Introduction

The ancient Greeks and Romans believed that the lens was the part of the eye responsible for the faculty of seeing. They theorized that the optic nerves were hollow channels through which “visual spirits” traveled from the brain to meet visual rays from the outside world at the lens, which they thought was located in the center of the globe. The visual information would then flow back to the brain. This concept was known as the emanation theory of vision. Celsus (25 BC–AD 50) drew the lens in the center of the globe, with an empty space called the locus vacuus anterior to it, in AD 30 (Fig I-1).

These erroneous ideas about lens position and function persisted through the Middle Ages and into the Renaissance, as shown by the drawing of the Belgian anatomist Andreas Vesalius in 1543 (Fig I-2). The true position of the crystalline lens was illustrated by the Italian anatomist Fabricius ab Aquapendente in 1600 (Fig I-3), and the Swiss physician Felix Plater (1536–1614) was the first to postulate that the retina, not the lens, was the part of the eye responsible for sight.

Figure I-1 The eye, after Celsus.

(From Gorin G. History of Ophthalmology. Wilmington: Publish or Perish, Inc; 1982.)

Today, many areas of lens physiology and biochemistry are still subjects of active research. Lens regeneration has been studied since the 18^th century. This regeneration, from epithelial cells from the lens, cornea, or iris, has succeeded repeatedly in several vertebrate species, especially newts and rabbits. In 2016, an experimental trial in children with congenital cataracts was able to regenerate working lenses from intact lens epithelial stem cells within six to eight months. However, no medical treatment can yet prevent the formation or progression of cataract in the lens of the otherwise healthy adult eye, and theories about cataract formation and innovative forms of management continue to be controversial. Although various risk factors for cataract development have been identified (discussed in Chapter 1), data to develop guidelines for reducing the risk of cataract remain inconclusive.

Figure I-2 Schematic eye from De fabrica corporis humani of Andreas Vesalius (1514–1564).

(Reproduced with permission from the Ophthalmic Publishing Company. Feigenbaum A. Early history of cataract and the ancient operation for cataract. Am J Ophthalmol. 1960;49:307.)

Figure I-3 Sketch from De oculo of Fabricius ab Aquapendente (1537–1619), showing the correct position of the lens within the eyeball.

(Reproduced with permission from the Ophthalmic Publishing Company. Feigenbaum A. Early history of cataract and the ancient operation for cataract. Am J Ophthalmol. 1960;49:307.)

The prevalence of lens disorders and continuing developments in their management make the basic and clinical science of this structure a vital component of ophthalmology training. The goal of Section 11 is to provide a curriculum for the study of the structure and function of the normal lens, the features of diseases involving the lens, and the surgical management of cataract.

• Lin H, Ouyang H, Zhu J, et al. Lens regeneration using endogenous stem cells with gain of visual function. Nature. 2016;531(7594):323–328.

Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.