The lacrimal gland (tear gland) is an exocrine gland located above the eyeball, in the anterior part of the upper outer aspect of each orbit. It secretes lacrimal fluid (tear fluid), a watery fluid isotonic to plasma, onto the surface of the eyeball. This fluid forms the aqueous portion of a multilayered tear film that lubricates, protects and provides nutrients to the conjunctiva and cornea. The lacrimal fluid ultimately drains through a series of ducts into the nasal cavity. When produced in excess, lacrimal fluid forms tears.
The lacrimal gland consists of two connecting parts: the larger orbital part and the smaller palpebral part. The gland together with its associated drainage system of ducts form the lacrimal apparatus.
This article will discuss the anatomy and function of the lacrimal gland.
|Structure||Two parts: Orbital and palpebral|
Arterial: lacrimal artery (from ophthalmic artery)
Venous: superior ophthalmic vein
|Lymphatic drainage||Superficial parotid lymph nodes|
Sensory: lacrimal nerve (from ophthalmic nerve)
Parasympathetic: greater petrosal nerve (from facial nerve)
Sympathetic: deep petrosal nerve (from internal carotid plexus)
|Functions||Lacrimal fluid production: lubrication, protection and nutrition of the ocular surface|
|Pathway of tears||Lacrimal gland → lacrimal ducts → ocular surface → lacrimal puncta → lacrimal canaliculi → lacrimal sac → nasolacrimal duct → inferior nasal meatus → nasopharynx|
- Blood supply
The lacrimal gland is an almond-shaped structure, about 2 cm in length. It is located in the anterior, superotemporal aspect of the orbit, within the lacrimal fossa of the frontal bone. The gland is split into two contiguous parts (lobes) by the lateral aponeurotic fibers of the levator palpebrae superioris muscle into an orbital part and a palpebral part. This division is only partial due to a posterior wall of parenchyma between the lobes.
- The larger orbital part rests above the aponeurosis of the levator palpebrae superioris muscle and the lateral rectus muscle laterally. It attaches to the periosteum of the orbit superiorly and the fascial sheath of the levator palpebrae superioris aponeurosis inferiorly. Anteriorly, it is bounded by the orbital septum, with the orbital fat serving as its posterior border.
- The palpebral part of the gland lies below the levator palpebrae superioris aponeurosis and projects into the superolateral aspect of the upper eyelid to attach to the superior conjunctival fornix. This part of the lacrimal gland can be observed clinically by everting the eyelid.
The lacrimal gland contains about 12 main excretory ducts. Ducts from the orbital part of the gland accompany those of the palpebral part by piercing through the levator palpebrae superioris aponeurosis to empty into the conjunctival sac. These ducts open along the lateral aspect of the superior fornix of the conjunctiva. In addition to the lacrimal gland, several accessory lacrimal glands (glands of Krause and Wolfring) are usually present in the palpebral conjunctiva. Although these glands are considerably smaller, they are structurally similar to the main lacrimal gland. The accessory glands account for about 10% of the total lacrimal secretion. Compared to the upper eyelid, the lower eyelid has fewer accessory lacrimal glands.
Pathway of lacrimal fluid
The lacrimal gland continuously secretes lacrimal fluid into the lateral aspect of the superior conjunctival fornix via the lacrimal ducts. The fluid is spread over the entire eye surface, from lateral to medial, with each blink. At the medial canthal region of the eye, the fluid collects in a triangular space called the lacrimal lake. The fluid is subsequently drained by capillary action into the lacrimal canaliculi which are small channels within each eyelid, via the lacrimal puncta. The lacrimal puncta are the openings of the lacrimal canaliculi which are located on the lacrimal papilla at the medial end of the upper and lower eyelids.
From the lacrimal canaliculi, lacrimal fluid drains medially into the lacrimal sac, the dilated upper aspect of the nasolacrimal duct. The lacrimal sac lies in a fossa formed by the lateral surface of the lacrimal bone and the frontal process of the maxilla. From the lacrimal sac, lacrimal fluid is carried by the nasolacrimal duct which opens at the anterior end of the inferior nasal meatus. This duct traverses a bony canal (nasolacrimal canal) formed by the lacrimal bone, maxilla and the inferior nasal concha. The fluid subsequently drains posteriorly through the nasal cavity to the nasopharynx, where it mixes with mucus and is ultimately swallowed. A small amount of lacrimal fluid is lost by evaporation or absorption across the conjunctiva.
The lacrimal gland receives sensory, parasympathetic, as well as sympathetic innervation.
- The lacrimal nerve, from the ophthalmic branch of the trigeminal nerve provides sensory innervation to the gland.
- Parasympathetic secretomotor neurons stimulate the secretion of lacrimal fluid. Preganglionic parasympathetic fibers from the central nervous system (CNS) reach the pterygopalatine ganglion via the greater petrosal nerve (a branch of the facial nerve) and the nerve of the pterygoid canal. These fibers synapse on postganglionic parasympathetic fibers within the ganglion, which then travel within the zygomatic and zygomaticotemporal branches of the maxillary nerve to supply the gland.
- Sympathetic innervation is from the superior cervical ganglion. From this ganglion, the nerve fibers reach the pterygopalatine ganglion via the internal carotid plexus and the deep petrosal nerve. The sympathetic fibers, subsequently, without synapsing in the ganglion, travel along the same path as the parasympathetic fibers supplying the lacrimal gland. Sympathetic innervation regulates blood flow through the lacrimal gland and glandular secretions.
Both the postganglionic parasympathetic secretomotor fibers and the postganglionic sympathetic fibers ultimately reach the lacrimal gland via the lacrimal nerve.
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Stimulation of the cornea and conjunctiva activates a reflex pathway that triggers an increase in tear production from the lacrimal gland. This phenomenon is referred to as the lacrimation reflex. The main goal of this reflex is to wash out the cause of irritation.
The afferent (sensory) limb of this reflex is carried by the lacrimal branch of the ophthalmic nerve to the brain. Here, impulses are relayed by interneurons to parasympathetic fibers of the superior salivary nucleus in the pons and to sympathetic fibers within the thoracic region of the spinal cord. The efferent limb of the reflex involves preganglionic parasympathetic secretomotor fibres carried by the greater petrosal nerve and postganglionic sympathetic fibres carried by the deep petrosal nerve to the lacrimal gland.
A variety of emotional states may stimulate an increase in tear fluid production in the absence of a precipitating physical irritant. This phenomenon is what is commonly called crying.
The main arterial supply of the lacrimal gland is by the lacrimal artery, which is a branch of the ophthalmic artery (from the internal carotid artery). The gland is also sometimes supplied by the infraorbital artery which is a branch of the maxillary artery (from the external carotid artery). The lacrimal gland is drained by the superior ophthalmic vein into the cavernous sinus. The lymphatic drainage of the lacrimal gland is by the superficial parotid lymph nodes which drain into the superior deep cervical nodes.
The main function of the lacrimal gland is the production of lacrimal fluid, which is involved in the maintenance of a healthy ocular surface. Lacrimal fluid contains proteins, antimicrobial agents, water and electrolytes that ensure adequate lubrication, protection and nutrition of the ocular surface.
Lacrimal fluid contains a significant amount of water that keeps the surface of the eye moist as well as dissolved elements that are necessary for normal cellular functioning. The fluid also contains antimicrobial agents including phospholipase, lysozyme, peroxidase, lactoferrin and immunoglobulins that provide defence against invading pathogens. In addition, the lacrimal gland produces numerous other substances and proteins including retinol (derived from Vitamin A) and growth factors such as epidermal, fibroblast and keratinocyte growth factors. These are involved in corneal regeneration and maintenance of corneal avascularity and transparency.
Preocular tear film
Lacrimal fluid forms the aqueous component of the tear film, which is a three-layered coating of the ocular surface. The tear film is composed of an inner mucin layer (largely from conjunctival goblet cells), a middle aqueous component and an outer lipid layer (from the Meibomian glands). The trilaminar tear film performs several important functions including:
- Protecting the ocular surface from pathogens
- Removing debris and metabolic waste
- Creating an air-tissue interface for gaseous exchange in order to provide oxygen to the avascular cornea
- Providing an even optical surface for light transmission at the air-cornea interface
- Promoting the integrity of the transparent cornea by providing nutrients and metabolites
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