820 



THE OKGANS OF SENSE. 



FIG. 695. DIAGRAMMATIC REPRESENTATION OF THE RADIAL 



LINES OF THE F(ETAL LENS. 



A, Seen from the front ; B, From behind. 



respectively its anterior and posterior poles, a line joining" which is known as its 

 axis ; its peripheral circumference is named the equator. Its axial measurement 

 is 4 mm., and its transverse diameter from 9 to 10 mm. Its anterior surface is less 



curved than the posterior and on 

 it rests the pupillary margin of 

 the iris ; the central part of the 

 surface corresponds with the aper- 

 ture of the pupil and is directed 

 towards the anterior chamber ; the 

 peripheral part is separated from 

 the iris by the aqueous humour of 

 the posterior chamber. Its pos- 

 terior surface, more convex than 



^ anteriorj QCCUpicS the hyaloid 



fossa of the vitreous body. The 

 curvatures of its surfaces, especi- 

 ally that of the anterior, are constantly varying, during life, for the purpose of 

 focussing near or distant objects on the retina. 



The substantia lentis consists of a soft outer part, the substantia corticalis, easily 

 crushed between the finger and thumb, and of a dense central part, the nucleus 

 lentis. The refractive index of the substantia corticalis is 

 about 14 ; that of the nucleus lentis about 145. Faint radial 

 lines run from the anterior and posterior poles of the lens 

 towards its equator. In the foetus they are three in number, and 

 form angles of 120 with each other (Fig. 696). 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 fibres come into 

 contact, and are attached by a clear, amorphous substance. 

 The lens, when hardened, exhibits a series of concentrically 

 arranged laminae (Fig. 694), superimposed like the scales of an 

 onion and attached to each other by a clear, amorphous sub- 

 stance. Each lamina is split along the radiating lines, and 

 consists of a series of hexagonal, riband-like fibres, the fibrse 

 lentis, which are adherent to each other by their margins ; 

 those of the deeper laminae are small and serrated, but non- 

 nucleated ; while those of the superficial coats are large 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 inter- 

 vening fibres take up intermediate 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 SECTION THROUGH THE 

 gradually elongated and transformed into lens fibres, which, 

 when fully formed, lose all trace of their nuclei, except in Showing the gradual tean 

 the more superficial laminae. Each lens fibre represents, there- h^lens ^bres Rafter 

 fore, a greatly elongated columnar cell (Fig. 696). Babuchiu). 



In the foetus the lens is soft, of a pinkish colour, and 



nearly spherical ; while in old age it becomes somewhat flattened, and assumes a 

 yellowish tint. 



Cameras Oculi (Fig. 677). As already stated (p. 813), the space between the 

 cornea and the lens is divided by the iris into two unequal parts, viz., the camera 



FIG. 696. 



