pe eer aS ae Crystalline Lens in Man. 131 
The axis in the simplest form of lens as that of some fish and 
of amphibia is a streak or line of this interstitial substance travers- 
ing the centre of the lens. The lens fibres are grouped around it 
in the manner of the meridian lines upon a globe, the inner fibres 
progressively shortening towards the centre of the lens. In other 
fish, and in the porpoise and the rabbit amongst mammals, the axial 
streak is flattened. It extends im 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 two edges of this which may be dis- 
tinguished 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 back 
and on the front 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, while 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 mammals, 
is present also in the human fcetal lens, which has been already 
mentioned is nearly spherical. As the lens enlarges, the three 
primary planes detach secondary and tertiary ones, the multiplica- 
tion continues during the whole period of growth, until in the 
human adult the minor planes form an excessively complex frame. 
With 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 when artificially hardened can be split by a coarse dis- 
section into concentric lamina. 
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 
