Parotid gland

The parotid gland is a major salivary gland in many animals. In humans, the two parotid glands are present on either side of the mouth and in front of both ears. They are the largest of the salivary glands. Each parotid is wrapped around the mandibular ramus, and secretes serous saliva through the parotid duct into the mouth, to facilitate mastication and swallowing and to begin the digestion of starches. There are also two other types of salivary glands; they are submandibular and sublingual glands. Sometimes accessory parotid glands are found close to the main parotid glands. The venom glands of snakes are a modification of the parotid salivary glands.

Etymology The word parotid literally means "beside the ear". From Greek παρωτίς (stem παρωτιδ-) : (gland) behind the ear < παρά - pará : in front, and οὖς - ous (stem ὠτ-, ōt-) : ear.

Structure The parotid glands are a pair of mainly serous salivary glands located below and in front of each ear canal, draining their secretions into the vestibule of the mouth through the parotid duct. Each gland lies behind the mandibular ramus and in front of the mastoid process of the temporal bone. The gland can be felt on either side, by feeling in front of each ear, along the cheek, and below the angle of the mandible. The parotid duct, a long excretory duct, emerges from the front of each gland, superficial to the masseter muscle. The duct pierces the buccinator muscle, then opens into the mouth on the inner surface of the cheek, usually opposite the maxillary second molar. The parotid papilla is a small elevation of tissue that marks the opening of the parotid duct on the inner surface of the cheek. The gland has four surfaces – superficial or lateral, superior, anteromedial, and posteromedial. The gland has three borders – anterior, medial, and posterior. The parotid gland has two ends – a superior end in the form of a small superior surface and an inferior end (apex). A number of different structures pass through the gland. From lateral to medial, these are:

Facial nerve Retromandibular vein External carotid artery Superficial temporal artery Branches of the great auricular nerve Maxillary artery Sometimes accessory parotid glands are found as an anatomic variation. These are close to the main glands and consist of ectopic salivary gland tissue. Capsule of parotid gland A capsule of the parotid gland is formed from the investing layer of the deep cervical fascia. It is supplied by the great auricular nerve. The fascia splits to enclose the gland. This splitting occurs between the angle of the mandible and the mastoid process. The superficial lamina (parotidomassetric fascia) is thick and is attached to the zygomatic arch. The deep lamina is thin and is attached to the styloid process, tympanic plate, and the ramus of the mandible. The part of the deep lamina extending between the styloid process and the mandible is thickened to form a stylomastoid ligament. The stylomandibular ligament separates the parotid gland from the superficial lobe of the submandibular gland.

Relations Superficial or lateral relations: The gland is situated deep to the skin, superficial fascia, superficial lamina of investing layer of deep cervical fascia, and great auricular nerve (anterior ramus of C2 and C3). Anteromedial relations: The gland is situated posterolaterally to the mandibular ramus, masseter, and medial pterygoid muscles. A part of the gland may extend between the ramus and medial pterygoid, as the pterygoid process. Branches of the facial nerve and parotid duct emerge through this surface. Posteromedial relations: The gland is situated anterolaterally to mastoid process of temporal bone with its attached sternocleidomastoid and digastric muscles, styloid process of temporal bone with its three attached muscles (stylohyoid, stylopharyngeus, and styloglossus) and carotid sheath with its contained neurovasculature (internal carotid artery, internal jugular vein, and 9th, 10th, 11th, and 12th cranial nerves). Medial relations: The parotid gland comes into contact with the superior pharyngeal constrictor muscle at the medial border, where the anteromedial and posteromedial surfaces meet. Hence, a need exists to examine the fauces in parotitis. The facial nerve (CN VII) splits into its branches within the parotid gland, thus forming its parotid plexus. Nerves of this plexus then pass through the parotid gland without innervating the gland itself.

Vasculature Arterial supply The external carotid artery and its terminal branches within the gland, namely, the superficial temporal and the maxillary artery, also the posterior auricular artery supply the parotid gland. Venous drainage Venous return is to the retromandibular veins. Lymphatic drainage The gland is mainly drained into the preauricular or parotid lymph nodes which ultimately drain to the deep cervical chain.

Nerve supply The parotid gland receives both sensory and autonomic innervation. Sympathetic The cell bodies of the preganglionic sympathetic fibres that supply the gland usually lie in the lateral horns of upper thoracic spinal segments (T1-T3). Postganglionic sympathetic fibers from superior cervical ganglion reach the gland by passing along the external carotid artery and middle meningeal artery. They act to cause vasoconstriction. Parasympathetic Preganglionic parasympathetic fibers for the parotid gland arise in the brainstem in the inferior salivatory nucleus, and leave the brain in the glossopharyngeal nerve (CN IX), then pass in the tympanic nerve to the tympanic plexus, then from the tympanic plexus in the lesser petrosal nerve to the otic ganglion where they synapse. Postganglionic (post-synaptic) fibers from the ganglion then "hitch-hike" along the auriculotemporal nerve to reach the parotid gland. Sensory General sensory innervation to the parotid gland and its capsule is provided by the auriculotemporal nerve.

Histology

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