322 



is flattened. It extends in two opposite directions, and forms 

 a central plane, the ends of which make a linear stigma on 

 the front and back of the lens. The directions of these 

 stigmata do not coincide ; they intersect at a right angle as 

 they would if the plane of which they are the ends had been 

 twisted through 90"^ in passing through the lens. From the 

 edges of this, which may be distinguished as the primary 

 plane, others, secondary planes, run out in a complicated 

 manner towards the circumference of the lens. The primary 

 central planes in most mammalia diverge at equal distances 

 of 120° from the axis, and their ends form on the front and 

 back of the lens a trifid stigma. The rays of one stigma 

 intersect the angles included between those of the other 

 stigma, as they would do if the tripartite axis had been 

 twisted through 60°. In these lenses the arrangement of 

 the fibres with respect to the central planes is much more 

 complicated than in the simple amphibian lens. All its 

 details are probably not yet known to us ; but, so far as they 

 have been ascertained, it appears that the fibres pass between 

 the front and back of the lens, winding round its equatorial 

 edge in such a manner that a fibre starting from the interval 

 between two of the front planes falls behind on the edge of a 

 posterior plane. Mobile a fibre starting from the edge of one of 

 the front planes would fall behind in the angle made by two 

 of the posterior planes. The intervening fibres between these 

 extremes take intermediate positions on the planes. 



The tripartite division of the axis persistent in many mam- 

 mals is present also in the human foetal lens, which, as has 

 been already mentioned, is nearly spherical. As the lens 

 enlarges, the three primary planes detach secondary and 

 tertiary ones ; the multiplication continues during the whole 

 period of growth, until in the human adult the minor planes 

 form an excessively complex frame. 



Witli this excessive complexity of the planes the fibres 

 maintain a general direction between the front and the back 

 of the lens ; and, since the surfaces of the fibres cohere less 

 strongly than their edges, most lenses, Avhen artificially har- 

 dened, can be split by a coarse dissection into concentric 

 laminae. 



In youth the lens is soft, but with advancing years it 

 acquires greater consistence, becoming in aged persons really 

 hard. It is to this change, in consequence of which the lens 

 becomes less plastic as we advance in life, that presbyopia is 

 mainly due. In birds and in lizards the nucleus only is con- 

 centrically laminated ; and the outer fibres pass vertically and 

 obliquely between the capsule and nucleus. 



