Explain the digestive system and how each part of the system works?

food is introduced in the mouth in a process called ingestion and chemically broken down in the process known as mastication.

The mouth contains saliva which has mucus that lubricates the mouth, it also has salivary amylase and water which acts a solvent.

The toungue roles the food into bolus and pushed down the pharynx in a process known as peristalsis.

Then the food enters the stomach through a muscle known as Cardiac sphincter. In the stomach there are muscular contractions which converts the food into chyme.

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in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".