is a cephalometric landmark, just superior to the nasion. The term is derived from the Latin glabellus, meaning smooth; the skin of the glabella can be used to measure skin turgor in suspected cases of dehydration by pinching and lifting it. A dehydrated patient's glabella tends to remain extended rather than returning to its normal shape. Glabellar reflex
A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates; the sagittal crest serves for attachment of the temporalis muscle, one of the main chewing muscles. Development of the sagittal crest is thought to be connected to the development of this muscle. A sagittal crest develops during the juvenile stage of an animal in conjunction with the growth of the temporalis muscle, as a result of convergence and gradual heightening of the temporal lines. A sagittal crest tends to be present on the skulls of adult animals that rely on powerful biting and clenching of their teeth as a part of their hunting strategy. Skulls of some dinosaur species, including tyrannosaurs, possessed well developed sagittal crests. Among mammals, cats and many other carnivores have sagittal crests, as do some leaf eaters, including tapirs and some apes. Sagittal crests are found in robust great apes, some early hominins.
Prominent sagittal crests are found among male gorillas and orangutans, do occur but only in male chimpanzees such as Bili apes. The largest sagittal crest discovered in the human lineage belongs to the "Black Skull", Paranthropus aethiopicus field number KNM WT 17000, the earliest known robust hominid ancestor and the oldest robust australopithecine discovered to date; the prominence of the crest appears to have been an adaptation for the P. aethiopicus' heavy chewing, the Black Skull's cheek teeth are correspondingly large. Smaller sagittal crests are present on the skulls of other Paranthropines, including Paranthropus boisei and Paranthropus robustus. Mountain gorilla Paranthropus aethiopicus Phrenology Sagittal Keel "Black Skull" —ArcheologyInfo.com
Behind either condyle of the lateral parts of occipital bone is a depression, the condyloid fossa, which receives the posterior margin of the superior facet of the atlas when the head is bent backward. Occipital condyle Atlas This article incorporates text in the public domain from page 131 of the 20th edition of Gray's Anatomy Illustration
The clivus is a bony part of the cranium at the skull base, a shallow depression behind the dorsum sellæ that slopes obliquely backward. It forms a gradual sloping process at the anterior most portion of the basilar occipital bone at its junction with the sphenoid bone. On axial planes, it sits just posterior to the sphenoid sinuses. Just lateral to the clivus bilaterally is the foramen lacerum, proximal to its anastomosis with the Circle of Willis. Posterior to the clivus is the basilar artery; the pons sits on the clivus. Clivus is used as an abbreviated term for the clivus ocularis, the sloping inner wall of the retina as it dips into the foveola in the macula of the eye. For this reason, to disambiguate, the clivus is sometimes referred to as the Blumenbach clivus; the abducens nerve tracks along the clivus during its course. Increased intracranial pressure can cause signs of palsy. Clivus is the site for chordoma The clivus is an important landmark for checking for anatomical atlanto-occipital alignment.
Wackenheim's clivus line should be tangential to it. Pharyngeal tubercle This article incorporates text in the public domain from page 148 of the 20th edition of Gray's Anatomy Anatomy photo:22:os-0913 at the SUNY Downstate Medical Center - "Osteology of the Skull: Internal Surface of Skull" Diagram at uwo.ca