330 



these are more plentiful, and richer in pigment ; and in such 

 eyes the connective tissue corpuscles of the neighbouring 

 sclerotic are also not infrequently pigmented. This excess 

 of pigment expresses itself in the living eye by an incomplete 

 narrow brown or blackish circle around the nerve-disc. In 

 the plane of the choroid and of the inner third of the sclerotic 

 the nerve-opening is crossed by a fibrous web, the lamina 

 cribrosa, which peripherally merges in the connective tissues 

 of these two coats. The anterior surface of this perforated 

 lamina is concave, the posterior convex. In the living eye 

 the lamina reveals itself as a white tendinous spot striped 

 with minute grey dots, the bundles of nerve-fibres lying in 

 its meshes. These details of the lamina are recognizable in 

 the healthy nerve- disc in a small central area which corre- 

 sponds to a depression, which I shall presently describe, known 

 as the physiological pit. A sharply defined image of these 

 details of the lamina overstepping this limit and reaching 

 towards or even to the edge of the nerve-disc is a sign of 

 atrophy. 



The nerve-fibres in the trunk of the nerve behind the 

 lamina cribrosa are of the opaque or double bordered kind, 

 while in, and in front of the lamina, they are pale and trans- 

 parent. Behind the lamina, each nerve-fibre consists of an 

 axis cylinder, a delicate external tubular sheath (the homo- 

 logue of the sarcolemma of a primitive muscular fibre) and an 

 intermediate cortical substance or medulla — the white sub- 

 stance of Schwann. At the lamina the medulla ceases, the 

 axis cylinder with perhaps a very attenuated prolongation of 

 the sheath passing forward into the nerve disc and retina. The 

 greatly reduced bulk of the nerve-trunk in the lamina, and 

 the transparence of the nerve-bundles in the nerve-disc and 

 retina, are due to this change in the constitution of the pri- 

 mitive fibres. 



Exceptionally, as a congenital error, some bundles of 

 opaque nerve-fibres reach the inner surface of the nerve-disc, 

 and are even prolonged for some distance into the retina. It 

 is not a very uncommon defect. The opaque nerve-fibres 

 produce a white patch, which is to be distinguished from 

 other similar white patches due to exudations of its brush- 

 like feathered edge. After emerging from the anterior surface 

 of the lamina cribrosa, the bundles of transparent nerve-fibres 

 bend away on all sides (quoqueversally) from a central point, 

 and curving over the edge of the choroidal foramen spread 

 out on the inner surface of the retina. In doing this they leave 

 a central void, a little hollow — the physiological pit. This 

 pit is usually in the centre of the nerve-disc, but not always 



