Lens

Lens of the eye (from Latin lens crystallina - crystal lentil) has the shape of a biconvex lentil. Its anterior surface is less convex than the posterior surface; the radius of curvature of the anterior surface is 10 mm, of the posterior surface - 6 mm. The anteroposterior dimension of the lens is 4.5-5 mm, the equatorial - 9 mm. Weight 0.2 g, volume 0.25 cm³, specific gravity 1.121; refractive index 1.413. The lens is located in the eye behind the iris in a cup-shaped depression of the vitreous body (fossa patellaris); along the equator it is surrounded by ciliary processes and fixed to the ciliary body by the fibers of the so-called Zinn's ligament (zonula Zinnii), and to the vitreous body - by the hyaloideo-capsular membrane. The spaces formed between the posterior fibers of Zinn's ligament and the anterior surface of the vitreous body are called zonular spaces (spatia zonularia), or Petit's canal (canalis Petiti).-According to its embryonic development, the lens is an ectodermal formation. The lens is formed by means of a cup-shaped invagination of the epithelium at the top of the eye vesicle. This invagination of the epithelins becomes detached, transforming into the lens vesicle, initially hollow; then its cells grow and occupy the entire cavity, forming lens fibers. According to morphological structure, the following components are distinguished in the lens: the capsule (or sac), the anterior epithelium of the capsule and the lens fibers. The capsule (capsula lentis) covers the lens on all sides and in it one can distinguish the anterior part (on the anterior surface - the anterior sac) and the posterior part (on the posterior surface); they connect at the equator, forming a rounded angle. The lens capsule is homogeneous, structureless, strongly light-refracting, transparent elastic membrane; its thickness at the anterior pole is greater (in an adult 11 μ), than at the posterior (3 μ). The inner surface of the anterior capsule is covered with a single layer of cubic, polygonal epithelial cells (fig. 1), the posterior sac has no epithelium. Toward the equator, the epithelial cells of the anterior sac gradually become taller, in shape approach cylindrical, elongate in length and give rise to lens fibers, which are directed along the meridians to the posterior pole (fig. 1 and 2). The cell nuclei, as they

Figure 1.

Figure 2.

Figure 1. Transition of capsule epithelium into lens fibers. Figure 2. Formation of lens sutures: A-one sector without sutures; B-linear sutures connect sectors (shark lens); C-V-shaped sutures of the embryonic nucleus in humans; D-in an adult. bend inward into the lens in the form of an arc, forming the so-called nuclear belt; in the center of the lens there are no nuclei. The lens fibers have the shape of long ribbons, 7 to 10 mm each; on cross sections they appear as an elongated hexagon. The fibers connect with each other by means of a cementing substance into lens plates, which are directed from the anterior pole to the posterior (fig. 2) and back. When looking at fig. 2, the course of the lens fibers becomes completely clear. At the junction of individual sectors at the posterior and anterior poles, thin lines (embryonic sutures) are formed, which diverge in the form of rays of a star (lens star). The pattern of sutures changes during life, with the growth and layering of lens fibers; in embryonic life each lens star has three rays, and the rays of one star correspond to the intervals of another; later they are divided into additional ones and in an adult there are from 9 to 12 rays (fig. 3). The location of the sutures can be seen with a slit lamp; the lens star also clearly appears with the beginning of cataract. Being an ectodermal formation, the lens grows throughout life, with new layers of fibers concentrically growing on the old ones; the latter end up in the center of the lens and form the so-called embryonic nucleus, the boundaries of which are outlined,

Figure 3.

Figure 4.

Figure 3. Lens sutures in humans: A-anterior suture of the inner part of the embryonic nucleus; B-posterior suture of the inner part of the embryonic nucleus; C-suture of the outer part of the embryonic nucleus; D-sutures of the adult nucleus; E-another drawing of it; F-sutures of fibers in the cortex. Figure 4. Longitudinal section of a. hyaloideae at its bifurcation in the Cloquet canal; thickened fibers form the boundary of the canal with the so-called secondary vitreous body. the senile nucleus is formed (nucleus lentis crystallinae or lentiformis). With age the senile nucleus increases, the central and para-central fibers become less distinct, merge into homogeneous masses, the capsule thickens somewhat, the lens strongly reflects light, and the pupil on examination appears grayish (senile reflex), which sometimes gives occasion to erroneously suspect the beginning of cataract. When examining mirror images of the lens with a strong light source, in addition to reflexes from the anterior and posterior surfaces of the lens (see Eye Accommodation), from the anterior and posterior boundaries of the nucleus from the age of 24-30 years, two more weak images (anterior and posterior nuclear reflexes) are obtained. The substance of the lens, resembling crystal in appearance (hence the name of the lens - lens crystallina), is transparent as a whole, colorless in young age, soft to the touch; in an elderly person the peripheral parts (cortex) are soft, the central part (nucleus) is harder, the elasticity of the lens is gradually lost (see Presbyopia). The nucleus with age acquires a yellowish or brownish color. According to chemical composition, the lens contains 60% water, 35% soluble and 2.5% insoluble albuminoid, 2% fat with traces of cholesterol and 0.5% mineral substances. In embryonic life the lens is nourished by the embryonic artery (a. hyaloidea), which goes from the posterior end of the embryonic fissure to the posterior surface of the lens and gives a nourishing vascular network to it (fig. 4). After birth the lens is nourished from the side of the ciliary processes, aqueous humor and vitreous body - by filtration, diffusion and osmosis. As a strong biconvex lens, the lens together with the cornea and other media has great importance in the refraction (see) of the eye. From the point of view of dynamics, due to its elasticity, the lens has great importance as one of the components in the act of eye accommodation (see). Pathology of the lens is expressed as follows: anomalies of development are observed, changes in the position of the lens and its opacifications. Inflammations of the lens are usually not observed; however, old authors described inflammations of the lens (phakitis and periphakitis), meaning: a) those cases where after iridocyclitis developed cataract (cataracta cyclitica), fused with exudative membranes with the iris and ciliary body, and the products of inflammation penetrated into the capsule of the lens; b) cases after penetration of an infected foreign body into the lens. Congenital malformations of the lens include: 1. Microphakia - abnormally small size of the lens in microphthalmia (see Microphthalmus); very rarely and in normal eyes there is a small-sized lens and then with dilated pupil the edges of the lens are visible. 2. a) Anterior lenticonus; with this the middle segment of the anterior surface of the lens protrudes into the anterior chamber in the form of a cone (fig. 5). Vision is disturbed; in the center and periphery of the lens refraction is not the same. It is observed on one or both eyes; it is explained by adhesions of the lens with the cornea in the embryonic stage. b) Posterior lenticonus - cone-shaped protrusion on the posterior surface of the lens; origin is more often explained by tension of the posterior surface of the lens by remnants of a. hyaloidea. For the recognition of these anomalies, the sharp light reflex in the area of lenticonus in the form of an oil drop is important. 3. Coloboma of the lens (coloboma lentis) (see Coloboma), a very rare malformation, sometimes in combination with coloboma of the iris and choroid. With dilated pupil one can see a depression or notch on the edge of the lens, more often in the lower part. 4. Aphakia - congenital absence of the lens, extremely rare cases. 5. Congenital change in the position of the lens - dislocation; unlike traumatic ones it is called ectopia (ectopia lentis); it is inherited, mostly on both eyes; the symptoms are the same as in acquired (traumatic or on the basis of degenerative changes of the eye): uneven depth of the anterior chamber, partial iridodonesis (tremor of the iris); on examination with transmitted light - a red pupil, divided by a dark arc into two parts, two papillae of uneven size on ophthalmoscopy; a number of subjective symptoms - diplopia, macro- or micropsia and others. Such patients are always threatened with the danger that the displaced lens will completely dislocate into the vitreous body or into the anterior chamber. Therapy is difficult. Sometimes optical glasses help, sometimes careful surgical intervention. Axenfeld recommended the reclination of the dislocated lens, i.e.

Figure 5. Lenticonus anterior.

the operation of turning the L. over with a special needle into the vitreous body. The operation is unreliable—the L. from the vitreous body may rise again into the area of the pupil, and in the subsequent course complications are observed with iridocyclitis or glaucoma. Dislocation of the L. is more often acquired—from rupture of Zinn's ligament and lig. hyaloideo-capsularis due to trauma, operations, as a result of subluxation, and in eyes with degenerative phenomena, e.g., in high myopia, etc. The L. is dislocated into the anterior chamber of the eye, into the vitreous body, and under the conjunctiva of the eyeball. Symptoms: partial or complete aphakia, trembling of the iris, "oil drop" symptom if the L. is in the anterior chamber. Consequences for the eye: with dislocation into the anterior chamber—complication with glaucoma (in 92-95%) due to swelling of the L., closure of the angle of the anterior chamber, etc.; adhesions with the cornea and iris. With dislocation into the vitreous body—glaucoma (less often than with dislocation into the anterior chamber), inflammatory phenomena both immediately after dislocation and later. With dislocation under the conjunctiva the L. either resolves or becomes fixed here, without causing significant irritation phenomena. Therapy: with dislocation into the anterior chamber—means that constrict the pupil, extraction (accompanied by loss of the vitreous body). With dislocation into the vitreous body—no intervention if the eye tolerates the dislocation well—correction of aphakia; with a tendency to complications of iridocyclitis or glaucoma—extraction of the L. (with a loop), but the operation under these conditions is too difficult—the eye often perishes; with pain and absence of vision—enucleation. With dislocation under the conjunctiva there is no need to rush with surgical intervention: after some time, if the L. does not resolve, it can be easily removed. Opacification of the L.—see Cataract.