348 MANUAL OF HISTOLOGY. 



The optic nerve, after leaving the optic canal, passes through 

 the orbit surrounded by three coverings, continuations of the 

 cerebral membranes. 



The dural coat, composed of dense connective tissue with a 

 few elastic fibres, forms the outer covering ; the fibres are at- 

 tached to the periosteum, where the nerve leaves the bony canal, 

 and where it enters the eyeball they are continued directly into 

 the outer layers of the sclera. 



Within this covering, and separated from it by a very nar- 

 row space, are the delicate fibres of the arachnoidal coat, 

 and the lymph-space between the two is called the subdwral 

 space. 



Within the arachnoidal coat, and separated from it by a 

 wide lymph-space, is the pial coat closely surrounding the 

 nerve-fibres, and sending processes of connective-tissue be- 

 tween their bundles. This membrane passes into the inner 

 layers of the sclera, and also sends numerous fibres to the la- 

 mina cribrosa. Its outer surface is covered with endothelium, 

 and between it and the arachnoid coat is the subaraclmoid 

 space, which reaches to the inner layers of the sclera, and is 

 continuous with the same space in the brain. 



The optic nerve itself, closely surrounded by its vagina 

 fibrosa, passes forward through the orbit, receiving the central 

 artery and vein at about 15 to 20 mm. from the sclera. These 

 vessels pass to the centre of the nerve and lie in a connective- 

 tissue sheath until they emerge on the inner surface of the eye- 

 ball to branch over the retina. 



On cross-sections of the nerve, bundles of connective tissue 

 are seen to pass inward from the pial sheath and form a cross- 

 network, through the openings of which the nerve-fibres pass. 

 On longitudinal sections the connective tissue appears in 

 irregular fenestrated sheaths ; this tissue can also be demon- 

 strated by macerating thick sections in a | per cent, solution 

 of chromic acid and then brushing out the nerve-elements. 



These nerve-filaments themselves are extremely small, but 

 vary somewhat in size. They consist of an axis-cylinder sur- 

 rounded by its medullary sheath; they are grouped in large 

 bundles which pass through the meshes of the connective tis- 

 sue. The fibres appear to be held together by a kind of homo- 

 geneous albuminous substance neuroglia, and have on their 

 surface occasional nucleated corpuscles, distinguished from 



