While some snakes grab their food and constrict it, others prefer to inject deadly venoms into their prey. Either way though, all snakes must deal with the next problem. How to eat it.With no limbs to provide leverage, snakes can't easily tear up their food, they must instead walk into their food.
When a snake, say a cornsnake (Elaphe guttata guttata) captures a prey item, say a mouse, it then begins the amazing process of forcing itself over the mouse.First the snake locates the head. All snakes eat their prey headfirst, it makes the swallowing of limbs so much easier. There are occasions where a snake will go tail first, but they are hardly normal.
After locating the head, the cornsnake then unhinges it's jaw and expands it's mandibles to fit over the mouse. Once the basics are done and the mouse has started to enter the snake's mouth, it then alternately moves it's jaws forward and to the sides. The snake actually moves one mandible at a time.
This form of eating is so amazing and rare that it is only found in snakes. Amphisbaenians, blindworms and caecillians all achieve their dietary requirements by eating early and eating often. Due to their unique method of feeding, there are no living analogues to snakes in this category.
Well all of the above can be answered with a quick look at snake anatomy.
In a typical lizard we can see the loss of the Jugal bone which frees up the quadrate and allows for a more wide swing of the jaw (thus allowing the lizard to swallow larger food items). In snakes, the jugal and the upper temporal has been lost. Note also how the mandibles are no longer connected to eachother and can actually move freely of one another (a condition known as: mandibular liberation). The mandibles are only connected through a ligament that stretches between the distal end of each mandible. Along with that, each mandible has a joint midway down it which allows it to bow outwards. By having a stretchy lower jaw rather than a rigid one, snakes are capable of expanding their jaws to accomodate large prey.
Several pairs of muscles originating on the braincase, insert themselves along the two palatopterygoid bars (named for the uniting of the pterygoid bones with shorter palatine bones). These muscles flex the maxillary bones allowing the upper jaw to move forward, backward, up and down (prokinensis).Buy separately moving each mandible along with the outward and forward sweeping of the palatomaxillary arches, snakes are able to "walk" their heads over their prey.
Snakes only have one lung decreasing the chance of large prey reducing lung room. Snake tracheas open anteriorly rather than way back in the throat. By having such a anterior trachea, snakes are able to keep their air tubes clear while swallowing.That last thing that snakes have which allows them to swallow such large prey is their very elastic skin.