The
skull is a
bonyBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
structure found in the
headIn anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose, and mouth...
of many
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s. The skull supports the structures of the
faceThe term face refers to the central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can depending on the definition in the human case, include the hair, forehead, eyebrow, eyes, nose, ears, cheeks, mouth, lips, philtrum, teeth, skin, and chin....
and protects the head against injury.
The skull can be divided into two parts: the
cranium and the
mandibleThe mandible or inferior maxillary bone forms the lower jaw and holds the lower teeth in place. It also refers to both the upper and lower sections of the beaks of birds; in this case the "lower mandible" corresponds to the mandible of humans while the "upper mandible" is functionally equivalent...
. A skull that is missing a mandible is only a cranium; this is the source of a very commonly made error in terminology. Those animals having skulls are called
craniatesCraniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives. Craniata includes all animals with a skull, or cranium, as the name suggests.Craniata as an unranked taxon replaces the former use of Vertebrata...
.
Functions of the skull include protection of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
, fixing the distance between the eyes to allow stereoscopic vision, and fixing the position of the ears to help the brain use auditory cues to judge direction and distance of sounds. In some animals, the skull also has a defensive function (e.g. horned ungulates); the
frontal boneThe frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
is where horns are mounted.
Human skull
In humans, the adult skull is normally made up of 22
bonesBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
. Except for the
mandibleThe mandible or inferior maxillary bone forms the lower jaw and holds the lower teeth in place. It also refers to both the upper and lower sections of the beaks of birds; in this case the "lower mandible" corresponds to the mandible of humans while the "upper mandible" is functionally equivalent...
, all of the bones of the skull are joined together by sutures, rigid articulations permitting very little movement. Eight bones form the
neurocranium (braincase)—including the frontal, parietals,
occipital boneThe occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoid in shape and curved on itself...
,
sphenoidThe sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...
, temporals and ethmoid—a protective
vault surrounding the brainThe cranial vault is the space in the skull within the neurocranium. The brain is found in the cranial vault, and brain size may be affected by the size of the vault as shown in craniometry, but studies relating it to intelligence have been ambivalent....
. Fourteen bones form the
splanchnocranium, the bones supporting the face. Encased within the
temporal boneThe temporal bones are situated at the sides and base of the skull.The temporal bone supports that part of the face known as the temple.-Parts:Each consists of four parts:* Squama temporalis* Mastoid portion* Petrous portion* Tympanic part...
s are the six
ear ossiclesNot to be confused with ossicones.The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe...
of the
middle earThe middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
s, though these are not part of the skull. The
hyoid boneThe hyoid bone is a horseshoe shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra behind.It is the only bone in the human skeleton not...
, supporting the
tongueThe tongue is a muscle on the floor of the mouth that manipulates food for chewing and swallowing . It is the primary organ of taste, as much of the upper surface of the tongue is covered in papillae and taste buds. A secondary function of the tongue is speech, in which the organ assists...
, is usually not considered as part of the skull either, as it does not articulate with any other bones. The skull is a protector of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
.
The skull contains the sinus cavities, which are air-filled cavities lined with
respiratory epitheliumRespiratory epithelium is a type of epithelium found lining the respiratory tract, where it serves to moisten and protect the airways. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by action of the mucociliary escalator.-...
, which also lines the large airways. The exact functions of the sinuses are unclear; they may contribute to decreasing the weight of the skull with a minimal decrease in strength, or they may be important in improving the resonance of the voice. In some animals, such as the
elephantElephants are large land mammals in two genera of the family Elephantidae: Elephas and Loxodonta. Three species of elephant are living today: the African Bush Elephant, the African Forest Elephant and the Asian Elephant...
, the sinuses are extensive. The elephant skull needs to be very large, to form an attachment for muscles of the neck and trunk, but is also unexpectedly light; the comparatively small brain-case is surrounded by large sinuses which reduce the weight.
The
meningesThe meninges is the system of membranes which envelops the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater...
are the three layers, or membranes, which surround the structures of the
nervous systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
. They are known as the
dura materThe dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for keeping in the cerebrospinal fluid...
, the
arachnoid materThe arachnoid mater is one of the three meninges, the membranes that cover the brain and spinal cord. It is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater, and is separated from the pia mater by the subarachnoid space.The delicate,...
and the
pia materThe pia mater is the delicate innermost layer of the meninges—the membranes surrounding the brain and spinal cord....
. Other than being classified together, they have little in common with each other.
In humans, the anatomical position for the skull is the
Frankfurt planeThe Frankfurt plane was established at the World Congress on Anthropology in Frankfurt, Germany in 1884, and decreed as the anatomical position of the human skull...
, where the lower margins of the
orbitsIn anatomy, the orbital bone is the cavity or socket of the skull in which the eye and its appendages are situated.It can also mean the skin which surrounds the eye of a bird....
and the upper borders of the
ear canalThe ear canal , is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 26 mm in length and 7 mm in diameter.- Structure:...
s are all in a horizontal plane. This is the position where the subject is standing and looking directly forward. For comparison, the skulls of other species, notably primates and hominids, may sometimes be studied in the Frankfurt plane. However, this does not always equate to a natural posture in life.
Tetrapod skulls
The skull of the earliest
tetrapodTetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent...
s closely resembles that of their ancestors amongst the lobe-finned fishes. The top and sides of the skull are formed of a series of plate-like bones, including the
maxillaThe maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes , the maxilla is sometimes called "upper maxilla", with the mandible being the "lower maxilla"...
,
frontalsThe frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
,
parietalThe parietal bones are bones in the human skull and form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles.-External:The external surface [Fig...
s, and
lacrimalThe lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.- Lateral or orbital surface :...
s, among others. The various separate bones that compose the
temporal boneThe temporal bones are situated at the sides and base of the skull.The temporal bone supports that part of the face known as the temple.-Parts:Each consists of four parts:* Squama temporalis* Mastoid portion* Petrous portion* Tympanic part...
of humans are also part of this series. A further plate composed of four pairs of bones forms the roof of the mouth; these include the
vomerThe vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
and
palatine boneThe palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
s. The base of the cranium is formed from a ring of bones surrounding the
foramen magnumIn anatomy, in the occipital bone, the foramen magnum is one of the several oval or circular apertures in the base of the skull , through which the medulla oblongata enters and exits the skull vault.Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum...
and a median bone lying further forward; these are
homologousHomology may refer to:* Homology , analogy between human beliefs, practices or artifacts owing to genetic or historical connections* Homology , similarities between nucleic or amino acid sequences / structures owing to shared ancestry...
with the
occipital boneThe occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoid in shape and curved on itself...
and parts of the
sphenoidThe sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...
in mammals. Finally, the lower jaw is composed of multiple bones, only the most anterior of which (the dentary) is homologous with the mammalian mandible.
In living tetrapods, a great many of the original bones have either disappeared, or fused into one another in various arrangements. In mammals and birds, in particular, there have been modifications of the skull to allow for the expansion of the brain. The fusion between the various bones is especially notable in birds, in which the individual structures may be difficult to identify . Living
amphibianAmphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
s typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.
Temporal fenestrae
The temporal fenestrae are anatomical features of the skulls of several types of
amnioteThe amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include mammals, birds and reptiles, as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In humans,...
s, characterised by bilaterally symmetrical holes (fenestrae) in the temporal bone. Depending on the lineage of a given animal, two, one, or no pairs of temporal fenestrae may be present, above or below the
postorbitalThe postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some vertebrates, the postorbital is fused with the postfrontal...
and
squamosalThe squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
bones. The upper temporal fenestrae are also known as the supratemporal fenestrae, and the lower temporal fenestrae are also known as the infratemporal fenestrae. The presence and morphology of the temporal fenestra is critical for taxonomic classification of the synapsids, of which mammals are part.
Physiological speculation associates it with a rise in metabolic rates and an increase in jaw musculature. The earlier amniotes of the Carboniferous did not have temporal fenestrae but two more advanced lines did: The
SynapsidSynapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals"...
s (mammal-like reptiles) and the Diapsids (most reptiles and later birds). As time progressed, diapsids' and synapsids' temporal fenestrae became more modified and larger to make stronger bites and more jaw muscles. Dinosaurs, which are sauropsids, have large advanced openings and their descendants, the birds, have temporal fenestrae which have been modified. Mammals, which are synapsids, possess no fenestral openings in the skull, as the trait has been modified. They do, though, still have the temporal orbit (which resembles an opening) and the temporal muscles. It is a hole in the head and is situated to the rear of the orbit behind the eye.
Classification
There are four types of amniote skull, classified by the number and location of their fenestra. These are:
- Anapsid
An anapsid is an amniote whose skull does not have openings near the temples.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related:...
a - no openings
- Synapsid
Synapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals"...
a - one low opening (beneath the postorbital and squamosal bones)
- Euryapsida
Euryapsida is a polyphyletic group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of...
- one high opening (above the postorbital and squamosal bones); euryapsids actually evolved from a diapsid configuration, losing their lower temporal fenestra.
- Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
a - two openings
Evolutionary, they are related as follows:
- Amniota
- Class Synapsid
Synapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals"...
a
- Order Therapsida
Therapsida is a group of synapsids that include mammals and their immediate evolutionary ancestors. Other than the mammals, all lineages of the therapsids are extinct, with the last known non-mammalian therapsids dying out in the Early Cretaceous...
- Class Mammal
Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
ia - mammals
- Class Sauropsida - reptiles
- Subclass Anapsid
An anapsid is an amniote whose skull does not have openings near the temples.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related:...
a
- (unranked) Eureptilia
Eureptilia is one of the two major clades of the Sauropsida, the other being Anapsida . Eureptilia includes not only all Diapsids, but also a number of primitive Permo-Carboniferous forms previously classified under the Anapsida, in the old order "Cotylosauria".Primitive eureptilians were all...
- Subclass Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
a
- (unranked) Euryapsida
Euryapsida is a polyphyletic group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of...
- Class Aves
Birds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
- birds
Skulls in fish
Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living
lungfishLungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
es. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the
pterygoidPterygoid can refer to:* Pterygoid processes of the sphenoid bone** The Lateral pterygoid plate by it* a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle* a branch of the Mandibular nerve...
s and
vomerThe vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
s alone, all of which bear teeth. Much of the skull is formed from
cartilageCartilage is a stiff yet flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
, and its overall structure is reduced.
In the ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well-formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from the
premaxillaThe incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
, with the
maxillaThe maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes , the maxilla is sometimes called "upper maxilla", with the mandible being the "lower maxilla"...
itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.
Cartilaginous fish, such as
sharkSharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs....
s, have a much simpler, and presumably more primitive, skull structure. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large
fontanelleA fontanelle is an anatomical feature on an infant's skull. Fontanelles are soft spots on a baby's head which, during birth, enable the bony plates of the skull to flex, allowing the child's head to pass through the birth canal. The ossification of the bones of the skull cause the fontanelles to...
. The most anterior part of the cranium includes a forward plate of cartilage, the
rostrumA rostrum is an anatomical structure resembling a bird's beak, such as the snout of a crocodile or dolphin or the foremost extension of a crustacean's carapace...
, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the
inner earThe inner ear is the bony labyrinth, a system of passages comprising two main functional parts:* The cochlea is the hearing part of the inner ear.* The semicircular canals , the utricle and the saccule are the balance part of the inner ear...
. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single
condyleA Condyle is the knuckle of any joint, a round projection, rounded articular area A Condyle (Latin: condylus, from Greek: kondylos; knuckle) is the knuckle of any joint, a round projection, rounded articular area A Condyle (Latin: condylus, from Greek: kondylos; knuckle) is the knuckle of any...
, articulating with the first
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
. There are, in addition, at various points throughout the cranium, smaller
foramina-Other:* the apical foramen is the hole at the tip of the root of a tooth.* the foramen ovale is a hole between the venous and arterial sides of the fetal heart....
for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.
The structure is simpler still in
lampreyA lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. Translated directly, their name means stone lickers . While lampreys are well known for those species which bore into the flesh of other fish to suck their blood, these species make up the minority...
s, in which the cranium is represented by a trough-like basket of cartilagenous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws.
See also
- Anatomical terms of location
Standard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
- Bone terminology
- Craniometry
Craniometry is the technique of measuring the bones of the skull.It is distinct from phrenology, the study of personality and character, and physiognomy, the study of facial features...
- Head and neck anatomy
Head and neck anatomy focuses on the structures of the head and neck of the human body, including the brain, bones, muscles, blood vessels, nerves, glands, nose, mouth, teeth, tongue, and throat...
- Phrenology
Phrenology is a hypothesis stating that the personality traits of a person can be derived from the shape of the skull. It is now considered a pseudoscience. Developed by German physician Franz Joseph Gall in 1796, the discipline was very popular in the 19th century...
, the pseudoscientific process of determining personality from the shape of the head.
- Skull (symbolism)
Skull symbolism is the attachment of symbolic meaning to the human skull. The most common symbolic use of the skull is as a representation of death and mortality, but such a reading varies with changing cultural contexts....
External links