THE EYE. 259 



wards (/*'). It passes over into a nerve fibre of the optic 

 nerve-fibre layer, the stratum fibrillosum (i). 



In order to comprehend the latter we must commence with 

 the contents of the optic nerve. It has medullated nerve 

 fibres, 0.0045 t° 1. 0014 mm. thick. Having entered the ball 

 of the eye, their medullary sheath is lost, and they become 

 pale axis cylinders.* 



Having advanced into the retina, our optic nerve fibres di- 

 vide and reunite at acute angles into bundles, forming a nerve 

 plexus. In proportion as we follow their further course for- 

 wards, the fibre bundles become thinner and thinner, and the 

 distance between them is constantly increased. At last we 

 meet with only isolated axis cylinders. 



We have grounds for assuming that each optic-nerve fibre 

 penetrates the body of a ganglion cell as an axis-cylinder 

 process ; still we cannot prove this at the present time. 



The membrana limitans interna, of a connective-tissue na- 

 ture, has been previously mentioned. 



The best portion of the retina, the yellow spot or macula 

 lutea, requires a short mention. 



The connective-tissue frame-work substance, with the ex- 

 ception of the limitans interna, is undeveloped. The nerve- 

 fibre layer likewise disappears ; the layer of the ganglion 

 cells, still largely developed at the periphery, also disappears 

 completely in the centre of the fovea. The molecular and 

 inner granular layers also suffer the same fate. There re- 

 mains, therefore, only the (exclusively occurring) cones with 

 the stratum granulosum externum. 



The latter (Fig. 207) are no longer as of old. Their body 

 has at last become narrowed to 0.0028 to 0.0033 mm. 

 (Schultze) ; it has diminished to nearly the thinness of the 

 rod, and the cone rod too.ooi to 0.0009 mm. The cone fibre 

 appears to have participated but little in this thinning. The 



* It is remarkable that in individual human retinas the medullary sheath of the 

 nerve tubes is preserved. In the dog the same not unfrerruently occurs ; in rab- 

 bits and hares it is even the rule. 



