Lacrimal gland

Structure

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. 

Innervation

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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Blood supply

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.

Function

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.