354 MICROSCOPIC .LY.l/o.l/r OF THE ORGAX\ 



become finally a single layer of cylindrical epithelium. The 

 supporting cells of Miiller are well developed here. 



2. In the pars cUiaris retinae we find only two layers of 

 cells. Toward the outside there is pig men ted epithelium, while 

 on the inner side there is a layer of high cylindrical cells-, 

 which are derived from the neuro-epithelial layer. These 

 cylindrical cells take the place of the layer of visual cells and 

 the outer granular layer, which is still to be seen at the ora 

 serrata. 



3. Pars iridica retinas, see Iris. 



(4) The Optic Nerve. 



The optic nerve possesses three sheaths, which are to be 

 regarded as continuous with the membranes of the brain. The 

 dura mater forms the outermost sheath, the arachnoidea the 

 second, and that which lies immediately on the nerve is derived 

 from the pia mater and sends septa between the individual 

 fibre bundles. Between the processes of the dura mater and 

 the arachnoidea, and between the arachnoidea and the pia 

 mater, there are two spaces, of which the first is in communi- 

 cation with the subdural space, and the second with the 

 subarachnoid space. All three sheaths are bound together 

 by connective-tissue strands, which cross over through the 

 spaces. 



At the entrance of the optic nerve into the eyeball the 

 dural and pial sheaths pass over into the sclera. The arach- 

 noidea, on the contrary, breaks up into fibres before it reaches 

 the sclera, so that the subdural and subarachnoidal spaces com- 

 municate with one another. 



Where the optic nerve enters the eye, the sclera and choroid 

 are pierced and perforated, so that they are reduced to a lattice- 

 work tissue, which we call the lamina cribrosa. 



The fibres of the optic nerve are medulkted but possess no 

 sheath of Schwann. As the fibres pass through the chorioid 

 and sclera they lose their medullary sheath and pass over on 

 the inner surface of the retina as naked axis cylinders, which 

 form the optic nerve-fibre layer. In consequence of the loss 



