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so, and, when eccentric, the vasa centralia usually also per- 

 forate the disc eccentrically. A normal physiological pit 

 never, however, is so eccentric as to touch the contour of the 

 disc. 



The physiological pit is then a small, gentle, funnel-like 

 hollow in the centre of the nerve- disc perforated by the vasa 

 centralia, appearing as a bright hollow spot in which, with 

 an enlargement of 12 or 15 linear, the meshes of the lamina 

 cribrosa and the ends of the bundles of opaque nerve-fibres 

 are plainly discernible. 



Blood-vessels. — At a variable distance from the eyeball 

 the trunk of the optic nerve is pierced by a branch of the 

 arteria ophthal., which soon gains the axis of the nerve, and 

 running forAvards through the lamina cribrosa perforates the 

 optic nerve-disc, 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 intraocular 

 end are very numerous ; they are sufficiently so to redden the 

 disc when they are distended with blood — a thing which no 

 amount of hyperamia of the retinal capillaries ever does in 

 this membrane. 



The very slight diminution which the arteria centralis un- 

 dergoes from its origin to its final division in the disc, shows 

 that it mainly ministers to the nutrition of the retina. It 

 gives, however, in its course small twigs for the nutrition of 

 the nerve outside the eyeball, and these reinforced by others 

 derived from minute nameless arteries distributed to the 

 sheath, ramify in the septa between the nerve-bundles. 



The veinlets accompanying the primary branches of the 

 arteria centralis in the retina do not, as a rule, coalesce in a 

 single trunk in front of the lamina cribrosa, but they pierce 

 this separately and first unite in or behind it. 



Sheath. — In the arrangement of its sheath, the optic nerve 

 differs from all the other large nerve-trunks. They have but one 

 tightly fitting tube of connective tissue — the external neuri- 

 lemma ; but the optic nerve has a double sheath. It has a 

 thin tightly fitting sheath representing the sheath of other 

 nerves, from the inner surface of which, septa are produced 

 inwards between the nerve-bundles constituting the internal 

 neurilemma or frame which holds the bundles together and 

 carries the nutrient blood-vessels. 



In front of the lamina cribrosa in the nerve-disc, the 



