62A 



THE EYE. 



the other two downwards and to either side, whereas those on the 

 anterior surface are directed one directly downwards and the other 

 two upwards and to the sides (a). These lines are the edges of planes 

 or septa within the lens diverging from the axis, and receiving the ends 

 of the lens-fibres, which here abut against one another. As Tweedy 

 has pointed out, they may be seen, by the aid of the ophthalmoscope, 

 even during life. 



STRUCTURE OF THE LENS AND ITS CAPSULE. 



"When the lens has been hardened and the capsule removed, a succes- 

 sion of concentric laminae may be detached from it like the coats from 

 an onion. They are, however, not continuous all round as a rule, 

 being apt to separate into parts opposite the radiating lines above 

 described (fig. 435). The laminfB are composed of long, riband-shaped, 

 microscopic fibres, to^oo i^^ch broad, which adhere together by their 

 edges, the latter being often finely serrated (fig. 4oG, a), and pass in a 

 curved direction (fig. 436, c), from the intersecting planes of the anterior 

 half of the lens to those of the posterior half, or vice versd .- in this 

 course no fibre passes from one pole to the other, but those fibres which 

 begin near the pole or centre of one surface, terminate near the mar- 

 ginal part of a plane on the opposite surface, and conversely ; the inter- 

 vening fibres passing to their corresponding places bet ween. The 

 arrangement will be better understood by a reference to fig. 433, c, where 

 the course of the fibres is diagrammatically indicated. 



The lens fibres, as the history of their development shows, are to be 

 looked upon as very much elongated cells. In the young state each has 

 a clear oval nucleus, but in the fully-formed lens the nuclei have dis- 

 appeared from the fibres which form the more internal parts of the lens, 

 and only remain in the most superficial layers. Here they are found, 

 not quite in the middle of each fibre, but slightly nearer the anterior 

 end, their situation nearly corresponding in adjacent fibres, and they 

 form by their juxtaposition the so-called " nuclear zone" around the 

 lens. The superficial fibres further differ from the more deeply seated 

 ones in being softer, and in possessing a plain, unserrated margin. 

 The extremities of all the fibres are softer and more 

 readily acted on by reagents than the middle parts, 

 and the axial or more internal part of a fibre more 

 so than the external, but the transition is gradual 

 from one to the other, and there is no definite 

 membrane enclosing each fibre. The lens-fibres 

 when cut across are seen to be six-sided prisms 

 (fig. 43G, B). By reason of this shape, and the 

 serrations of their edges, they fit very exactly the 

 one to the other with but little interfibrillar 

 cementing substance between. This is met with 

 in rather larger quantity in the intersecting planes 

 between the ends of the fibres. In fishes, and some 

 other animals, the edges of the lens-fibres are 

 much more distinctly and regularly toothed than 

 in man. 



At the back of the lens the fibres are directly in contact with the 

 inner surface of the capsule, but in front they are separated from the 

 latter by a single layer of flattened, polygonal, nucleated cells (fig. 437), 



Yv'. 437 



<^lJ^ 



Fig. 437. — Cells ly- 

 ing BETWEEN THE 



Lens and its Cap- 

 sule (from Bow- 

 man). — 



