Chewing and swallowing

Some people eat to live, others live to eat. For both groups - the first step of digestion is chewing, or mastication.

This is where food gets moistened by saliva, and is broken down into smaller bits that are easy to swallow and pass through the esophagus.

It’s also the step that helps smear the food out over the tongue so that it can be fully tasted.

The journey of food starts in the oral cavity, which is the first part of the digestive tract.

The oral cavity is like an empty room, there’s the roof, which is formed by the hard and soft palate, the floor, which is formed by the tongue and the mylohyoid muscles, the lateral walls formed by the inside of the cheeks, and there’s the front which gets sealed off by the lips and teeth.

A layer of epithelial cells line the inside of the mouth and form the first line of defense against pathogens.

The surface of the epithelial cells is kept moist by mucus secreted by salivary glands.

The major salivary glands are actually located outside the oral cavity.

These are the parotid glands, found in front of each ear, the submandibular or submaxillary glands, found under the mandible and the sublingual glands that sit beneath the tongue, under the floor of the mouth.

Now, even though the salivary glands are not located inside the mouth, they have ducts that travel to the oral cavity so they can secrete saliva into it.

The secretion of saliva is mainly controlled by the parasympathetic nervous system through cranial nerves.

The parotid glands are innervated by the glossopharyngeal nerve, or cranial nerve nine, while the submandibular and sublingual glands are innervated by the facial nerve, or cranial nerve seven.

Saliva is mostly made of water, but it has other important components like salivary amylase, which is a digestive enzyme that breaks down starch, and mineral salts like sodium bicarbonate that help maintain a pH of 6.5 to 7.5 inside the mouth.