566 



THE ORGANS 



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the subdural space (Fig. 375, c). The pial sheath is divisible into 

 two sub-layers: an outer fibrous layer (the so-called arachnoid), and 

 an inner vascular layer. These two layers are separated by a narrow 

 space, the subarachnoid space. The optic nerve fibres, in passing 

 through the sclera and chorioid, separate the connective-tissue 



bundles so that they form 

 a lattice-work, the already 

 mentioned lamina crihrosa 

 (Fig. 375, }i)- The optic 

 nerve fibres are medullated, 

 but have no neurilemma. 

 As they pass through the 

 lamina cribrosa the medul- 

 lary sheaths are lost, the 

 fibres reaching the retina as 

 naked axones. 



Relations of Optic Nerve 

 TO Retina and Brain 



The rod and cone visual 

 cells are the neuro-epithelial 

 beginnings of the visual 

 tract (Fig. 373, A, 3, 4, 5, 

 and 6). By their expanded 

 bases in the outer molecular 

 layer, the rod and cone cells 

 communicate with the neu- 

 rone system No. I. of the 

 optic tract. This comprises 

 (fl) rod neurones, {h) cone 





Fig. 375. — Section through Entrance of Optic 

 Nerve into Eyeball. (Merkel-Henle.) a, Dural 

 sheath; b, pial sheath, inner and outer layers; c, 

 space between inner and outer layers of pia mater; 

 d, optic nerve; c, central artery of retina; a' , 

 sclera; /, chorioid; , retina; h, lamina cribrosa. 



neurones, (c) horizontal neurones. 



Neurone System No. I. — {a) Rod neurones. The cell bodies 

 of these neurones (Fig. 373, A, 20) lie in the inner nuclear layer. 

 Their dendrites enter the outer molecular layer where they form net- 

 works around the expanded bases of the rod cells. Their axones pass 

 through the inner molecular layer and end in arborizations around the 

 bodies and dendrites of cells of the nerve-cell layer (neurone system 

 No. II.). {h) Cone neurones (Fig. 373, ^, 7). These have their cell 

 bodies in the inner nuclear layer. Their dendrites pass to the outer 

 molecular layer where they form networks around the expanded bases 



