snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...

skeleton consists primarily of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

, and ribs, with only

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

remnants of the limbs.

Skull

The

of a

is a very complex structure, with numerous

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

s to allow the snake to swallow prey far larger than its head. The typical snake skull has a solidly

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

braincase, with the separate

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, pa ...

s and the united

parietal bone The parietal bones () are two bones in the skull which, when joined at a fibrous joint, form the sides and roof of the cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is nam ...

s extending downward to the basisphenoid, which is large and extends forward into a

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

extending to the ethmoidal region. The nose is less ossified, and the paired nasal bones are often attached only at their base. The

occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...

is either trilobate and formed by the basioccipital and the exoccipitals, or a simple knob formed by the basioccipital; the

supraoccipital The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

is excluded from the foramen magnum. The basioccipital may bear a curved ventral process or hypapophysis in the

vipers The Viperidae (vipers) are a family of snakes found in most parts of the world, except for Antarctica, Australia, Hawaii, Madagascar, and various other isolated islands. They are venomous and have long (relative to non-vipers), hinged fangs th ...

. The

prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern a ...

is situated, on each side, between the frontal bone and the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

, and may or may not be in contact with the nasal bone. The postfrontal bone, usually present, borders the

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

behind, rarely also above, and in the

pythons The Pythonidae, commonly known as pythons, are a family of nonvenomous snakes found in Africa, Asia, and Australia. Among its members are some of the largest snakes in the world. Ten genera and 42 species are currently recognized. Distribution ...

a supraorbital bone is intercalated between it and the prefrontal bone. The premaxillary bone is single and small, and as a rule connected with the maxillary only by ligament. The paired

vomer The vomer (; lat, vomer, lit=ploughshare) 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 max ...

is narrow. The palatine bone and

pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medi ...

are long and parallel to the axis of the skull, the latter diverging behind and extending to the quadrate or to the articular extremity of the mandible; the pterygoid is connected with the maxillary by the ectopterygoid or transverse bone, which may be very long, and the maxillary often emits a process towards the palatine, the latter bone being usually produced inwards and upwards towards the anterior extremity of the basisphenoid. The quadrate is usually large and elongate, and attached to the cranium through the supratemporal (often regarded as the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

). In rare cases, ('' Polemon'') the transverse bone is forked, and articulates with two branches of the maxilla. The quadrate and the maxillary and palatopterygoid arches are more or less movable to allow for the distension required by the passage of prey, often much exceeding the size of the mouth. For the same reason, the rami of the lower jaw, which consist of

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

splenial The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side (closest to the tongue) between the angular and surangular The suprangular or surangular is a jaw bone found in most land ver ...

, angular, and

articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two oth ...

elements, with the addition of a coronoid in the

boa Kwon Bo-ah (; born November 5, 1986), known professionally as BoA, is a South Korean singer, songwriter, dancer, record producer and actress. One of the most successful and influential Korean entertainers, she has been dubbed the " Queen of K- ...

s and a few other small families, are connected at the

symphysis A symphysis (, pl. symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing togethe ...

by a very extensible elastic ligament. The

hyoid apparatus The hyoid apparatus is the collective term used in veterinary anatomy for the bones which suspend the tongue and larynx. It consists of pairs of stylohyoid, thyrohyoid, epihyoid and ceratohyoid bones, and a single basihyoid bone. The hyoid appara ...

is reduced to a pair of cartilaginous filaments situated below the trachea, and united in front. There are various modifications according to the genera. A large hole may be present between the

s and the basisphenoid ('' Psammophis'', '' Coelopeltis''); the maxillary may be much abbreviated and movable vertically, as in the

Viperidae The Viperidae (vipers) are a family of snakes found in most parts of the world, except for Antarctica, Australia, Hawaii, Madagascar, and various other isolated islands. They are venomous and have long (relative to non-vipers), hinged fangs th ...

; the pterygoids may taper and converge posteriorly, without any connection with the quadrate, as in the Amblycephalidae; the supratemporal may be much reduced, and wedged in between the adjacent bones of the cranium; the quadrate may be short or extremely large; the prefrontals may join in a median suture in front of the frontals; the dentary may be freely movable, and detached from the articular posteriorly. The deviation from the normal type is much greater still when we consider the degraded wormlike members of the families

Typhlopidae The Typhlopidae are a family of blind snakes. They are found mostly in the tropical regions of Africa, Asia, the Americas, and all mainland Australia and various islands. The rostral scale overhangs the mouth to form a shovel-like burrowing str ...

and

Glauconiidae The Leptotyphlopidae (commonly called slender blind snakes or thread snakes) are a family of snakes found in North America, South America, Africa and Asia. All are fossorial and adapted to burrowing, feeding on ants and termites. Two subfamilies ...

, in which the skull is very compact and the maxillary much reduced. In the former this bone is loosely attached to the lower aspect of the cranium; in the latter it borders the mouth, and is suturally joined to the premaxillary and the prefrontal. In both the transverse bone and the supratemporal are absent, but the coronoid element is present in the mandible.

Joints of the snake skull

*Red A: the

between the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

and quadrate. It is

analogous Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ...

to the joint in mammal jaws. *Red B: the joint between the quadrate and the

supratemporal The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and latera ...

. It is highly mobile in most directions, allowing a wider gape (i.e., the snake can open its mouth wider) and greater jaw flexibility. *Red C: the joint between the prefrontal and

. It allows the maxilla to pivot in the plane of the photograph, and while it does not increase gape, it does facilitate the complex action by which the snake draws prey into its mouth. *Green A: the joint between the

and nasal bone. It allows the

nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes ...

to upturn slightly, increasing gape and assisting in swallowing. *Green B: allows the lower jaws to bow outwards, further increasing the gape. *Blue: the joint between the supratemporal and parietal. Immobile, except for '' Dasypeltis''.

Snake dentition

In most

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...

are located on the

of the

lower jaw In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

, the

, the palatine bone and the

lateral pterygoid plate The pterygoid processes of the sphenoid (from Greek ''pteryx'', ''pterygos'', "wing"), one on either side, descend perpendicularly from the regions where the body and the greater wings of the sphenoid bone unite. Each process consists of a me ...

. The latter form an "inner row" of teeth that can move separately from the rest of the jaws and are used to help "walk" the jaws over prey. Several snake lineages have evolved

venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a st ...

which is typically delivered by specialized teeth called fangs located on the

. Most snakes can be placed into one of four groups, based on their teeth, which correlate strongly with venom and lineage.

Aglyph

Aglyphous snakes (''lacking grooves'') have no specialized teeth; each tooth is similar in shape and often size. When teeth vary in size, as in some bird eaters, they do not vary in shape. Most aglyphous snakes are non-venomous; some, like '' Thamnophis'', are considered mildly venomous. The feature is not a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

Opisthoglyph

Opisthoglyphous ("rearward grooves") snakes possess venom injected by a pair of enlarged teeth at the back of the maxillae, which normally angle backward and are grooved to channel venom into the puncture. Since these fangs are not located at the front of the mouth, this arrangement is vernacularly called "rear-fanged". In order to envenomate prey, an opisthoglyphous snake must move the prey into the rear of its mouth and then penetrate it with its fangs, presenting difficulties with large prey although they can quickly move smaller prey into position. The opisthoglyphous dentition appears at least two times in the history of snakes. The venom of some opisthoglyphous snakes is strong enough to harm humans; notably,

herpetologist Herpetology (from Greek ἑρπετόν ''herpetón'', meaning "reptile" or "creeping animal") is the branch of zoology concerned with the study of amphibians (including frogs, toads, salamanders, newts, and caecilians ( gymnophiona)) and rep ...

s Karl Schmidt and Robert Mertens were killed by a boomslang and a

twig snake The twig snakes (genus ''Thelotornis''), also commonly known as bird snakes or vine snakes, are a genus of rear-fanged venomous snakes in the family Colubridae. The genus is native to Africa. All species in the genus have a slender and elonga ...

, respectively, after each underestimated the effects of the bite and failed to seek medical help. Opisthoglyphous snakes are found mostly in the families Colubridae and Homalopsidae.

Proteroglyph

Proteroglyphous snakes (''forward grooved'') have shortened maxillae bearing few teeth except for a substantially enlarged fang pointing downwards and completely folded around the venom channel, forming a hollow needle. Because the fangs are only a fraction of an inch long in even the largest species these snakes must hang on, at least momentarily, as they inject their venom. Some

spitting cobra A spitting cobra is any of several species of cobras that can defensively spray a toxic secretion - functioning as both a venom (that can be injected via a wound) and a toxungen (that can be sprayed on the target surface) - from their fangs in ...

s have modified fang tips allowing them to spray venom at an attacker's eyes. This form of dentition is unique to

elapids Elapidae (, commonly known as elapids ; grc, ἔλλοψ ''éllops'' "sea-fish") is a family of snakes characterized by their permanently erect fangs at the front of the mouth. Most elapids are venomous, with the exception of the genus Emydocep ...

Solenoglyph

Solenoglyphous snakes (''pipe grooved'') have the most advanced venom delivery method of any snake. Each maxilla is reduced to a nub supporting a single hollow fang tooth. The fangs, which can be as long as half the length of the head, are folded against the roof of the mouth, pointing posteriorly. The skull has a series of interacting elements that ensure that the fangs rotate into biting position when the jaws open. Solenoglyphous snakes open their mouths almost 180 degrees, and the fangs swing into a position to allow them to penetrate deep into the prey. While solenoglyph venom is typically less toxic than that of proteroglyphs, this system allows them to deeply inject large quantities of venom. This form of dentition is unique to vipers.

Exceptions

A few snakes do not conform to these categories. '' Atractaspis'' is solenoglyphous but the fangs swing out sideways, allowing it to strike without opening its mouth, perhaps allowing it to hunt in small tunnels.

Scolecophidia The Scolecophidia, commonly known as blind snakes or thread snakes, are an infraorder of snakes. They range in length from . All are fossorial (adapted for burrowing). Five families and 39 genera are recognized. The Scolecophidia infraorder is mo ...

(blind burrowing snakes) typically have few teeth, often only in the upper jaw or lower jaw.

Informal or popular terminology

Common names for the various types of snake dentition originate largely from older literature, but still are encountered in informal publications. Aglyphous snakes are commonly called fangless; opisthoglyphous snakes rear-fanged or back-fanged; and both Proteroglyphous and Solenoglyphous snakes are referred to as front-fanged.Rose, Walter; The reptiles and amphibians of southern Africa; Pub: Maskew Miller, 1950Engelmann, Wolf-Eberhard. Snakes (No. 05352). Publisher Bookthrift 1982.

Taxonomic key of skull modifications

Modifications of the skull in the European genera: *I. Quadrate articulating with the cranium, supratemporal absent; mandible much shorter than the skull, with coronoid bone; maxillary small, on lower aspect of cranium; pterygoids not extending to quadrate; nasals forming long sutures with the premaxillary, prefrontals, and frontal: ''

Typhlops ''Typhlops'' is a genus of blind snakes in the family Typhlopidae. The genus is endemic to the West Indies. Some species which were formerly placed in the genus ''Typhlops'' have been moved to the genera ''Afrotyphlops, Amerotyphlops, Anilios, ...

''. *II. Quadrate suspended from the supratemporal; mandible at least as long as the skull; pterygoids extending to quadrate or mandible. :*A. Mandible with coronoid bone; nasals in sutural contact with frontals and prefrontals; transverse bone short, not projecting much beyond cranium; maxillary not half as long as mandible, which is not longer than skull (to occiput): '' Eryx''. :*B. No coronoid bone; nasals isolated. ::*1. Maxillary elongate, not movable vertically. :::*a. Maxillary half as long as mandible. ::::*Supratemporal half as long as skull, projecting far beyond cranium; mandible much longer than skull: '' Tropidonotus''. ::::*Supratemporal not half as long as skull, projecting far beyond cranium; mandible much longer than skull: '' Zamenis''. ::::*Supratemporal not half as long as skull, projecting but slightly beyond cranium; mandible much longer than skull: '' Coluber''. ::::*Supratemporal not half as long as skull, not projecting beyond cranium; mandible not longer than skull: ''

Coronella ''Coronella'' is a genus of harmless snakes in the family Colubridae. The genus is endemic to Europe, North Africa and West Asia. Two species are currently recognized as being valid.. www.reptile-database.org. :''Common names: Smooth snakes.'' ...

, Contia''. :::*b. Maxillary not half as long as mandible, which is longer than skull; supratemporal not half as long as skull, projecting beyond cranium. ::::*Quadrate longer than supratemporal; maxillary much longer than quadrate, nearly straight in front of prefrontal; a large vacuity between the frontal bones and the basisphenoid: '' Coelopeltis''. ::::*Quadrate not longer than supratemporal; maxillary little longer than quadrate, strongly curved in front of prefrontal:'' Macroprotodon'' ::::*Quadrate longer than supratemporal; maxillary little longer than quadrate, nearly straight in front of prefrontal: '' Tarbophis'' ::*2. Maxillary much abbreviated and erectile; supratemporal not half as long as skull; mandible much longer than skull; basioccipital with a strong process. :::*Maxillary bone solid: '' Vipera''. :::*Maxillary bone hollow: '' Ancistrodon''. :::*The vertebrae number 130 to 500 - in the European forms 147 ('' Vipera ursinii'') to 330 ('' Coluber leopardinus'').

Vertebrae and ribs

The

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

consists of an

atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

(composed of two vertebrae) without ribs; numerous precaudal vertebrae, all of which, except the first or first three, bear long, movable, curved ribs with a small posterior

tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projection ...

at the base, the last of these ribs sometimes forked; two to ten so-called

lumbar vertebrae The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse p ...

without ribs, but with bifurcate transverse processes (lymphapophyses) enclosing the

lymphatic vessel The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph ve ...

s; and a number of ribless caudal vertebrae with simple transverse processes. When bifid, the ribs or transverse processes have the branches regularly superposed. The

centra Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the stores are all owned by individual franchisees. The chain has three different ...

have the usual ball and socket joint, with the nearly hemispherical or transversely elliptic

condyle A condyle (;Entry "condyle"
in
procoelous vertebrae), while the

neural arch The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

is provided with additional articular surfaces in the form of pre- and post- zygapophyses, broad, flattened, and overlapping, and of a pair of anterior wedge-shaped processes called zygosphene, fitting into a pair of corresponding concavities, zygantrum, just below the base of the neural spine. Thus the vertebrae of snakes articulate with each other by eight

s in addition to the cup-and-ball on the centrum, and interlock by parts reciprocally receiving and entering one another, like the mortise and tenon joints. The precaudal vertebrae have a more or less high neural spine which, as a rare exception ( Xenopholis), may be expanded and plate-like above, and short or moderately long transverse processes to which the ribs are attached by a single facet. The centra of the anterior vertebrae emit more or less developed descending processes, or haemapophyses, which are sometimes continued throughout, as in '' Tropidonotus'', '' Vipera'', and '' Ancistrodon'', among

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...

an genera. In the caudal region, elongate transverse processes take the place of ribs, and the haemapophyses are paired, one on each side of the

haemal canal A haemal arch also known as a chevron, is a bony arch on the ventral side of a tail vertebra of a vertebrate. The canal formed by the space between the arch and the vertebral body is the haemal canal. A spinous ventral process emerging from the hae ...

. In the rattlesnakes the seven or eight last vertebrae are enlarged and fused into one.

Vestigial limbs

No living snake shows any remains of the pectoral arch, but remains of the pelvis are found in: *

Boa Kwon Bo-ah (; born November 5, 1986), known professionally as BoA, is a South Korean singer, songwriter, dancer, record producer and actress. One of the most successful and influential Korean entertainers, she has been dubbed the " Queen of K- ...

s and

Pythons The Pythonidae, commonly known as pythons, are a family of nonvenomous snakes found in Africa, Asia, and Australia. Among its members are some of the largest snakes in the world. Ten genera and 42 species are currently recognized. Distribution ...

: a long ilium, attached to the lower branch of the first bifurcate transverse process of the lumbar vertebrae, bearing three short bones, the longest of which, regarded as the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

, terminates in a claw-like

pelvic spur Pelvic spurs are the externally visible portion of the vestigial remnants of legs found on each side of the cloaca in primitive snakes, such as boas and pythons. The remnants of a pelvis and femur, which have no connection with the spine, simply ...

which usually appears externally on each side of the cloaca. *

Leptotyphlopidae The Leptotyphlopidae (commonly called slender blind snakes or thread snakes) are a family of snakes found in North America, South America, Africa and Asia. All are fossorial and adapted to burrowing, feeding on ants and termites. Two subfamilies ...

: ilium, pubis, and ischium, and rudimentary femur, the ischium forming a ventral

. * Aniliidae *

: a single bone on each side.

References

* George Albert Boulenger. ''

The Snakes Of Europe ''The Snakes of Europe'' is a book by the Belgium, Belgian-British people, British zoologist George Albert Boulenger, published in 1913, which is described in the author's preface as the first book written in English language, English describing ...

'', 2nd edition. London: Methuen & Co., Ltd., 1913. {{Reflist

External links

Snake Anatomy
External and Internal snake anatomy with postmortem images. Snake anatomy Skeletons