694 THE ORGANS OF SENSE. 
anterior and posterior poles, a line joining which is known as its axis (axis lentis) ; 
its peripheral circumference is named the equator (zquator lentis). Its axial 
measurement is + mm. and its transverse diameter from 9-10 mm. Its anterior 
surface (facies anterior lentis) is less curved than the posterior, and its central 
part corresponds with the aperture of the pupil and is directed towards the anterior 
chamber. Around the aperture of the pupil the pupillary margin of the iris 
rests upon the lens, but its peripheral part is separated from it by the aqueous 
humour of the posterior chamber. Its posterior surface (facies posterior lentis), 
more convex than the anterior, occupies the fossa patellaris of the vitreous body. 
The relations of its equator to the suspen- 
sory ligament and the canal of Petit have 
already been referred to. The superficial 
part of the lens possesses a refractive index 
of about 1-4 and its central part or nucleus 
one of about 1:45. The curvatures of its 
surfaces, especially that of the anterior, are 
constantly varying during life for the pur- 
pose of focussing rays from near or distant 
objects on the retina. 
The lens substance (substantia lentis) 
consists of a soft outer part, the substantia 
Fig, 504.—LENS HARDENED IN FORMALIN AND orticalis, easily crushed between the finger 
DISSECTED TO SHOW ITS CONCENTRIC LAMINAE , = 
(enlarecd): and thumb, and of a dense central part, the 
nucleus lentis. aint lines (radi lentis) 
radiate from its anterior and posterior poles towards its equator. In the foetus they 
are three in number and form with each other angles of 120° (Fig. 505). From the 
anterior pole one ray ascends vertically and the other two diverge downwards, 
while from the posterior pole one ray descends vertically and the other two 
diverge upwards. In the adult the rays may be increased to six or more. They 
represent the free edges of a corresponding number of septa, which dip into the 
substance of the lens, and along which the extremities of the different groups of 
lens tibres come into contact and are attached by a clear, amorphous substance. The 
lens, when hardened, exhibits a 
series of concentrically-arranged 
lamin (Fig. 504), superimposed 
like the coats of an onion and 
attached to each other by a clear, 
amorphous substance. — _Kach 
lamina is spht along the radiat- 
ing lines and consists of a series 
of hexagonal, riband-like fibres, 
the fibre lentis, which are ad- th 
herent to each other by their Fic. OT PaDH eee gE EEO Or THE Rabi 
: 5 JENTIS OF THE Fara LENs. 
Margins , those of the deeper A, Seen from the front ; B, From behind, 
laminze are smaller and serrated, 
but non-nucleated; while those of the superficial coats are larger and nucleated, 
but non-serrated. The fibres extend in a curved manner from the rays on the 
anterior surface to the rays on the posterior surface, but no fibre extends from 
pole to pole. Fibres which start at or near one pole end at or near the equator on 
the opposite surface, and vice versa, while the imtervening fibres take up inter- 
mediate positions. Between the substantia lentis and the anterior part of the 
capsule there is a layer of nucleated columnar epithelial cells, the epithelium 
lentis. On being traced towards the equator its cells become gradually elongated 
and transformed into lens fibres, which, when fully formed, lose all trace of their 
nuclei, except in the more superficial laminve. Each lens fibre represents, therefore, 
a greatly elongated columnar cell (Fig. 506). 
In the foetus the lens is soft, of a pinkish colour, and nearly spherical; while in 
old age it becomes more flattened than in the adult, and, losing its transparency, 
assumes a yellowish tint. 
Nucleus | 
lentis 
