A dog's mouth works differently from a human's. For starters, he has 42 teeth and nearly 2,000 taste buds, while a person typically has 32 teeth and 9,000 taste buds. The reason for this reflects a basic difference between species: While we can take our time and enjoy the taste of a meal, getting as much enjoyment from the flavour as from filling our stomachs, a dog generally cannot. In the wild, survival means finding and eating as quickly as possible. Taste is secondary. So a dog has more teeth with which to quickly rip apart meat and chew bone into small pieces. In fact, his teeth are not capable of grinding like ours are. The sole focus of the dog's mouth is to get the food down, through the oesophagus and into the stomach as quickly and efficiently as it can.
Hard to Stomach
Another big difference between a human's digestive system and a dog's is in the stomach. When we chew our food, we also produce saliva, which contains an enzyme that helps break down the food before it gets to our stomach. For a dog, which will swallow whole large pieces of raw meat and crushed bone, all of the work has to be done in the stomach. The dog's pancreas will begin producing enzymes that will help in breaking down the food; the stomach wall also has glands that produce necessary acids. Since what goes into a dog's stomach can be anything from grass to bone, the acids must be particularly powerful (in comparison, the acid in a dog's digestion is about three times stronger than in a human's). A dog's stomach will work on breaking down food for roughly eight hours before passing it into the small intestine.
End of the Line
The broken-down food will remain in the small intestine for up to about two days, depending on how difficult it is to break down further. Simple food, such as corn, will pass in a matter of a few hours, but bone or more complex material takes quite a bit longer. We may think of dogs as carnivores, but because of their powerful digestive acids, they can eat almost anything. A human who picks something off the floor might be in danger of bacterial infection because our digestive system is not as powerful and food tends to take longer to digest. For a dog, the danger is minimal because of the speed and power of his insides. Whatever remains after the dog's small intestine if finished with it (and there is rarely much besides waste) is then processed in a few hours by the large intestine, moved through the colour and out. In all, the entire process, from the time a dog bites into his food to the time waste is produced, can take anywhere from around 10 hours to a couple of days.