THE EYE 15*9 



firmer, and the central portion, which constitutes the nucleus, is 

 very firm and hard. The laminae are arranged concentrically, 

 and, after boiling or immersion in alcohol, they may be peeled off, 

 like the coats of an onion. The fibres of which the laminae are 

 composed terminate upon septa within the lens, of which septa 

 the radiating lines on the surfaces, already referred to, are the 

 free margins. The concentric laminae are therefore not continuous 

 all round, but are split up along these lines. The lens-flbres, which 

 are disposed in a curved manner, are of small size, and have serrated 

 edges, which fit closely to each other. In transverse section the 

 fibres appear as hexagonal prisms. The fibres are the elongated cells 

 which line the posterior part of the epiblastic vesicle (lens -vesicle), 

 from which the lens is developed. In early life each fibre has a 

 nucleus, but, after the lens has attained its full development, only 

 the outermost fibres are nucleated. 



The lens is non-vascular in the adult. 



Capsule of the Lens. — ^This is a transparent, homogeneous, elastic, 

 and brittle membrane, which surrounds and encloses the lens. Its 

 anterior wall is thicker and more elastic than the posterior. In 

 the adult it is non-vascular, but in the foetus it receives the hyaloid 

 branch of the arteria centralis retinae, which reaches it through the 

 canal of Stilling in the vitreous body. 



Epithelium of the Lens. — ^The posterior surface of the lens is 

 devoid of epithelium, and is in direct contact with the posterior 

 wall of the capsule. The anterior surface is covered by a single 

 layer of columnar cells, which intervenes between the anterior 

 surface and the anterior wall of the capsule. Towards the equator 

 these cells become elongated, and pass into short fibres, which 

 become continuous with the superficial lens-fibres. 



Crystalline Lens at Different Ages. — The characters of the lens 

 at different ages are as follows : 



Foetal Lens. Adult Lens. Lens in Old Age. 



Almost spherical. Biconvex. Flattened. 



Pinkish colour. Colourless. Amber colour. 



Semitransparent. Transparent. Opaque, more or less. 



Soft in consistence. Firm in consistence. Very firm in consistence. 



Vitreous Body. — This body occupies about four-fifths of the 

 space within the eyeball, and is situated between the crystalline 

 lens and the retina. It is transparent, and gelatinous in consist- 

 ence, and is composed of HoO, holding in solution a small quantity 

 of sodium chloride and albuminous matter. It is surrounded by a 

 transparent, homogeneous envelope, called the hyaloid membrane. 

 This membrane is in contact with the retina, except anteriorly, 

 in which latter situation it presents an excavation called the fossa 

 fatellaris, into which the posterior surface of the crystalline lens 

 is received. 



Towards its circumference the vitreous body is laminated, the 

 laminae being arranged concentrically. Laminae are also saicj to 



