360 NORMAL HISTOLOGY. 



The lamina cribrosa consists of five to eight lamellae, composed 

 of transversely extending fibrous trabeculse, the direct pro- 

 longations of the scleral tissue. These bands bridge across what 

 otherwise would be a canal, and unite in such manner that the 

 openings occupied by the nerve-bundles present less area than the 

 intervening fibrous tissue. The fibrous lamellae, additionally con- 

 nected with one another by vertical bands, pass from the margins 

 of the scleral ring to the connective tissue supporting the blood-ves- 

 sels within the optic nerve. The lamina cribrosa marks the nar- 



FIG. 382. 



- 9 e 



Longitudinal section through optic entrance of human eye : a, a, bundles of optic fibres, which 

 spread over retina at a 1 ' , a! ; b, layers of retina terminating at edge of optic papilla ; c, choroid ; d, 

 sclera, continued across optic nerve as lamina cribrosa ; e, g, t, respectively pial, arachnoidean, and 

 dural sheaths, including subdural and subarachnoidean lymph-spaces; /, I', retinal vessels cut longi- 

 tudinally. 



rowest diameter of the optic nerve, the loss of the medullary 

 substance, together with the decrease in the neuroglia, reducing the 

 size of the nerve about one-half. On arriving at the margin of the 

 optic papilla, the bundles of nerve-fibres bend over its edges, con- 

 stituting a thick layer, which rapidly thins away during its radial 

 distribution over the retinal area. 



The centre of the optic papilla not infrequently presents a funnel- 

 shaped depression, at the bottom of which the retinal vessels 

 enter ; this depression, variable in size and form, but always retain- 

 ing sloping walls, is known as the physiological excavation, as 

 distinguished from those possessing the vertical or overhanging 

 walls indicative of grave pathological change. 



At some distance (15-20 mm.) from the eyeball the retinal blood- 

 vessels pierce the exterior of the optic nerve to take up a central 



