Utilisateur:Goodshort/Squelette du serpent

Un squelette de serpent.

Le squelette d'un serpent est constitué du crâne, des vertèbres et des côtes, ainsi que de membres vestigiaux.

Crâne

Le crâne d'un Python reticulatus.

Le crâne d'un serpent est une structure très complexe, avec de nombreuses articulations qui lui permettent d'avaler des proies bien plus grosses que sa tête.

The typical snake skull has a solidly ossified braincase, with the separate frontal bones and the united parietal bones extending downward to the basisphenoid, which is large and extends forward into a rostrum 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 is either trilobate and formed by the basioccipital and the exoccipitals, or a simple knob formed by the basioccipital; the supraoccipital is excluded from the foramen magnum. The basioccipital may bear a strong, curved ventral process or hypapophysis in the vipers.

The prefrontal bone is situated, on each side, between the frontal bone and the maxilla, and may or may not be in contact with the nasal bone.

The postfrontal bone, usually present, borders the orbit behind, rarely also above, and in the pythons 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 is narrow.

The palatine bone and pterygoid 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).

In rare cases, (Miodon, 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, splenial, angular, and articular elements, with the addition of a coronoid in the boas and a few other small families, are connected at the symphysis by a very extensible elastic ligament.

The hyoid apparatus 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 frontal bones and the basisphenoid (Psammophis, Coelopeltis); the maxillary may be much abbreviated and movable vertically, as in the Viperidae; 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 and Glauconiidae, 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.

Articulations du crâne

Fichier:Snake skull joints lateral view.jpg

Lateral view of the skull of a Burmese python, with visible kinetic joints labeled. Red = highly mobile, green = slightly mobile, blue = immobile.

Red A: the joint between the mandible and quadrate. It is analogous to the joint in mammal jaws.

Red B: the joint between the quadrate and the supratemporal. 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 maxilla. 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 frontal bone and nasal bone. It allows the nose 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.

Denture

In most snakes, teeth are located on the dentary of the lower jaw, the maxilla, the palatine bone and the lateral pterygoid plate. 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. While most snakes are not hazardous to humans, several lineages have evolved venom which is typically delivered by specialized teeth called fangs located on the maxilla.

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

Denture aglyphe

Un crâne de Python molurus, un serpent aglyphe.

Les serpents aglyphes ne possèdent pas de crochet à venin ni appareil venimeux. Ils n'ont également pas de dents spécialisées, toutes leurs dents étant généralement semblables tant du point de vue de la forme que de la taille.Ils sont en général inoffensifs, mais attention à certains grands boïdés : 19 genres -3 sous-familles- 74 espèces et pythonidés 8 genres renfermant 26 espèces qui sont constricteurs, c’est-à-dire qui étouffent leurs proies par constriction, et qui peuvent, de par leur taille tuer un homme et avaler un enfant (un cas vérifié). Mais tous les serpents aglyphes ne sont pas constricteurs.Dans la famille des couleuvres colubridés, la couleuvre à collier (natrix natrix) par exemple avale ses proies vivantes.Ainsi que la couleuvre vipérine (natrix maura), qui se nourrit de poissons, de grenouilles et qui les avale avec une grande rapidité. La proie meurt étouffée en passant dans le tube digestif.

Denture opisthodonte

L'opisthodontie caractérise une forme de denture primitive, les familles des Elapidés et des Vipéridés ayant évolué à partir d'ancêtres opisthodontes , ne présentant pas de dent spécialisée capable d'inoculer du venin, et ce malgré la présence d'une glande de Duvernoy. Le venin produit est mélangé à la alive du serpent qui est de ce fait considéré comme peu dangereux pour l'homme.

Denture opisthoglyphe

An opisthoglyphous snake. A hognose snake skull (Heterodon nasicus)

Un serpent opisthoglyphe est un serpent qui possède dans la partie postérieure de son maxillaire un ou plusieurs crochets à venin. En raison de cette position en fond de la gueule, il est rare de se faire envenimer par un tel serpent. Un risque existe cependant face aux serpents du genre Oxybelis qui ont la possibilité d'ouvrir la gueule à 180°.

Exemples de serpents opisthoglyphes

• La couleuvre de Montpellier - Malpolon monspessulanus
• Oxybelis aeneus
• Oxyrhopus aff. Melanogenys
• Erythrolamprus aesculapii

Opisthoglyphous snakes (rearward grooves) possess weak venom injected by a pair of enlarged teeth at the back of the maxillae which normally angle backwards and are grooved to channel venom into the puncture. Since these fangs and 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 (or a human handler's finger) into position.

While the venom of most opisthoglyphous snakes is too weak to harm humans, sometimes this is not the case. Notably, herpetologists Karl Schmidt and Robert Mertens were killed by a boomslang and twig snake, respectively, after each underestimated the effects of the bite and failed to seek medical help.^{[réf. nécessaire]} Opisthoglyphous snakes are found in family Colubridae.

Denture proteroglyphe

A proteroglyphous snake. A king cobra skull (Ophiophagus hannah)

Le protéroglyphe est un serpent présentant un petit crochet fixe (également appelé glyphe), relié à la glande à venin, à l'arrière du maxillaire. Ce crochet est toujours dans la même position, que la gueule soit ouverte ou fermée (par opposition aux solénoglyphes). Ce sont les serpents considérés comme les plus vénimeux au monde (Cobra).

Certains d'entre eux sont marins, et leur venin est considéré comme deux fois plus puissant que celui du cobra royal. On les trouve sur les côtes du Pacifique Sud ainsi que dans le Golfe Persique.

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, the most toxic of all snakes. Some spitting cobras have modified fang tips allowing them to spray venom at an attacker's eyes. This form of dentition is unique to elapids.

Denture solenoglyphe

A solenoglyphous snake. A rattlesnake skull (Crotalus sp.)

Les serpents solénoglyphes présentent de longs crochets mobiles, reliés à la glande à venin, à l'avant du maxillaire. Ces crochets se replient quand la gueule est fermée, épousant la forme du palais . Ils se redressent quand le serpent ouvre la gueule pour mordre.

Les autres serpents sont appelés protéroglyphes ou Opistoglyphes ou Aglyphes, (des plus évolués aux moins évolués). Les aglyphes ne disposent pas de crochets à venins (colubridés, boïdés principalement). Cette famille de serpents est souvent confondus avec la suivante. Les opistoglyphes possèdent sur la partie postérieur du maxilaire, un ou plusieurs crochets caniculés reliés à la glande à venin. Risque d'envenimation faible surtout si il s'agit de petites espèces. Certaines espèces plus grosses, plus agressives peuvent potentiellement être dangereuses (colubridés). Les protéroglyphes disposent d'un petit crochet fixe et caniculé situé à l'avant du maxilaire. Relié à la glande à venin, ce crochet est suivi de petites dents pleines (élapidés).

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

Références

• George Albert Boulenger. The snakes of Europe, 2nd edition. London: Methuen & Co., Ltd., 1913.