The lacrimal gland is an exocrine gland located in the superolateral orbit within the fossa for the lacrimal gland. Embryologic development of the lateral horn of the levator aponeurosis indents the lacrimal gland, dividing it into orbital and palpebral lobes (see Chapter 1, Fig 1-7) as it traverses laterally to insert on the Whitnall tubercle. The superior transverse ligament (Whitnall ligament) forms septa through the stroma of the gland, with some fibers also projecting onto the lateral orbital wall several millimeters above Whitnall tubercle, as noted in Chapter 1 of this volume.
Between 8 and 12 major lacrimal ducts empty into the superior cul-de-sac approximately 5 mm above the lateral tarsal border after passing posterior to the levator aponeurosis, and through the Müller muscle and conjunctiva. Because the lacrimal excretory ducts pass through the palpebral portion of the gland, removal of the palpebral lobe may reduce secretion from the entire gland. Therefore, biopsy of the lacrimal gland is preferentially performed on the orbital lobe.
Figure 14-1 Nonmotor pathways of cranial nerve (CN) VII, including the efferent pathway to the lacrimal gland.
(Illustration by Christine Gralapp.)
Ocular surface irritation activates tear production from the lacrimal gland. The ophthalmic branch of the trigeminal nerve, cranial nerve (CN) V, provides the sensory (afferent) pathway in this reflex tear arc. The efferent pathway is more complicated. Parasympathetic fibers, originating in the superior salivatory nucleus of the pons, exit the brainstem with the facial nerve, CN VII (Fig 14-1). Lacrimal fibers leave CN VII as the greater superficial petrosal nerve and pass into the sphenopalatine ganglion. From there, they are believed to enter the lacrimal gland via the superior branch of the zygomatic nerve, through an anastomosis between the zygomaticotemporal nerve and the lacrimal nerve. It is unclear whether this anastomosis is uniformly present. The role of the sympathetic nervous system in lacrimation is not well understood.
The accessory glands of Krause and Wolfring are exocrine glands located deep within the superior fornix and just above the superior border of the tarsus, respectively. Aqueous lacrimal secretion has traditionally been divided into basal secretion and reflex secretion. Previously, it was thought that the accessory glands predominate in basal tear secretion, and that the lacrimal gland predominates in reflex tearing. It is now believed that all lacrimal glands respond as a unit.
The tear film is an extremely complex composition of mucin, aqueous, and oily layers of variable osmolality with components including lysozyme, lactoferrin, immunoglobulins, and prostaglandins (see BCSC Section 2, Fundamentals and Principles of Ophthalmology, for a more detailed discussion of the tear film).
Saleh GM, Hussain B, Woodruff SA, Sharma A, Litwin AS. Tear film osmolarity in epiphora. Ophthalmic Plast Reconstr Surg. 2012;28(5):338–340.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.