The parotid gland: anatomy, contents, blood and nerve supply
The parotid gland: anatomy
Is the largest of the salivary glands, almost totally serous in secretion.
The gland is a pyramid with an irregular triangular base directed laterally
*The pyramid is wedged in the slit between the external auditory meatus posteriorly & the mandible anteriorly.
*The base of the pyramid lies laterally in the preauricular region, & the three borders are:
– Superior: along the zygomatic arch
– Posterior: anterior to the EOM & mastoid process
– Anterior: actually it is antero-inferior overlies the posterior part of masseter.
*The apex of the pyramid lies deep in the slit reaching the carotid sheath.
* Superiorly: The zygomatic arch & the skull base above
– Mastoid process sandwiched by the sternomastoid muscle & the posterior belly of digastric
– The EOM, styloid process & stylohyoid are also posterior relations.
* Antero-inferiorly: The angle of the mandible sandwiched between masseter & medial pterygoid muscles
* Laterally: The fatty tissue of the side of the face
* Medially: The carotid sheath & lateral pharyngeal space
The parotid duct:
– The duct leaves the anterior border of the gland 1 cm below the zygomatic arch.
– It courses over masseter in a horizontal line that could be marked on the surface as the middle 1/3 of a line passing from the intertragic notch to the point midway between the red margin of the lip & the lower border of the nose.
– At the anterior border of masseter, the duct dips medially piercing the buccal pad of fat & buccinator opposite to the last molar tooth.
– After a short course in buccinator (which acts as a valve), the duct opens in the vestibule of the mouth opposite to the upper second molar tooth.
The parotid isthmus:
– Is the thinnest portion of the gland that lies between the mandible & the mastoid process
– The part of the gland superficial to the isthmus is called (wrongly!) the superficial lobe & the part deep to it called the deep lobe.
– Actually, VII is the structure which separates the gland into its two lobes
The accessory parotid gland:
From the duct over masseter sometimes a piece of parotid tissue lies & may extend high up to the temporal fossa. This is the accessory parotid g. or sometimes named (glenoid lobe).
The parotid fascia:
Is derived from the investing cervical fascia which splits to enclose the gland.
The superficial layer of the fascia will be attached above to the zygomatic arch & after it crosses the gland it fuses with masseteric fascia.
The deep layer is attached above to the lower border of the tympanic plate & reaches medially the carotid sheath with which it fuses.
A thickening in the deep layer stretches between the styloid process & the angle of the mandible called the stylomandibular ligament which separates the gland from the submandibular gland.
The fascia is supplied by the great auricular nerve.
Contents of the gland:
Many important structures lie within or very near to the parotid gland, structures which lie within the substance of the gland are; from superficial to deep:
1-The facial nerve:
– After leaving the stylomastoid foramen in its way to the face this nerve enters the posterior surface of the gland medially then passes very superficial in its substance where it gives its terminal branches from the anterior border of the gland to the face.
– The facial branches are almost horizontal at their beginning and could be injured in parotid incisions.
2-The retromandibular vein:
– The two maxillary veins leave the ITF & go forward medial to the neck of the mandible to enter the parotid deep to the facial nerve
– After receiving the superficial temporal vein, the retromandibular vein is formed within the gland.
– It drains the parotid & divides into:
a) Anterior division: receives the anterior facial vein to form the common facial vein in the submandibular triangle.
b) Posterior division: receives the posterior auricular vein to form the EJV behind the angle of the mandible.
3-The external carotid artery:
– Enters the gland from below & ascends in it deep to the above two structures
– When reaches the neck of the mandible the ECA divides into its two terminal divisions which leave the gland from its anterior & superior borders.
5- The auriculo-temporal nerve:
In its way from the ITF to the temporal fossa, it passes through the upper part of the gland to accompany the superficial temporal artery & go to the scalp.
– Superficial temporal a.
– Posterior auricular a.
1- Sensory & secretomotor: Lesser petrosal fibers from the otic ganglion via the auriculotemporal nerve.
2- Parotid fascia is supplied by the great auricular nerve.
• Malignant tumors of the parotid sometimes involve the facial nerve forming lower motor facial palsy.
• Incisions in the substance of the gland should be horizontal along the facial nerve branches because they are superficial to the vessels of the gland & paralysis could occur by cutting the facial nerve before a serious bleeding is encountered.
• Inflammation of the parotid as in mumps usually obliterate the angle between the lobule of the auricle & the angle of the mandible.