The salivary glands are several exocrine glands located within the soft tissues of the head and neck near or within the oral cavity. They produce and excrete a watery substance called saliva that contributes to the digestion of food. The salivary glands excrete saliva in the mouth via their ducts.

All salivary glands are classified regarding their size and duct types. They can be divided into two main groups - minor and major salivary glands. The major salivary glands are responsible for the production of about 90 - 95% of saliva. 

Minor salivary glands

Each person has numerous minor salivary glands that are embedded in the mucosa of the oral cavity. Overall, the oral cavity contains about 800 - 1000 dispersed minor salivary glands. They are classified depending on their location:

• Labial glands - located in the lips;
• Buccal glands - found within the cheeks;
• Palatine glands - situated in the palate; they are further subdivided into glands of the hard palate and glands of the soft palate;
• Molar glands - positioned between the masseter and buccinator muscles;
• Lingual glands - found in the tongue.

The minor salivary glands mainly function for the lubrication of the oral cavity. The ducts of the buccal and labial salivary glands open in the oral vestibule, but the ducts of the palatine and lingual glands open in the oral cavity proper.

Major salivary glands

The major salivary glands are primary glands that are responsible for the initiation of digestion and moistening, lubrication and protection of the oral cavity. They are a collection of exocrine tissue that secrete saliva through one central duct. Each person has three pairs of major salivary glands:

• Parotid gland
• Submandibular gland
• Sublingual gland

Parotid gland

The parotid gland is the largest of all salivary glands, and it has an inverted pyramid shape. It produces around 60 - 65% of daily saliva. The parotid gland is a serous gland lying on each lateral side of the face in front of the auricle of the ear. It is composed of two lobes - superficial and deep. Both lobes are separated by the facial nerve (CN VII).

Anterior to the parotid gland lies the masseter muscle, while posterior is the external ear and sternocleidomastoid muscle. In the superior direction, the parotid gland extends to the zygomatic arch, while in the inferior direction, it reaches the mandible and its angle. The parotid gland has a fibrous capsule called the parotid sheath formed by the parotid fascia that covers the gland from all sides.

Parotid duct

The parotid gland secretes its produced saliva via the parotid duct, also known as the Stensen's duct. The parotid duct goes through the cheeks and opens in the oral cavity through a papilla located opposite the second maxillary molar tooth. 

The parotid duct goes from the anterior border of the gland to the zygomatic arch. It extends below the zygomatic arch, wraps around the posterior border of the masseter muscle, and passes horizontally over its surface. Next, the parotid duct pierces the buccinator muscle and reaches the oral cavity, where it opens with the papilla of the parotid duct. 

Accessory parotid gland

In about 50% of people in the population, the parotid gland has an accessory parotid gland referred to as the accessory lobe of the parotid gland. It is located anterior and more in the superior aspect of the gland, and its inferior part connects with the superior aspect of the parotid duct. The accessory parotid gland can have one or several smaller ducts connected with the parotid duct.

Submandibular gland

The submandibular gland is the second largest salivary gland. It is a mixed (seromucous) gland that lies below the muscles of the oral diaphragm within the submandibular fossa of the mandible. It appears elongated, a bit flattened and has the shape of the letter "U" (horseshoe-shaped).

Without the stimulation of the parotid gland, the submandibular gland can produce around 70% of the daily saliva. But when the parotid gland receives stimuli, the submandibular gland produces about 10 - 30% of daily saliva.

Each submandibular gland is located next to the inner surface of the mandible and the anterior and posterior bellies of the digastric muscle. Like the parotid gland, the submandibular gland also has two lobes - superficial and deep. Both lobes are continuous with each other, and they are separated by the mylohyoid muscle. Fibers from the posterior border of the mylohyoid extend into the gland, and the gland wraps around them.

Submandibular duct

The saliva from the submandibular gland reaches the oral cavity proper via the submandibular duct - a central excretory duct of the submandibular gland. It is also known as Wharton's duct. 

The submandibular duct curves around the posterior border of the mylohyoid muscle and gets above the muscles of the oral cavity floor. Then it passes medially across the diaphragm of the oral cavity and along the upper surface of the mylohyoid. It opens within the oral cavity proper via a small papilla called the sublingual caruncle that is found on the undersurface of the tongue. On the way to the caruncle, it crosses with the lingual nerve of the mandibular nerve (CN V3).

Sublingual gland

The sublingual gland is the smallest of the major salivary glands. It is a mucous gland, and it contributes only about 5% of daily saliva. It is directed longitudinally and has a lentiform shape, or simply - the sublingual salivary gland resembles an almond.

Each gland lies above the muscles of the oral diaphragm. Above the gland is the lateral side of the tongue. The inferior aspect of the sublingual gland lies on the superior surface of the mylohyoid muscle. In the medial direction, it connects with the genioglossus and hyoglossus muscles.

Along the gland on its medial side goes the lingual nerve and submandibular duct. The lateral and anterior aspects of the gland face the mandible and lie in the sublingual fossa located on the inner surface of the mandibular body. Both sublingual glands are located very close to each other within the mental spine area of the mandible.

Sublingual duct

The sublingual gland is classified as the major salivary gland, and therefore, it should have only one central duct. However, it has one major and several minor ducts. The main sublingual duct is called the duct of Bartholin, and the minor ducts are known as the Rvinus ducts. The duct of Bartholin joins with the submandibular duct near the sublingual caruncle and also opens to the oral cavity via the sublingual caruncle. 

 

Saliva

The major and minor salivary glands produce a seromucous transparent and stretchy liquid with a pH of 6.2 - 7.6 called saliva. Overall, it starts the chemical food procession and helps in digestion, it lubricates food during swallowing and helps in bolus creation. It contains water, mucus, electrolytes, antibacterial substances and enzymes. However, It mainly contains water - around 99.5%. All salivary glands produce about 0.5 - 1.5 liters of saliva per day. The main functions of saliva include:

• Regulation of the balance between acids and alkalines
• Initiation of chemical food procession
• Food breakdown 
• Regulation of ingested food temperature
• Lubrication and moistening of ingested food and oral mucosa
• Initiation of swallowing
• Protection against various antigens and microbes
• Teeth protection from bacterial decay