52 



MICROSCOPY OF THE LIVING EYE 



After what has been said about the cornea, Httle need be said 

 on the subject of direct iUumination in the lens, either by the 

 wide beam, or by the thin optical section, or by the small cylin- 

 drical beam ; the first and last are depicted in Figs. 16 and 49, b ; 

 thin optical sections of the lens are shown diagrammatically in 

 Fig. 30, A, B, C, the patient being supposed to be looking some- 

 what upwards in A, horizontally in B, and somewhat down- 

 wards in C. Sections depict how the posterior capsular surface 

 has a curvature of shorter radius than the anterior capsular 

 surface. The human lens, in thin optical section, reveals 

 anatomical zones defined by thin curved lines of increased relu- 

 cency. First a subcapsular line almost concentric with, and very 

 close beneath, each lens-capsule ; the anatomical significance of 

 this line is not quite certain because the space between it and the 

 surface far exceeds the thickness of the capsule. Next in order, 

 proceeding towards the centre of the lens from either the anterior 

 or posterior capsule, comes a senile or adult demarcating zone repre- 

 senting the limit of the new cortex laid on beneath the capsule 

 in adult life ; next a boundary (at the approximate level of the 

 upright and inverted Y sutures respectively) which marks the 

 demarcation between the cortex of childhood and that — the most 

 central — of the intranatal period. These zones, with the exception 

 of the subcapsular, are not always clearly defined with ease. 



It is important to remember that anatomical disturbances 

 early in the course of the optical section of a relucent transparent 

 tissue may produce artificial optical effects in the deeper region of 

 the section which must not be misinterpreted as being anatomical 

 features. Thus, partly obstructive features, like blood vessels 

 cutting the thin optical section of the cornea rob the light from the 

 region of the section axially beyond them and produce " shadow- 

 streaks " (Fig. 43, X) running back parallel with the optical axis 

 of the slit-lamp. These may be distinguished from shadow- 

 streaks due to dust or particles bridging the jaws of the slit of the 

 lamp by the fact that they are stationary when the focusing lens 

 is moved up and down, whereas they move with the lens when due 

 to particles in the slit. Shadow-streaks in the optical section of 

 the lens caused by a pathological change in the anterior cortex 



