H72 



HUMAN ANATOMY. 



Fragments of isolated lens-fibres; A, from 

 superficial layers ; B, from deeper layers ; C, 

 young fibres with nuclei. X 275. 



for distant objects ; these radii are reduced to about 6 and 5 mm. respectively in 

 accommodation for near objects. The anterior surface is therefore more affected in 

 the act of accommodation, the lens becomes more convex and its antero-posterior 

 diameter increases from 4 to 4.4 mm. The superficial portion of the lens beneath 

 the capsule is composed of soft compressible material, the substantia corticalis ; the 

 consistency gradually increases toward the centre, especially in later life, so that the 

 central portion, the nucleus lentis, is much firmer and dryer. 



The structure of the lens includes the capsule and its epithelium and the lens 

 substance. The capsule, which entirely surrounds the lens, is a transparent, struc- 

 tureless, highly elastic membrane, which, while 

 resistant to chemical reagents, cuts easily and 

 then rolls outward. It is thickest on the anterior 

 surface, where it measures from .010 .015 mm., 

 and thinnest at the posterior pole (.005-. 007 

 mm. ). In the adult the lens is devoid of blood- 

 vessels, but during a part of foetal life it is 

 surrounded by a vascular net-work, the tunica 

 vasculosa lentis, which is supplied chiefly by the 

 hyaloid artery. This temporary vessel is the 

 terminal branch of the central artery of the retina 

 and passes from the optic disc forward through 

 the hyaloid canal or canal of Cloquet in the vit- 

 reous to the posterior surface of the lens. The 

 vascular lens tunic and the hyaloid artery are 

 temporary structures and usually disappear be- 

 fore birth. Exceptionally they may persist, 

 the tunic being represented by the pupillary 

 membrane and the artery by a fibrous strand within the vitreous, stretching from 

 the optic disc towards the lens. The capsule probably represents an exudation 

 product of the cuticular elements from which the lens- 

 substance is developed. 



The anterior portion of the capsule is lined by a sin- 

 gle layer of flat polygonal cells, the epithelium of the lens 

 capsule, which represents morphologically the anterior 

 wall of the original lens- vesicle (page 1481). On ap- 

 proaching the equator of the lens, these cells become 

 elongated, and gradually converted into the young lens- 

 fibres, the nuclei of which form a curved line, with its 

 convexity forward, in the superficial part of the lens. 



The lens-substance is composed of long flattened fibres, the cross-sections of 

 which have a compressed hexagonal outline, from .oo5-.on mm. broad and from 



.002 .004 mm. thick, held 



FIG. 1231. together by an interfibrillar 



cement substance. These 

 fibres are modified epithelial 

 elements, which develop by 

 the elongation of the original 

 ectoblastic cells of the poste- 

 rior layer of the lens-vesicle. 

 The subsequent growth of 

 the lens depends upon a 

 similar modification of the 

 anterior capsule-cells, the re- 

 gion where this transforma- 

 tion occurs being known as 

 the transitional zone. The individual lens-fibres vary greatly in length, those form- 

 ing the outer layers being longer and thicker than those which constitute the nucleus 

 of the lens. The edges of the fibres are finely serrated, and, as the points of the 

 serrations of adjacent fibres are in contact, fine intercellular channels are left for the 



FIG. 1230. 



Lens-fibres seen in transverse section. 

 X 280. 



Adult crystalline lens, showing lens-stars; A, anterior; fi, posterior 

 surface ; radiating lines of juncture meet at central area. X 4. (At until. ) 



