Friday, March 13, 2009

Skeleton and Teeth:

Snake skeleton is not very complex because it does not have any appendages (limbs). A few species, such as boas and pythons, retain some vestigial structures similar to pelvic bones. In some species, these can be seen externally and are called "spurs." All other snakes simply have vertebrae, ribs, and a skull. Snakes can have between 130-500 vertebrae, with ribs attached to each one. This does not include the bones in the tail section.
Adaptations in the skull allow snakes to eat prey much larger than themselves. The bones are connected by elastic ligaments, allowing a lot of stretch and the joint of the upper and lower jaws is placed very posterior (far back) in the skull, allowing the mouth to open as wide as possible. Also, the bones of the lower jaw are not fused together at the front, which means, which can move laterally when the snake is swallowing its prey. In addition, a snake has an additional loosely-attached bone called a "quadrate" on each side. This provides a "double hinge" at the joint and as a snake swallows, it alternately moves the jaws on each side of the face and "walks" over the prey into its mouth. Also, the backwards curve of the teeth helps the snake to swallow prey, as these act as hooks to prevent live prey from escaping free. Snake teeth are both acrodont (attached to the bone) and polyphydont (able to grow back when lost), and a snake may have several sets of teeth throughout its lifetime. This is necessary, because teeth are often lost while feeding. The type of teeth a snake has differs depending on the method used to capture and kill prey. There are three kinds of snake dentition:

Constrictor dentition: Most snakes have two rows of teeth on each upper jaw and one row on each lower jaw. All of the teeth are short and hook-like. All non-poisonous snakes have constrictor dentition, regardless of whether or not they actually constrict their prey. Poisonous snakes have either grooved fangs or hollow fangs.

Groove fanged: Venomous snakes have only one row of teeth on each upper jaw, plus a pair of fangs. The fang has a groove that serves as a path for the venom to flow into the prey through the puncture.

Hollow fangs: The teeth of the hollow-fanged snakes serve the same purpose as grooved fangs, but the fangs are more like a hypodermic needle through which the venom flows and fangs can be either erectile or fixed. The erectile teeth are retracted into a groove on the roof of the mouth and extend when the mouth opens to strike, but fixed fangs are always extended.

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