ANATOMY OF THE HUMAN EYE

Anatomy of the eye includes lacrimal gland, cornea, conjunctiva, uvea (iris, choroid & ciliary body), lens, blood supply, retina, vitreous & optic-nerve. For ophthalmologists, optometrists, medical, dental, and optometry students, eye-anatomy forms the basis for eye-pathology in diseases: dry eye, retinal detachment, macular degeneration, diabetic retinopathy, eye-trauma etc.

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Monday, February 13, 2006

Lacrimal Excretory System





Tears are produced in the lacrimal gland (#1 in diagram to the left)exit the ocular surface via the puncta at the medial portion of the eyelids (#2 Diagram left). Each punctum is a small, round, or transversely oval aperture situated on a slight elevation. The puncta can be seen to be roughly in line with the openings of the Meibomian glands, the nearest of which is only 0.5 to 1 mm away.
Each puncta empties into a canaliculus (Diagram left #3)which has at first a vertical (2 mm length) and then a horizontal (8 mm length) segment. The canaliculi join in the common canaliculus and may dilate to be called the sinus of Maier which then transitions to the lacrimal sac.
In the photo below, the puncta and canaliculi are lined by stratified squamous epithelium (
Photomicrographs below #'s 1 and 2). Click to enlarge the photograph. The puncta are positioned between conjunctiva (Photo below 3) and skin (Photo below #4) at the border of the eyelid.

The lacrimal sac is placed in the lacrimal fossa (formed by the lacrimal bone and the frontal process of the maxilla) which lies in the anterior part of the medial wall of the orbit. The sac is closed above (Diagram above #4) and open below, where it is continuous with the naso-lacrimal duct (Diagram above #5). The upper portion of the lacrimal sac is called the fundus and the lower portion is called the body and the length of these segments, somewhat arbitrarily distinguished are about 3 and 10 mm respectively. The lacrimal sac joins the nasolacrimal duct which measures about 12 mm in length and 3-4 mm in diameter. The lacrimal sac and duct are both lined by two layers of epithelium, the superficial of which is columnar, the deeper flattened. The epithelium of the lacrimal sac "stratified columnar" and may have areas that are ciliated and areas in which the superficial layer contains only Goblet cells and mucous. The bases of the columnar cells pass through the deeper layer to reach the basement membrane. The lacrimal sac has a papillary appearance with numerous infoldings. Occasional oncocytic cells may be evident particularly in lacrimal sacs that are chronically inflamed and are probably the source for oncocytomas that may affect the lacrimal sac. The nasolacrimal duct enters the nose at the inferior nasal meatus. The point of entry is called the lacrimal osteum and it is covered by a fold of nasal mucosa, the plica lacrimalis or if you prefer the valve of Hasner, that prevents mucous from entering the system in a retrograde fashion from the nose (with sneezing or "blowing one's nose against a handkerchief"). The "valve" of Hasner is closed in about 70% of newborns but opens spontaneously by 6-12 months. This is an important clinical point one must consider in an infant with tearing.
By the way there is no end to eponyms for valves in this system if one wants to seem erudite. They are all folds of mucous membranes which have no real valvular function. The valves of Foltz or Bochdalek occur at the junction of the puncta and cannaliculus; the valve of Rosenmuller or Huschke occurs as the common cannaliculus enters the fundus of the lacrimal sac; the valve of Beraud or Krause occur at the junction of the fundus and body of the sac; the valve of Taillefer occurs at the sac-duct junction and of course the valve of Hasner also is called the valve of Cruveilhier or Bianchi.






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