358 NORMAL HISTOLOGY. 



The nerve-fibre and the ganglion-cell layer having already ended 

 before reaching the ora, the sudden reduction is caused principally 

 by the abrupt termination of the two reticular strata. The 



FIG. 379. 



Section of human retina through ora serrata : A, B, visual and ciliary portion of 

 retina; a, vacuoles ; b, robust fibres of Miiller; c, remains of nuclear layers; d, 

 termination of supporting fibres ; e, transformation of inner nuclear layer into colum- 

 nar cells within continuation of pigment-layer. 



region of the ora serrata is also noteworthy on account of the re- 

 markable development of the radial fibres of Miiller, which 

 here occur not only in unusual numbers but also of exceptional 

 strength. 



Beyond the ora serrata the retinal laminae are continued as the 

 pars ciliaris and the pars iridica retinae. These prolongations 

 consist of an outer and an inner lamina. The outer layer is 

 the direct and only slightly modified extension of the retinal 

 pigment ; the inner lamina, the attenuated representative of the 

 remaining retinal layers, consists of a single row of slender colum- 

 nar cells, which originate at the ora by the transformation of the 

 elements of the inner nuclear layer. A delicate cuticle, the 

 limitans interna, extends over the posterior surface of both the 

 ciliary body and the iris ; this membrane is a true cuticular for- 

 mation, and begins at the ora as a new structure not present 

 within the optical part of the retina. 



THE OPTIC NERVE. 



The optic nerve corresponds to a highly-developed single fu- 

 niculus, enveloped by stout connective-tissue sheaths, which are 

 prolongations of the brain-membranes. Externally the optic nerve 

 is invested by a robust fibrous membrane, the dural sheath, de- 

 rived directly from the dura ; this covering extends the entire length 

 of the nerve, and on the entrance of the latter into the eyeball be- 

 comes continuous with the outer part of the sclera. The surface 

 of the optic nerve is closely invested with the pial sheath, an 

 extension of the pia, while between the latter and the dural covering 

 lies a delicate partition from the arachnoid, constituting the arach- 

 noidean sheath. The clefts included between these sheaths con- 



