Appetite, hunger, and thirst, warn us that the hody needs restoration; pain, that universal monitor, never ceases to torment us if we do not obey it.
Then comes eating and drinking which are ingestion, an operation which begins as soon as the food is in the mouth, and enters the oesophagus.
During its passage, through a space of a few inches much takes place.
The teeth divide solid food, the glands which line the inside of the mouth moisten it, the tongue mingles the food, presses it against the palate so as to force out the juice, and then collects the elements in the centre of the mouth, after which, resting on the lower jaw, it lifts up the central portion forming a kind of inclined plane to the lower portion of the mouth where they are received by the pharynx, which itself contracting, forces them into the oesophagus.
One mouthful having thus been treated, a second is managed in the same way, and deglutition continues until appetite informs us that it is time to stop. It is rarely, though, that it stops here, for as it is one of the attributes of man to drink without thirst, cooks have taught him to eat without hunger.
To ensure every particle of food reaching the stomach, two dangers must be avoided.
It must not pass into the passage behind the nose, which luckily is covered by a veil.
The second is that it must not enter the trachea. This is a serious danger, for any particle passing into the trachea, would cause a convulsive cough, which would last until it was expelled.
An admirable mechanism, however, closes the glottis while we swallow, and we have a certain instinct which teaches us not to breathe during deglutition. In general, therefore, we may say, that in spite of this strange conformation, food passes easily into the stomach, where the exercise of the will ceases, and digestion begins.