224 The Ciliary Muscle and [ Sours, Oce aeaee 
the lamina they are pale and transparent. Behind the lamina each 
nerve-fibre consists of an axis-cylinder—a delicate external tubular 
sheath (the homologue of the sarcolemma of a primitive muscular 
fibre), and of an intermediate cortical substance or medulla—the 
white substance of Schwann. At the lamina the medulla ceases, 
the axis-cylinder with perhaps a very attenuated prolongation of 
the sheath passing forwards into the nerve-disc and retina. The 
greatly reduced bulk of the nerve in the lamina, and the transpa- 
rence of the nerve-bundles in the nerve-disc and retina, are due to 
this change in the constitution of the primitive nerve-fibres. Hx- 
ceptionally, as a congenital error, some bundles of opaque nerve- 
fibres reach the inner surface of the nerve-disc, and are even pro- 
longed for some distance into the retina. This is not a very 
uncommon defect. The opaque nerve-fibres produce a white par- 
ticle, which is to be distiguished from similar particles, due to 
exudations, by 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, qua- 
quaversally, 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 small hollow—the phy- 
siological pit. This pit is usually in the centre of the nerve-dise, 
but not always so, and when excentric the vasa centralia usually 
also pierce the disc excentrically. A normal physiological pit never, 
however, is so excentric as to touch the contour of the disc. 
The physiological pit is then a small funnel-like hollow in 
the centre of the nerve-disc, perforated by the vasa centralia, ap- 
pearing as a bright, hollow spot, in which, with an enlargement 
of twelve or fifteen diameters, the meshes of the lamina cribrosa 
and the ends of the bundles of opaque nerve-fibres are plainly dis- 
cernible. 
Blood-vessels.—At a variable distance from the eye-ball the 
trunk of the nerve is pierced by a branch of the ophthalmic artery, 
which soon gains the axis of the nerve, and running forwards 
through the lamina cribrosa, perforates the nerve-dise in which it 
divides into two primary branches which bifurcate, and passing 
across the boundary of the disc are distributed to the retina. In 
the disc we can distinguish, first, a short vertical piece of the 
arterial trunk, and next, the branches making a large angle with 
the trunk and following the surface of the disc. The capillaries of 
the nerve-trunk and those distributed to its disc-like interocular 
end are very numerous; they are sufficiently abundant to redden 
the disc when distended with blood, a thing which no amount of 
hypercemia of the retinal capillaries ever does in this membrane. 
The very slight diminution which the arteria centralis undergoes 
from its origin to its final termination in the disc, shows that it 
