The human lens is a surprising structure. It is avascular and yet it is actively growing throughout life, albeit extremely slowly. It receives its nourishment from the aqueous fluid that bathes it. The lens is enclosed in an elastic capsule and, for this reason, tends to assume a spherical shape, or would do if the moulding of the lens fibres allowed. In situ the shape of the lens is maintained by a series of taut fibres known as the zonule. The fibres exert radial tension on the lens but the tension is reduced when the circular part of the ciliary muscle contracts. The reduced tension of the zonule allows the lens to assume a more spherical shape and hence the anteroposterior diameter of the lens increases. As a result, the refracting power of the lens increases, that is to say, light rays are more bent and the eye becomes focused on near objects. This process of accommodation, which is produced by relaxation of the lens but contraction of the ciliary muscle, gradually becomes less effective as we grow older, probably because the lens becomes less malleable rather than because the ciliary muscle is becoming weaker. This reduction in the range of accommodation explains why the little child will present an object close to an adult's eyes and expect him or her to see it clearly. It also explains why, in the mid-40s, it becomes necessary to hold a book further from one's eyes if it is to be read easily and also the subsequent inability to read without the assistance of a spectacle lens, which provides additional converging power. The need for reading glasses occurs in people with normal eyes at about the age of 45 (presbyopia) but this is only a milestone in a slowly progressive path of deterioration that begins at birth.
Histological section of the lens reveals that beneath the capsule there is an anterior epithelium with a single layer of cells, but no such layer is evident beneath the posterior capsule. Furthermore, if one follows the single-layered anterior epithelium to the equator of the lens, the epithelial cells can be seen to elongate progressively and lose their nuclei as they are traced into the interior of the lens. Thus, one can deduce from histological sections that the lens fibres are being continuously laid down from the epithelial cells at the equator. The actual arrangement of the lens fibres is quite complex, each fibre being made up of a prismatic six-sided band bound to its fellow by an amorphous cement substance.
Slit-lamp examination of the lens reveals the presence of the lens sutures, which mark the points of junction of the end of the lens fibres. Two such sutures are usually seen, both often taking the form of the letter "Y", the posterior suture being inverted. The lens fibres contain proteins known as "crystallins" and have the property of setting up an antigen-antibody reaction if they are released into the eye from the lens capsule. One other feature of the lens, which can usually be seen with the slit-lamp microscope, is an object looking like a pig's tail, which hangs from the posterior capsule. This is the remains of the hyaloid artery, a vessel that runs in the embryonic eye from the optic disc to the vascular tunic of the lens, which is present at that stage (Figure 11.1).
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