272 



PHYSIOLOGY 



CHAP. 



The anterior or ciliary part consists only of a simple layer of a 

 different constitution, destitute of nerve-fibres, which runs from 

 the ora serrata to the apex of the ciliary processes, where it is 

 limited to the black pigment layer, which is continued on to the 

 posterior surface of the iris. The thickness of the retina diminishes 

 from behind, forwards, from 0'5 mm. at the entrance of the optic 

 nerve to 01 mm. near the ora serrata. We shall elsewhere 

 discuss its different layers and more intricate structure. 



FIG. 111. Section through the optic nerve at its entrance (/>) and an ophthalmoscopic view of the 

 disc (A) to show the corresponding parts. (Jaeger.) c, d, lines of correspondence ; 6, pit in 

 centre of disc ; r., retina ; ch., choroid ; si, so, inner and outer parts of sclera ; ci., a ciliary 

 artery cut longitudinally; a., -w., central artery and vein; sd., sub-dural space; sa., sub- 

 arachnoid space; du., dural sheath; ar., arachnoid sheath of nerve; p., pial sheath; ., 

 nerve-bundles ; se., septa between them. 



The optic nerve penetrates the eyeball, by perforating the 

 sclerotic and choroid, and spreads out in the retina at a point 

 which does not correspond with the posterior pole, but lies inside 

 the geometric axis of the eye. Viewed superficially with the 

 ophthalmoscope, the entrance of the optic nerve is seen as a disc 

 known as the papilla nervi optici, its periphery is slightly elevated, 

 but the centre, through which the retinal vessels emerge, forms 

 a depression or pit (Fig. Ill, A). In section, a number of 



