The snake begins the feeding process by locating the head of the prey to be ingested. This makes swallowing an animal with projecting limbs much easier since they can fold against the body and prevent impediment of the swallowing process. Notice how there is very little movement of the body advancing over the prey item; rather the prey item is carried into the gullet like a conveyer belt. The rodent is then moved down toward the stomach via rhythmic, lateral undulations.
The feeding motion that is seen in crocodilians and in some lizards is very animated; they toss and chomp at prey in order to arrange the meal for swallowing. Unlike snakes, crocodilians and lizards have larger heads to accommodate for prey size, limbs to aid in movement of body position and the prey, and the ability to rip, tear, and twist off flesh into manageable pieces if needed. Additionally, the heads of crocodilians and lizards are rigidly constructed, which means they lack the flexibility characteristic to snake skulls.
The relative immobility of the snake’s fore body during prey ingestion is made possible by the lose construction of the skull in concert with ligaments that allow for elasticity, this is called cranial kinesis or skull movement. A snake’s jaw does not come “unhinged,” despite the popular misconception for why snakes can ingest large prey with such small mouths.
The bottom jaw, or mandible, is not fused by bone at the front, as is the case in crocodilians and mammals; it is instead connected with flexible ligaments that allow the bottom jaw to spread outward and over a girthy prey item. The gape is also facilitated via elasticity. The bones of a snake’s jaw are themselves rather flexible, which is advantageous when force is being exerted on the mouth from all directions as large prey is engulfed. The upper and lower jaw in pit vipers is connected via a quadrate bone, this further aids in the vertical flexibility of the mouth over the body of large prey.